Diagnostic Information

Passive and Active Diagnostic Tests
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 EGR diagnostic active test will force the EGR valve open during closed throttle decel and/or force the EGR valve closed during a steady state. Either action should result in a change in manifold pressure.

Warm-up Cycle
A warm-up cycle means that engine temperature must reach a minimum of 70°C (160°F) and rise at least 22°C (40°F) over the course of a trip.

Freeze Frame
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 MIL is commanded on. This data can help to identify the cause of a fault.

Diagnostic Information
The diagnostic tables and functional checks are designed to locate a faulty circuit or component through a process of logical decisions. The tables 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 complemented by the diagnostic procedures contained in this service information. 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) ("Service Engine Soon" or "Check Engine") lamp is illuminated on some applications.

Malfunction Indicator Lamp (MIL)
The Malfunction Indicator Lamp (MIL) looks the same as the MIL you are already familiar with ("Service Engine Soon" or "Check Engine") lamp. However, OBD II requires that it illuminate under a strict set of guidelines. Basically, the MIL is turned on when the ECM detects a DTC that will impact the vehicle emissions.

The MIL is under the control of the ECM. 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 three consecutive trips, with no emissions-related faults.

If the vehicle is experiencing a misfire malfunction which may cause damage to the Three-Way Catalytic Converter (TWC), the MIL will flash once per second. This will continue until the vehicle is outside of speed and load conditions which could cause possible catalyst damage, and then the MIL will stop flashing and remain on steady.

Extinguishing the MIL
When the MIL is on, the ECM will turn off the MIL after three 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 and Freeze Frame until forty (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:

^ The diagnostic tests that are passed must occur within 375 RPM of the RPM data stored at the time the last test failed.
^ Plus or minus ten (10) percent of the engine load that was stored at the time the last test failed.
^ Similar engine temperature conditions (warmed up or warming up) as those stored at the time the last test failed.

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

The MIL is on the instrument panel and has the following functions:

^ It informs the driver that a fault that affects vehicle emission levels has occurred and that the vehicle should be taken for service as soon as possible.
^ As a bulb and system check, the MIL will come "ON" with the key "ON" and the engine not running. When the engine is started, the MIL will turn "OFF."

When the MIL remains "ON" while the engine is running, or when a malfunction is suspected due to a driveability or emissions problem, an On-Board Diagnostic (OBD) System Check must be performed. These checks will expose faults which may not be detected if other diagnostics are performed first.

DTC Types
Each DTC is directly related to a diagnostic test. The ECM sets DTCs based on the failure of the tests during a trip or trips. Certain tests must fail two (2) consecutive trips before the DTC is set. The following are the four (4) types of DTCs and the characteristics of those codes:

^ Type A
- Emissions related
- Requests illumination of the MIL on the first trip with a fail
- Stores a History DTC on the first trip with a fail
- Stores a Freeze Frame (if empty)

^ Type B
- Emissions related
- "Armed" after one (1) trip with a fail
- "Disarmed" after one (1) trip with a pass
- Requests illumination of the MIL on the second consecutive trip with a fail
- Stores a History DTC on the second consecutive trip with a fail (the DTC will be armed after the first fail)
- Stores a Freeze Frame on the second consecutive trip with a fail (if empty) (Some special conditions apply to misfire and fuel trim DTCs).

^ Type C (if the vehicle is so equipped)
- Non-Emissions related
- Requests illumination of the Service Lamp or the service message on the Driver Information Center (DIC) on the first trip with a fail
- Stores a History DTC on the first trip with a fail
- Updates the Fail Record each time the diagnostic test fails

^ Type D (Type D non-emissions related are not utilized on certain vehicle applications).
- Non-Emissions related
- Does not request illumination of any lamp
- Stores a History DTC on the first trip with a fail
- Does not store a Freeze Frame

Special Cases of Type B Diagnostic Tests
Unique to the misfire diagnostic, the ECM has the capability of alerting the vehicle operator to potentially damaging levels of misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the ECM will command the MIL to "flash" at a rate of once per second during those the time that the catalyst damaging misfire condition is present.

Storing And Erasing Freeze Frame Data
Regulations require that engine operating conditions be captured whenever the MIL is illuminated. The data captured is called Freeze Frame data. The freeze frame data is very similar to a single record of operating conditions. Whenever the MIL is illuminated, the corresponding record of operating conditions is recorded to the freeze frame buffer.

Freeze frame data can only be overwritten with data associated with a misfire or fuel trim malfunction. Data from these faults take precedence over data associated with any other fault. The freeze frame data will not be erased unless the associated DTC is cleared.

Intermittent Malfunction Indicator Lamp
In the case of an "intermittent" fault, the MIL (Service Engine Soon or Check Engine) may illuminate and then (after three trips) go "OFF." However, the corresponding diagnostic trouble code will be stored in the memory. When unexpected diagnostic trouble codes appear, check for an intermittent malfunction.

A diagnostic trouble code may reset. Consult the "Diagnostic Aids" associated with the diagnostic trouble code. A physical inspection of the applicable sub-system most often will resolve the problem.