The engine control module (ECM) is located behind the passenger kick panel. The ECM is responsible for the control of all engine functions that affect vehicle operation and performance. The ECM also performs the diagnostic functions of the engine control systems. The ECM is responsible for the OBD II diagnostic tests of the emission related systems.

The ECM supplies either 5 or 12 volts to the engine control sensors or switches. These supply circuits are buffered and have a high internal circuit resistance that requires the use of a digital voltmeter with a 10 megaohm input impedance. The ECM controls most components with an electronic switch that completes a ground circuit when commanded ON. The electronic switch is commonly referred to as an output driver. This control module uses a device called a quad-driver, that operates several components simultaneously.

There are no serviceable parts in the ECM. The calibrations are stored in the ECM in either a programmable read only memory (PROM) or an erasable programmable read only memory (EPROM).

Comprehensive component monitoring diagnostics are required in order 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. An example of a rationality concern is a throttle position (TP) sensor that indicates high throttle position at low engine loads. 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. 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. Some active diagnostic tests, are also intrusive tests. An intrusive diagnostic test is any on-board test, run by the diagnostic management system, that has 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. Freeze Frame 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.

Diagnostic

When used as a noun, the word diagnostic refers to any on-board test run by the vehicle's diagnostic management system. A diagnostic is simply a test run on a system or component in order to determine if the system or component is operating according to specification. Some of the required diagnostics are:

Enable Criteria

The term enable criteria is engineering language for the conditions necessary for a given diagnostic test to run. Each diagnostic has a specific list of conditions which must be met before the diagnostic will run.

Enable criteria is another way of saying conditions required.

The enable criteria for each diagnostic is listed on the first page of the diagnostic trouble code (DTC) description under the heading Conditions for Setting the DTC. Enable criteria varies with each diagnostic and typically includes, but is not limited to, the following items:

Trip

Technically, a trip is a key-on run key-off cycle in which all the enable criteria for a given diagnostic are met, allowing the diagnostic to run. Unfortunately, this concept is not quite that simple. A trip is official when all the enable criteria for a given diagnostic are met. But because the enable criteria vary from one diagnostic to another, the definition of trip varies as well. Some diagnostics are run when the vehicle is at operating temperature, some when the vehicle first starts up, some require that the vehicle be cruising at a steady highway speed, some run only when the vehicle is at idle, and some diagnostics function with the torque converter clutch (TCC) disabled. Some run only immediately following a cold engine startup.

A trip then, is defined as a key-on run key-off cycle in which the vehicle was operated in such a way as to satisfy the enables criteria for a given diagnostic, and this diagnostic will consider this cycle to be one trip. However, another diagnostic with a different set of enable criteria, which were not met, during this driving event, would not consider it a trip. No trip will occur for that particular diagnostic until the vehicle is driven in such a way as to meet all the enable criteria.

Diagnostic Information

The diagnostic charts and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The charts are prepared with the requirement that the vehicle functioned correctly at the time of assembly and that there are not multiple faults present.

There is a continuous self-diagnosis on certain control functions. This diagnostic capability is complimented by the diagnostic procedures contained in this manual. The language of communicating the source of the malfunction is a system of diagnostic trouble codes (DTCs). When a malfunction is detected by the control module, a DTC is set and the malfunction indicator lamp (MIL) is illuminated.

Malfunction Indicator Lamp (MIL)

The malfunction indicator lamp (MIL) is located in the instrument panel cluster. The MIL will display as either SERVICE ENGINE SOON or one of the following symbols when commanded ON:

The MIL is under the control of the diagnostic executive. The MIL will be turned ON if an emissions-related diagnostic test indicates a malfunction has occurred. It will stay ON until the system or component passes the same test, for 3 consecutive trips, with no emissions related faults.

The MIL indicates that an emissions related fault has occurred and vehicle service is required. The following is a list of the modes of operation for the MIL:

Extinguishing the MIL

When the MIL is ON, the diagnostic executive will turn OFF the MIL after 3 consecutive trips that a test passed has been reported for the diagnostic test that originally caused the MIL to illuminate. Although the MIL has been turned OFF, the DTC will remain in the ECM memory, both Freeze Frame and Failure Records, until 40 warm-up cycles after no faults have been completed.

If the MIL was set by either a fuel trim or misfire-related DTC, additional requirements must be met. In addition to the requirements stated in the previous paragraph, these requirements are as follows:

Meeting these requirements ensures that the fault which turned ON the MIL has been corrected.

Data Link Connector (DLC)

The provision for communicating with the control module is the data link connector (DLC). The DLC is used to connect to a scan tool. Some common uses of the scan tool are listed below:

There are primary system based diagnostics which evaluate system operation and its effect on vehicle emissions. The primary system-based diagnostics are listed below with a brief description of the diagnostic function.

