Part 3

ENGINE OBD II MONITORS

Part 3 of 3

Continued From Part 2 Part 2

Low Data Rate System (LDR)

The LDR misfire monitor uses a low data rate crankshaft position signal, one position reference signal at 10 degrees before top dead center (BTDC) for each cylinder event. The PCM uses the CKP signal to calculate the crankshaft speed and acceleration for each cylinder. The crankshaft acceleration is then processed to detect a sporadic, single-cylinder misfire patterns. The changes in overall engine RPM are removed by subtracting the median engine acceleration over a complete engine cycle. The resulting deviant cylinder acceleration values are used in evaluating misfire. See Generic Misfire Processing below.

High Data Rate System (HDR)

The HDR misfire monitor uses a high data rate crankshaft position which indicates 18 position references per crankshaft revolution. This high resolution signal is processed using 2 different algorithms. The first algorithm is optimized to detect hard misfires on one or more continuously misfiring cylinders. The low pass filter filters the high-resolution crankshaft velocity signal to remove some of the crankshaft torsional vibrations that degrade signal to noise. Two low pass filters are used to enhance detection capability - a base filter and a more aggressive filter to enhance single-cylinder capability at higher RPM. This significantly improves detection capability for continuous misfires on single cylinders up to redline. The second algorithm, called pattern cancellation, is optimized to detect low rates of misfire. The algorithm learns the normal pattern of cylinder accelerations from the mostly good firing events and is then able to accurately detect deviations from that pattern. Both the hard misfire algorithm and the pattern cancellation algorithm produce a deviant cylinder acceleration value, which is used in evaluating misfire in the Generic Misfire Processing below. Due to the high data processing requirements, the HDR algorithms may be implemented by the PCM in a separate chip. The chip carries out the HDR algorithm calculations and sends the deviant cylinder acceleration values to the PCM microprocessor for additional processing as described below. The chip requires correct operation of the CKP and camshaft position (CMP) sensor inputs. DTC P1336 will be set if the chip detects noise on the CKP sensor input or if the chip is unable to synchronize with the missing tooth location. A DTC P1336 points to noise present on the CKP sensor input or a lack of synchronization between the CMP and CKP sensors.

Generic Misfire Processing

The acceleration that a piston undergoes during a normal firing event is directly related to the amount of torque that cylinder produces. The calculated piston/cylinder acceleration value(s) are compared to a misfire threshold that is continuously adjusted based on inferred engine torque. Deviant accelerations exceeding the threshold are conditionally labeled as misfires. The calculated deviant acceleration value(s) are also evaluated for noise. Normally, misfire results in a nonsymmetrical loss of cylinder acceleration. Mechanical noise, such as rough roads at high RPM with light load conditions, will produce symmetrical acceleration variations. Cylinder events that indicate excessive deviant accelerations of this type are considered noise. Noise-free deviant acceleration exceeding a given threshold is labeled a misfire. The number of misfires are counted over a continuous 200 revolution and 1,000 revolution period. The revolution counters are not reset if the misfire monitor is temporarily disabled such as for negative torque mode. At the end of the evaluation period, the total misfire rate and the misfire rate for each individual cylinder is computed. The misfire rate is evaluated every 200 revolution period (Type A) and compared to a threshold value obtained from an engine speed/load table. This misfire threshold is designed to prevent damage to the catalyst due to sustained excessive temperature 899°C (1,650°F) for Pt/Pd/Rh conventional advanced washcoat, 982°C (1,800°F) for Pt/Pd/Rh advanced washcoat and 982°C (1,800°F) for Pd-only high tech washcoat. If the misfire threshold is exceeded and the catalyst temperature model calculates a catalyst mid-bed temperature that exceeds the catalyst damage threshold, the MIL blinks at a 1 Hz rate while the misfire is present. If the threshold is again exceeded on a subsequent driving cycle, the MIL is illuminated. If a single cylinder is determined to be consistently misfiring in excess of the catalyst damage criteria, the fuel injector to that cylinder shuts off to prevent catalyst damage for a calibrated period of time, typically 30 to 60 seconds. Up to 2 cylinders may be disabled at the same time on 6- and 8-cylinder engines and one cylinder on 4 cylinder engines. After the calibrated period of time has elapsed, the injector is re-enabled. If misfire on that cylinder is detected again after 200 revs (about 5 to 10 seconds), the fuel injector is shut off again and the process repeats until the misfire is no longer present. Note that ignition coil primary circuit failures trigger the same type of fuel injector disablement. See Comprehensive Component Monitor (CCM). The misfire rate is also evaluated every 1,000 revolution period and compared to a single (type B) threshold value to indicate an emission-threshold concern, which can be either a single 1,000 over-rev event from startup or 4 subsequent 1,000 over-rev events on a drive cycle after start-up. Many vehicles will set DTC P0316 if the type B threshold is exceeded during the first 1,000 revolutions after engine startup. This DTC is stored in addition to the normal P03xx DTC that indicates the misfiring cylinder. If the misfire is detected but it cannot be attributed to a specific cylinder, a P0300 is stored.

