On-Board Diagnostics

The powertrain control module (PCM) has been programmed to monitor many different circuits of the fuel injection system. This monitoring is called "on-board diagnosis."

Certain criteria, or "arming conditions," must be met for a trouble code to be entered into the PCM memory. The criteria may be a range of: engine rpm, engine temperature, and/or input voltage to the PCM. If a problem is sensed with a monitored circuit, and all of the criteria or arming conditions are met, a trouble code will be stored in the PCM.

It is possible that a trouble code for a monitored circuit may not be entered into the PCM memory even though a malfunction has occurred. This may happen because one of the trouble code criteria (arming conditions) has not been met.

The PCM compares input signal voltages from each input device with specifications (the established high and low limits of the range) that are programmed into it for that device. If the input voltage is not within specifications and other trouble code criteria (arming conditions) are met, a trouble code will be stored in the PCM memory.

The On Board Diagnostics have evolved to the second Generation of Diagnostics referred to as OBDII. These OBDII Diagnostics control the functions necessary to meet the requirements of California OBDII and Federal OBD regulations. These requirements specify the inclusion of a malfunction indicator light (MIL) located on the instrument panel for all 1994 and subsequent model-year passenger cars, light duty trucks, and medium-duty vehicles. The purpose of the MIL is to inform the vehicle operator in the event of malfunction of any powertrain components which can affect emissions and which provide input to, or receive output from, the engine controller module.

Control of OBDII functions is separated into two areas: the Task Manager and the Comprehensive Component Manager.

TASK MANAGER
The Task Manager controls the execution of some component/systems tests, control of MIL illumination, storage of fault codes, and storage of vehicle data at time of fault detection (freeze frame). This routine performs the following monitoring requirements:

Fuel System Monitoring
Misfire Detection
Exhaust Gas Recirculation (EGR) System Monitor
Oxygen Sensor Upstream Heater Monitor
Oxygen Sensor Upstream Monitor
Evaporative System Monitor
Catalyst Monitor
Purge Flow Monitor
O2 Heater Downstream Monitor
O2 Sensor Downstream Monitor

Another function of the task manager is control of the Disable Tables. These tables are used to discontinue testing when a condition specified in the tables are met. Testing of a component or system is disabled if a fault or failure occurs which would render the test useless. (For example, the "Catalyst Monitor" test will not be run unless the "O2 Sensor" test has passed.)

COMPREHENSIVE COMPONENT MANAGER
The Comprehensive Component Manager controls the testing of all powertrain components/systems which can affect emissions and were not otherwise tested by the Task Manager. This routine is organized into three areas:

Power Supply Circuit Checks - Performed on sensor to determine out-of-range or high/low input conditions. Performed on outputs to check for open/short conditions. The components tested are:

Idle Air Control Motor (IAC) Open/Short Circuit
Generator Field
Torque Converter Clutch (TCC) Solenoid
Fan Relay
A/C Relay
Shift Inter-Lock Solenoid
Transmission Relay Output
ASD Relay
Fuel Pump Relay
EVAP Purge Solenoid
EGR Solenoid
Ignition Coils (1 thru 5)
Injector Failure (1-10)

Ground Circuit Checks - Performed on sensor inputs connected to ground to determine out-of-range or high/low input conditions. Performed on outputs to check for open/short conditions. The components tested are:

Manifold Absolute Pressure (MAP) Sensor Voltage (high/low)
Intake Air Temperature Sensor Voltage (high/low)
Coolant Temperature Sensor Voltage (high/low)
Throttle Position Sensor Voltage (high/low)
MAP Sensor 5 Volt Supply Check
Battery Temperature Sensor Voltage (high/low)
Fuel Level Sensor Voltage (high/low)
Transmission Temperature Sensor Voltage (high/low)
Governor Pressure Sensor Voltage (high/low)
Governor Pressure Solenoid (open/short)
ASD Relay Voltage (out of range)
Battery Voltage (too high)
Overdrive Override Switch (fault)
Leak Detection Pump Solenoid (open/short)
3/4 Shift Solenoid (open/short)
Speed Control Vac and Vent Solenoids (open/short)

Functional/Rationality Checks - These tests determine if a component is working correctly through methods other than voltage limit tests and open/short tests. Solenoids in this section are checked for proper operation (functionality) and sensors are checked for meaningful output values (rationality). The Components/systems checked are:

Cam Sensor Pulses Present
Crank Sensor Pulses Present
MAP Sensor Output in Range
Coolant Temperature in Range
Throttle Position Sensor in Range
Vehicle Speed Output Correct
Park/Neutral Switch Working
Fuel Level Sensor Output Correct
Transmission Temperature Sensor Output Correct
Transmission Governor Pressure in Range
Output Shaft Speed Sensor Output Correct
IAC Motor Working
Lock-up Solenoid Working
3/4 Shift Solenoid Working
Governor Pressure Solenoid Working
O2 Sensor 1/1 Output Voltage Low
O2 Sensor 1/2 Output Voltage Low
O2 Sensor 1/3 Output Voltage Low
O2 Sensor 2/1 Output Voltage Low
O2 Sensor 2/2 Output Voltage Low
Charging System Working
Power Steering Switch Working
Thermostat Working
O2 Sensor 1/1 (open circuit or high voltage)
O2 Sensor 1/2 (open circuit or high voltage)
O2 Sensor 1/3 (open circuit or high voltage)
O2 Sensor 2/1 (open circuit or high voltage)
O2 Sensor 2/2 (open circuit or high voltage)