The engine control module (ECM) is the control center of the engine controls system. The ECM constantly monitors information from various sensors and controls the systems that affect vehicle performance and emissions. The ECM also performs diagnostic tests on various parts of the system. The ECM can detect operational malfunctions and alert the driver via the malfunction indicator lamp (MIL). When the ECM detects a malfunction, it stores a diagnostic trouble code (DTC). The condition is identified by the particular DTC that is set. This aids the technician in making repairs.

The electronically erasable programmable read only memory (EEPROM) is memory that is a physical part of the control module. The EEPROM contains program and calibration information that the ECM needs in order to control the engine operation. Special equipment, as well as the correct program and calibration for the vehicle, are required in order to reprogram the control module.

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

The ECM can withstand the normal current draws that are associated with normal vehicle operation. Care must be used in order to avoid overloading any of these circuits during diagnosis. When testing for opens or shorts do not ground or apply voltage to any of the ECM circuits unless the diagnostic procedure instructs you to do so. The circuits should only be tested with a DMM.

The electronic components that are used in the control systems are often designed to carry very low voltage. The electronic components are susceptible to damage caused by electrostatic discharge. Less than 100 volts of static electricity can cause damage to some electronic components. By comparison, it takes as much as 4,000 volts for a person to even feel the effect of a static discharge.

There are several ways for a person to become statically charged. The most common methods of charging are by friction and by induction. An example of charging by friction is a person sliding across a car seat.

Charging by induction occurs when a person with well insulated shoes stands near a highly charged object and momentarily touches ground. Charges of the same polarity are drained off leaving the person highly charged with the opposite polarity. Static charges can cause damage, therefore, it is important to use care when handling and testing electronic components.

The ECM can supply 5 volts or 12 volts to the various sensors or switches. This is done through pull-up resistors to the regulated power supplies within the control module. In some cases, even an ordinary shop voltmeter will not give an accurate reading because the resistance is too low. A DMM with at least 10 megaohms input impedance is required in order to ensure accurate voltage readings.

The ECM controls the output circuits by controlling the ground or the power feed circuit through the transistors or a device called an output driver module.

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. Input components are also tested for rationality. A rationality condition occurs when the signal from a sensor is not reasonable. For example, a throttle position (TP) sensor that indicates high throttle position at low engine loads, or manifold absolute pressure (MAP) voltage, will set a rationality DTC. 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 circuits:

A passive test is a diagnostic test which simply monitors a vehicle system or component. 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. Freeze Frame stores information only for emission-related faults that command the MIL ON. This data can help to identify the cause of a fault.

Failure Records data is an enhancement of the Freeze Frame feature. Failure Records store the same vehicle information as Freeze Frame, but it will store that information for any fault which is stored in onboard memory, even if the MIL is not commanded ON. This data can help to identify the cause of a fault.