Rough Road Detection

The misfire monitor may include a rough road detection system to eliminate false misfire indications due to rough road conditions. The rough road detection system uses data from the ABS wheel speed sensors for estimating the severity of rough road conditions. This is a more direct measurement of rough road over other methods which are based on driveline feedback via crankshaft velocity measurements. It improves accuracy over these other methods since it eliminates interactions with actual misfire.
In the event of an rough road detection system failure, the rough road detection output is ignored and the misfire monitor remains active. An rough road detection system failure could be caused by a failure in any of the input signals to the algorithm. This includes the ABS wheel speed sensors, brake pedal sensor, or controller area network (CAN) bus hardware failures. Specific DTCs indicate the source of these component failures.
A redundant check is also carried out on the rough road detection system to verify it is not stuck high due to other unforeseen causes. If the rough road detection system indicates rough road during low vehicle speed conditions where it is not expected, the rough road detection output is ignored and the misfire monitor remains active.

Profile Correction

Profile correction software is used to learn and correct for mechanical inaccuracies in the crankshaft position wheel tooth spacing. Since the sum of all the angles between the crankshaft teeth must equal 360 degrees, a correction factor can be calculated for each misfire sample interval that makes all the angles between individual teeth equal. The LDR misfire system learns one profile correction factor per cylinder (for example, 4 correction factors for a 4-cylinder engine), while the HDR system learns 36 or 40 correction factors depending on the number of crankshaft wheel teeth (for example, 36 for V6/V8 engines).
The corrections are calculated from several engine cycles of misfire sample interval data. The correction factors are the average of a selected number of samples. In order to ensure the accuracy of these corrections, a tolerance is placed on the incoming values such that an individual correction factor must be repeatable within the tolerance during learning. This is to reduce the possibility of learning corrections on rough road conditions which could limit misfire detection capability and to help isolate misfire diagnoses from other crankshaft velocity disturbances.
To prevent any fueling or combustion differences from affecting the correction factors, learning is done during deceleration fuel shut-off (DFSO). This can be done during closed-throttle, non-braking, de-fueled decelerations in the 97 to 64 km/h (60 to 40 mph) range after exceeding 97 km/h (60 mph) (likely to correspond to a freeway exit condition). In order to minimize the learning time for the correction factors, a more aggressive deceleration fuel shut-off strategy may be used when the conditions for learning are present. The corrections are typically learned in a single 97 to 64 km/h (60 to 40 mph) deceleration, but may take up to 3 such decelerations or a higher number of shorter decelerations.
Since inaccuracies in the wheel tooth spacing can produce a false indication of misfire, the misfire monitor is not active until the corrections are learned. In the event of battery disconnection or loss of keep alive memory (KAM), the correction factors are lost and must be relearned. If the software is unable to learn a profile after three, 97 to 64 km/h (60 to 40 mph) deceleration cycles, DTC P0315 is set.

