System Overview

The engine control module (ECM) (1) uses a built-in on-board diagnostic (OBD) function to monitor itself, input and output signals, and the function of emission-related systems. Each time the engine control module runs through all on-board diagnostic functions it is said to have performed a trip. This usually takes approx. 30 minutes although it depends on how the car is being driven.

If the engine control module detects a fault, two parameters (depending on the diagnostic trouble code (DTC) these parameters vary) are frozen and substitute (limphome) values are adopted to allow the system to continue operating.

Diagnostic trouble code, type of fault and frozen values are stored in the engine control module memory in a predetermined schedule, different for different diagnostic trouble codes. The engine control module will permanently store the diagnostic trouble code and light the malfunction indicator lamp (MIL) (for emission-related faults) if the fault is present in the car for a specified number of periods. If the fault disappears before the period has run out the code is erased from the engine control module.

In principle there are three periods:

^ Trip (all diagnostic functions have been carried out).
^ Operating cycle (fuel trim has started).
^ Time (seconds).

If for some reason a fault disappears after the diagnostic trouble code has been permanently stored in the engine control module, information about the fault is stored in the engine control module. Every time the fault recurs a first counter counts it. For every trip that is driven without the fault recurring a second counter counts down. It starts on 3 and counts to 0. When the second counter reaches zero - all engine control module diagnostic functions have been carried out three times and the fault has still not recurred - the malfunction indicator lamp goes out (if it was lit) and a third counter starts counting down.

The third counter counts warm-up cycles and starts at 40. A warm-up cycle means a start with engine coolant temperature under 40°C (104°F) and not exceeding 84°C (183.2°F). If the engine starts at an engine coolant temperature above 40°C (104°F), the warm-up cycle cannot be completed. When the third counter has counted down to 0 the diagnostic trouble code is erased from the engine control module memory.

If the fault recurs the second and third counters are reset and the first counter counts up one.

Faults diagnosed by the automatic transmission control module (TCM) may be emission-related. For this reason the automatic transmission control module can request that the lighting of the malfunction indicator lamp via the engine control module.