Monitors
OBD II MONITORSThe ECM performs a battery of tests on specific vehicle systems to determine if they are operating within the parameters set by OBD II. The Diagnostic System Manager (DSM) software ensures that the tests are performed at specific times and in the correct sequence in order to produce valid results. Testing the vehicle while cold, or in unusual operating conditions (such as during evaporative purge) could produce false readings that would illuminate the vehicle's Malfunction Indicator Lamp (MIL) unnecessarily.
Comprehensive Component Monitor
ECM inputs and outputs are checked frequently during engine operation. As in the original OBD application, these components and their circuits are checked for operation. Tests for shorts and opens are performed. Some tests require system or component actuation so a change of state can be observed. DTCs and MIL illumination occur when a fault is recorded.
OBD II requires even more careful review of these input and output components by not only determining if they are operating, but also by performing rationality checks. By comparing the readings from other sensors, the GEMS can determine if a sensor reading is appropriate for the current operating conditions. An example of this is a throttle position sensor signal indicating the throttle is half open when other inputs and outputs (RPM, IAC) suggest the engine is at idle.
A specific DTC is stored as soon as a fault is detected. The system must fail the test on two consecutive drive cycles before the MIL is illuminated.
The GEMS will continue testing failed or out of range components, even after the MIL is illuminated. Should the system pass the test on three consecutive trips, the MIL will turn off. The DTC will remain stored however, for 40 more warm-up cycles.
Fuel System Monitor
The ECM continually adjusts fuel trim when in closed loop operation. If a system malfunction occurs, requiring an amount of fuel trim compensation that exceeds standards set in the GEMS program, a DTC will be stored. The ECM monitors the fuel system continuously once it is operating in closed loop.
Should fuel trim requirements fall outside of the acceptable parameters on a second consecutive trip, the MIL will be illuminated. If the system concern does not repeat itself for three consecutive trips, the MIL will turn off. The DTC will remain stored for 40 more warm-up cycles.
NOTE: It is important to understand that two consecutive trips is not the same as two warm-up cycles. Two consecutive trips could occur two weeks apart, with dozens of warm-up cycles in between.
Catalyst Efficiency Monitor
The Three-Way Catalyst (TWC) or Catalytic Converter, is a central device in the vehicle's emissions control system. Over time, deterioration of a catalyst's operating efficiency can lead to an increase in hydrocarbon emissions. OBD II requires that the ECM monitor operation of the vehicle's TWCs to ensure that they are operating within specification. This is accomplished by monitoring signals produced by oxygen sensors mounted ahead of (upstream) and below (downstream) of each TWC.
A properly functioning three-way catalyst stores oxygen during lean engine operation and gives up that stored oxygen during rich engine operation to consume unburned hydrocarbons. Catalyst efficiency is estimated by monitoring the oxygen storage capacity of the catalyst during closed-loop operation.
The GEMS monitors the switching frequency of the downstream Heated Oxygen Sensor (HO2S) during the test. Because the sensor switches in the presence of oxygen, it should have a significantly lower switching frequency than the sensor mounted ahead of the catalyst.
A frequency approaching that of the upstream sensor would indicate that the TWC is not storing oxygen during lean operation. This lack of stored oxygen renders the TWC incapable of burning off excess hydrocarbons produced during the rich cycle. The result is excessive hydrocarbon emissions.
Catalyst efficiency is tested once each drive cycle. The first time the system fails a self-test, the ECM will store a DTC. The system must fail the test on two consecutive drive cycles before the MIL is illuminated.
The Diagnostic System Manager will continue testing for catalyst efficiency once each drive cycle, even after the MIL is illuminated. Should the system pass the test on three consecutive drive trips, the MIL will turn off. The DTC will remain stored, however, for 40 more warm-up cycles.
Misfire Monitor
Cylinder misfire poses a serious threat to the vehicles emissions system. Misfires produce concerns ranging from open ignition circuits to fouled spark plugs. As a cylinder misfires, the raw hydrocarbons (HC) that should have been consumed during ignition are forced out of the exhaust manifold. Obviously, this adversely affects vehicle emissions. Worse however, is what happens after these raw HCs leave the engine and enter the Three-Way Catalyst (TWC).
As these raw hydrocarbons move into the catalyst, the internal temperature of the converter increases. Continued operation can cause the catalytic honeycomb to melt into a solid mass, destroying the catalyst's ability to function. Eventually, the TWC may cause so much restriction that the excessive backpressure prevents the engine from running. Obviously, detecting and preventing engine operation under misfire conditions is a high priority of an emissions control system.
The ECM detects engine misfire by measuring the contribution each cylinder makes to engine performance. This is calculated from measurements of crankshaft acceleration for each cylinder provided by the crankshaft position sensor.
The acceleration for each cylinder is determined from the crankshaft rotation velocity. The GEMS performs a series of calculations to determine the acceleration rates of the individual cylinders. When a cylinder's acceleration falls outside of a predetermined range, the GEMS takes a closer look at the signal.
For example, operating conditions such as rough roads or high rpm/light load operation can provide misfire-like changes in crankshaft acceleration. Internal programming in the GEMS is designed to filter out these look-alike signals and focus on real misfire. The GEMS separates misfire into two classifications, and has a different response for each.
Type A Misfire:
This is a serious misfire situation where raw fuel entering the TWC can cause excessive catalyst temperatures. This could quickly cause permanent damage to the TWC. In this situation, the MIL lamp illuminates immediately and flashes to attract the driver's attention. Continued operation at this point will damage the TWC.
Type B Misfire:
A second type of response occurs when the GEMS detects a low-level misfire. At lower levels, misfire will not significantly raise TWC temperature but will produce excessive vehicle emissions. In this situation, the GEMS records a DTC. The GEMS will illuminate the MIL if this failure is repeated during a second consecutive drive cycle where operating conditions (engine warm-up, rpm and load) are approximately the same. Should the misfire not reappear under these conditions on three consecutive trips, the MIL will turn OFF.