Neutral Profile Correction and Non-Volatile Memory

The neutral profile correction strategy is only available on selected vehicles. The 60-40 mph decel profile learning algorithm is active on all vehicles in current production.
Neutral profile learning is used at end of line to learn profile correction through a series of one or more neutral engine RPM throttle snaps. This allows the misfire monitor to be activated at the assembly plant. A scan tool command is required to enable neutral profile correction learning. Learning profile correction factors at high-speed (3,000 rpm) neutral conditions versus during 60-40 mph decels optimizes correction factors for higher rpms where they are most needed and eliminates driveline/transmission and road noise effects. This improves signal-to-noise characteristics which means improved detection capability.
The profile correction factors learned at the assembly plant are stored into non-volatile memory. This eliminates the need for specific customer drive cycles. However, misfire profiles may need to be relearned using a scan tool procedure if major engine work is done or the PCM is replaced. Re-learning is not required for a reflash.

Misfire Monitor Specifications

Misfire monitor operation: DTCs P0300 to P0310 (random and specific cylinder misfire), P1336 (noisy crank sensor, no crankshaft/camshaft sensor synchronization), P0315 (crankshaft position system variation not learned), P0316 (misfire detected on startup [first 1000 revolutions]). The monitor execution is continuous, misfire rate calculated every 200 or 1,000 revolutions. The monitor does not have a specific sequence. The CKP, CMP, MAF, and ECT or CHT sensors have to be operating correctly to run the monitor. The monitoring duration is the entire driving cycle (see disablement conditions below).
Typical misfire monitor entry conditions: Entry condition minimum/maximum time since engine start-up is 0 seconds, engine coolant temperature is -7°C to 121°C (20°F to 250°F), RPM range is (full-range misfire certified, with 2 revolution delay) 2 revolutions after exceeding 150 RPM below drive idle RPM to redline on tach or fuel cutoff, profile correction factors are learned in KAM are Yes, and the fuel tank level is greater than 15%
Typical misfire temporary disablement conditions: Closed throttle deceleration (negative torque, engine being driven), Fuel shut-off due to vehicle-speed limiting or engine-RPM limiting mode, and a high rate of change of torque (heavy throttle tip-in or tip out) and rough road conditions.
The profile learning operation includes DTC P0315 - unable to learn profile in 3, 97 to 64 km/h (60 to 40 mph) decelerations; monitor execution is once per profile learning sequence; The monitor sequence: profile must be learned before the misfire monitor is active; The CKP and CMP sensors are required to be OK; CKP/CMP signals must be synchronized. The monitoring duration is 10 cumulative seconds in conditions (a maximum of 3, 97 to 64 km/h (60 to 40 mph) de-fueled decelerations).
Customer drive cycle typical profile learning entry conditions: Entry conditions from minimum to maximum: Engine in deceleration fuel shut-off mode for four engine cycles, the brakes are not applied, the engine RPM is 1,300 to 3,700 RPM, the change is less than 600 RPM, the vehicle speed is 48 to 121 km/h (30 to 75 mph), and the learning tolerance is 1%.
Assembly plant or repair facility typical profile learning entry conditions: Entry conditions from minimum to maximum: Engine in deceleration fuel shut-off mode for four engine cycles, the vehicle in park/neutral gear, the engine RPM is 2,000 to 3,000 RPM, the learning tolerance is 1%.

Positive Crankcase Ventilation (PCV) System Monitor

The PCV monitor consists of a modified PCV system design. The PCV valve is installed into the rocker cover using a quarter-turn cam-lock design to prevent accidental disconnection. High retention force molded plastic lines are used from the PCV valve to the intake manifold. The diameter of the lines and the intake manifold entry fitting are increased so that inadvertent disconnection of the lines after a vehicle is repaired causes either an immediate engine stall or does not allow the engine to be restarted. In the event that the vehicle does not stall if the line between the intake manifold and PCV valve is inadvertently disconnected, the vehicle has a large vacuum leak that causes the vehicle to run lean at idle. This illuminates the MIL after 2 consecutive driving cycles and stores one or more of the following DTCs: Lack of HO2S sensor switches, bank 1 (P2195), Lack of HO2S sensor switches bank 2 (P2197), fuel system lean, bank 1 (P0171) or fuel system lean, bank 2 (P0174).
For additional PCV information, see Electronic Engine Control (EEC) System.

