The engine control module (ECM), located inside the passenger kick-panel, is the control center of the fuel injection system. It constantly looks at the information from various sensors and controls the systems that affect the vehicle's performance. The ECM also performs the diagnostic functions of the system. It can recognize operational problems, alert the driver through the malfunction indicator lamp (MIL), and store DTCs which identify problem areas to aid the technician in making repairs.

There are no serviceable parts in the ECM. The calibrations are stored in the ECM in the programmable read only memory (PROM).

Notice: Refer to Handling Electrostatic Discharge Sensitive Parts Notice in the Preface section.

The ECM supplies either 5 or 12 volts to power the sensors or switches. This is done through resistances in the ECM which are so high in value that a test light will not come on when connected to the circuit. In some cases, even an ordinary shop voltmeter will not give an accurate reading because its resistance is too low. You must use a digital voltmeter with a 10 megohm input impedance to get accurate voltage readings. The ECM controls output circuits such as the fuel injectors, the idle air control valve, the A/C clutch relay, etc., by controlling the ground circuit through transistors or a device called a quad-driver.

Comprehensive component monitoring diagnostics are required to monitor emissions-related input and output powertrain components.

Input components are monitored for circuit continuity and out-of-range values. This includes rationality checking. Rationality checking refers to indicating a fault when the signal from a sensor does not seem reasonable, i.e. throttle position (TP) sensor that indicates high throttle position at low engine loads or manifold absolute pressure (MAP) voltage. Input components may include, but are not limited to, the following sensors:

In addition to the circuit continuity and rationality check, the ECT sensor is monitored for its ability to achieve a steady state temperature to enable closed loop fuel control.

Output components are diagnosed for proper response to control module commands. Components where functional monitoring is not feasible will be monitored for circuit continuity and out-of-range values if applicable. Output components to be monitored include, but are not limited to the following circuit:

A passive test is a diagnostic test which simply monitors a vehicle system or component. Conversely, an active test, actually takes some sort of action when performing diagnostic functions, often in response to a failed passive test. For example, the exhaust gas recirculation (EGR) diagnostic active test will force the EGR valve open during closed throttle deceleration and/or force the EGR valve closed during a steady state. Either action should result in a change in manifold pressure.

This is any on-board test run by the Diagnostic Management System which may have an effect on vehicle performance or emission levels.

A warm-up cycle means that engine temperature must reach a minimum of 70°C (160°F) and rise at least 22°C (72°F) over the course of a trip.

Freeze Frame is an element of the diagnostic management system which stores various vehicle information at the moment an emissions-related fault is stored in memory and when the malfunction indicator lamp (MIL) is commanded ON. These data can help to identify the cause of a fault.

Failure Records data is an enhancement of the OBD Freeze Frame feature. Failure Records store the same vehicle information as does Freeze Frame, but it will store that information for any fault which is stored in on board memory, while Freeze Frame stores information only for emission-related faults that command the malfunction indicator lamp (MIL) ON.