The fuel control front heated oxygen sensor (HO2S) 1 is diagnosed for the following conditions:

The catalyst monitor rear heated oxygen sensor (HO2S) 2 is diagnosed for the following conditions:

If the oxygen sensor pigtail wiring, connector, or terminal are damaged, the entire oxygen sensor assembly must be replaced. Do not attempt to repair the wiring, connector, or terminals. In order for the sensor to function properly, it must have clean reference air provided to it. This clean air reference is obtained by way of the oxygen sensor wires. Any attempt to repair the wires, connector, or terminals could result in the obstruction of the reference air and degrade oxygen sensor performance.

The misfire monitor diagnostic is based on crankshaft rotational velocity, reference period, and the variations between the two. The engine control module (ECM) determines crankshaft rotational velocity using the crankshaft position (CKP) sensor and the camshaft position (CMP) sensor. When a cylinder misfires, the crankshaft slows down momentarily. By monitoring the CKP and CMP sensor signals, the ECM can calculate when a misfire occurs.

For a non-catalyst damaging misfire, the diagnostic will be required to monitor a misfire present for between 1,000-3,200 engine revolutions.

For catalyst-damaging misfire, the diagnostic will respond to misfire within 200 engine revolutions.

Rough roads may cause false misfire detection. A rough road will cause torque to be applied to the drive wheels and drive train. This torque can intermittently decrease the crankshaft rotational velocity. This may be falsely detected as a misfire.

A rough road sensor, or G sensor, works together with the misfire detection system. The G sensor produces a voltage that varies along with the intensity of road vibrations. When the ECM detects a rough road, the misfire detection system is temporarily disabled.

Whenever a cylinder misfires, the misfire diagnostic counts the misfire and notes the crankshaft position at the time the misfire occurred. The misfire counters provide a record of each engine cylinder. A current and a history misfire counter are maintained for each cylinder. The misfire current counters, Misfire Cur #1-4, indicate the number of firing events out of the last 200 cylinder firing events which were misfires. The misfire current counter will display real time data without a misfire diagnostic trouble code (DTC) stored. The misfire history counters, Misfire History #1-4, indicate the total number of cylinder firing events which were misfires. The misfire history counters will display 0 until the misfire diagnostic has failed and a DTC P0300 is set. Once the misfire DTC P0300 is set, the misfire history counters will be updated every 200 cylinder firing events. A misfire counter is maintained for each cylinder.

If the misfire diagnostic reports a failure, the diagnostic executive reviews all of the misfire counters before reporting a DTC. This review, enables the diagnostic executive to report the most current information.

When crankshaft rotation is erratic, a misfire condition will be detected. Because of this erratic condition, the data that is collected by the diagnostic can sometimes incorrectly identify which cylinder is misfiring.

Use diagnostic equipment to monitor misfire counter data on On-Board Diagnostic (OBD) compliant vehicles. Knowing which specific cylinders misfired can lead to the root cause, even when dealing with a multiple cylinder misfire. Using the information in the misfire counters, identify which cylinders are misfiring. If the counters indicate cylinders numbers 1 and 4 misfired, look for a circuit or component common to both cylinders number 1 and 4.

The misfire diagnostic may indicate a fault due to a temporary fault not necessarily caused by a vehicle emission system malfunction. Examples include the following items:

This system monitors the averages of short-term and long-term fuel trim values. If these fuel trim values stay at their limits for a calibrated period of time, a malfunction is indicated. The fuel trim diagnostic compares the averages of short-term fuel trim values and long-term fuel trim values to rich and lean thresholds. If either value is within the thresholds, a pass is recorded. If both values are outside their thresholds, a rich or lean diagnostic trouble code (DTC) will be recorded.

The fuel trim system diagnostic also conducts an intrusive test. This test determines if a rich condition is being caused by excessive fuel vapor from the evaporative emission (EVAP) canister. In order to meet OBD requirements, the control module uses weighted fuel trim cells to determine the need to set a fuel trim DTC. A fuel trim DTC can only be set if fuel trim counts in the weighted fuel trim cells exceed specifications. This means that the vehicle could have a fuel trim problem which is causing a problem under certain conditions, i.e., engine idle high due to a small vacuum leak or rough idle due to a large vacuum leak, while it operates fine at other times. No fuel trim DTC would set, although an engine idle speed DTC or heated oxygen sensor (HO2S) 2 DTC may set. Use a scan tool to observe fuel trim counts while the problem is occurring.

A fuel trim DTC may be triggered by a number of vehicle faults. Make use of all information available, other DTCs stored, rich or lean condition, etc., when diagnosing a fuel trim fault.

No fuel trim DTC will set regardless of the fuel trim counts in cell 0 unless the fuel trim counts in the weighted cells are also outside specifications. This means that the vehicle could have a fuel trim problem that is causing a driveability concern but the DTC does not set. For example the engine idles high due to a small vacuum leak or the engine idles rough due to a large vacuum leak. In either example the engine operates fine at any other time, and no fuel trim DTC would set. Yet ECM could set an engine idle speed DTC or heated oxygen sensor (HO2S) 2 DTC because of the decline in engine performance. Use a scan tool to observe fuel trim counts when the problem is occurring.