Thermostat Monitor





The thermostat monitor is designed to verify correct thermostat operation. This monitor is executed once per drive cycle and has a monitor run duration of 300-800 seconds. If a malfunction is present, DTC P0125 or P0128 is set and the malfunction indicator lamp (MIL) is illuminated.
The monitor checks the engine coolant temperature (ECT) or cylinder head temperature (CHT) sensor to warm up in a predictable manner when the engine is generating sufficient heat. A timer is initialized while the engine is at moderate load and the vehicle speed is above a calibrated limit. The target timer value is based on ambient air temperature at start-up. If the timer exceeds the target time and ECT or CHT has not warmed up to the target temperature, a malfunction is indicated. The test runs if the start-up intake air temperature from the intake air temperature (IAT) sensor is at, or below the target temperature. A 2-hour engine off soak time is also required to enable the monitor and to prevent erasing of any pending DTCs during a hot soak. This soak time feature also prevents false-passes of the monitor when the engine coolant temperature rises after the engine is turned off during a short engine off soak period.The target temperature is calibrated to -11°C (20°F) the thermostat regulating temperature. For a typical 90°C (195°F) thermostat, the target temperature would be calibrated to 79°C (175°F). Some vehicle calibrations may lower the target temperature to less than 27°C (50°F) for vehicles that do not warm-up to thermostat regulating temperatures in the 11°C (20°F) to 27°C (50°F) ambient temperature range.
1. Inputs: ECT or CHT, IAT, engine LOAD (from MAF sensor) and vehicle speed input.
a. Typical monitor entry conditions:
i. vehicle speed greater than 24 km/h (15 mph)

ii. intake air temperature at start-up is between -7°C (20°F) and target thermostat temperature

iii. engine load greater than 30%

iv. engine off (soak) time greater than 2 hours

2. Output: MIL.

Variable Camshaft Timing (VCT) Monitor

The VCT output driver in the PCM is checked electrically for opens and shorts. The VCT system is checked functionally by monitoring the closed loop camshaft position error correction. If the correct camshaft position cannot be maintained and the system has an advance or retard error greater than the calibrated threshold, a VCT control concern is indicated.See Electronic Engine Control (EEC) System Variable Camshaft Timing (VCT) System.

Malfunction Indicator Lamp (MIL)





The MIL notifies the driver that the PCM has detected an on board diagnostic (OBD) emission-related component or system malfunction. When this occurs, an OBD DTC sets.

- The MIL is located in the instrument cluster and is labeled CHECK ENGINE, SERVICE ENGINE SOON or the international standards organization (ISO) standard engine symbol.
- The MIL is illuminated during the instrument cluster prove out for approximately 4 seconds.
- The MIL remains illuminated after instrument cluster prove out if:
- If the MIL remains on after the bulb check:

- an emission-related malfunction and DTC exists.
- the PCM does not send a control message to the instrument cluster (applications with the MIL controlled through the communication link).
- the PCM is operating in the hardware limited operation strategy (HLOS).
- The MIL remains off during the instrument cluster prove out if an indicator or instrument cluster malfunction is present.
- To turn off the MIL after a repair, a reset command from the scan tool must be sent, or 3 consecutive drive cycles must be completed without a malfunction.
- For all MIL malfunctions, see Diagnostic Index.
- If the MIL flashes at a steady rate, a severe misfire condition may exist.
- If the MIL flashes erratically, the PCM can reset while cranking if the battery voltage is low.
- If the MIL flashes erratically, the PCM can reset while cranking if the battery voltage is low.
- The MIL flashes after a period of time with the key in the RUN position (engine not running) if DTC P1000 is set.

Powertrain Control Software





The first 4 bits of a DTC do no convert directly into hex digits. The conversion into different types of DTCs (P, B, C and U) is defined by SAE J2012. This standard contains DTC definitions and formats.





ISO 14229 sends 2 additional bytes of information with each DTC, a failure-type byte and a status byte.





All ISO 14229 DTCs are 4 bytes long instead of 3 or 2 bytes long. Additionally, the status byte for ISO 14229 DTCs is defined differently than the status byte for previous applications with 3 byte DTCs.

Failure-Type Byte

The failure-type byte is designed to describe the specific failure associated with the basic DTC. For example, an failure-type byte of 1C means circuit voltage out of range, 73 means actuator stuck closed. When combined with a basic component DTC, it allows one basic DTC to describe many types of failures.





For example, DTC P0110:1C-AF means intake air temperature sensor circuit voltage out of range. The base DTC P0110, means intake air temperature circuit, while the failure-type byte 1C means circuit voltage out of range. This DTC structure was designed to allow manufactures to more precisely identify different kinds of faults without always having to define new DTC numbers.The PCM does not use failure-type bytes and always sends a failure-type byte of 00 (no sub-type information). This is because OBD II regulations require manufacturers to use 2 byte DTCs for generic scan tool communications. Additionally, the OBD II regulations require the 2 byte DTCs to be very specific, so there is no additional information that the failure-type byte could provide.A list of failure-type bytes is defined by SAE J2012 but is not described here because the PCM does not use the failure-type byte.

Status Byte

The status byte is designed to provide additional information about the DTC, such as when the DTC failed, when the DTC was last evaluated, and if any warning indication has been requested. Each of the 8 bits in the status byte has a precise meaning that is defined in ISO 14229.
The protocol is that bit 7 is the most significant bit and is the left-most bit while bit 0 is the least significant bit and is the right-most bit.






DTC status Bit Definitions

Refer to the following status bit descriptions:

Bit 7
- 0 - The ECU is not requesting warning indicator to be active
- 1 - The ECU is requesting warning indicator to be active

Bit 6
- 0 - The DTC test completed this monitoring cycle
- 1 - The DTC test has not completed this monitoring cycle

Bit 5
- 0 - The DTC test has not failed since last code clear
- 1 - The DTC test failed at least once since last code clear

Bit 4
- 0 - The DTC test completed since the last code clear
- 1 - The DTC test has not completed since the last code clear

Bit 3
- 0 - The DTC is not confirmed at the time of the request
- 1 - The DTC is confirmed at the time of the request

Bit 2
- 0 - The DTC test completed and was not failed on the current or previous monitoring cycle
- 1 - The DTC test failed on the current or previous monitoring cycle

Bit 1
- 0 - The DTC test has not failed on the current monitoring cycle
- 1 - The DTC test failed on the current monitoring cycle

Bit 0
- 0 - The DTC is not failed at the time of request
- 1 - The DTC is failed at the time of request

For DTCs that illuminate the MIL, a confirmed DTC means the PCM has stored a DTC and has illuminated the MIL. If the fault has corrected itself, the MIL may no longer be illuminated but the DTC still shows a confirmed status for 40 warm up cycles at which time the DTC is erased.
For DTCs that do not illuminate the MIL, a confirmed DTC means the PCM has stored a DTC. If the fault has corrected itself, the DTC still shows a confirmed status for 40 warm up cycles at which time the DTC is erased.
To determine if a test has completed and passed, for example, after a repair, information can be combined from 2 bits as follows:
If bit 6 is 0 (the DTC test completed this monitoring cycle), and bit 1 is 0 (the DTC test has not failed on the current monitoring cycle), then the DTC has been evaluated at least once this drive cycle and was a pass.
If bit 6 is 0 (the DTC test completed this monitoring cycle) and bit 0 is 0 (the DTC test is not failed at the time of request), then the most recent test result for that DTC was a pass.
The status byte bits can be decoded as a 2-digit hexadecimal number, and can be displayed as the last 2 digits of the DTC, for example for DTC P0110:1C-AF, AF represents the status byte info.