Emissions-Related Misfire

Theory of Diagnostic Operation
To determine if the misfire is not severe enough to harm the catalyst but would raise emissions to an unacceptable level, we use a different strategy. The ECM constantly counts misfires during sample periods which are 1,000 crankshaft revolutions in length. Once again, it compares the number of misfires for each period with the values stored in its memory for similar operating conditions. The MIL is not turned on the first time that this type of misfire is detected. A misfire code will be set in the TRIONIC or MOTRONIC Intellegant Saab Tester (ISAT) diagnostic menu but not in the ISAT GST (Generic Scan Tool) menu which, by law, cannot register a code until the MIL is turned on. The ECM will prepare to turn the light on if the misfire returns on a subsequent trip by creating what is usually described as a "pending" code.

NOTE:
The term "pending code" is used to describe a concept rather than a real code. At this time the ISAT GST menu does not display pending codes.

Think of it in terms of getting a warning ticket for having a burned out brake light. You won't get a real ticket unless the same officer catches you again and the light is still out. If you are smart and fix the light you can tear up the warning ticket.

The ECM will memorize the operating conditions which were present when it first spotted the misfire and begin counting trips, warm-up and driving cycles. There are several possible results depending on what happens next. Here are the options:

1. The "pending" code will be erased as soon as, on a subsequent driving cycle, there is no misfire when similar operating conditions are present. The "pending" code will also be erased if 80 driving cycles take place without ever encountering driving conditions similar to those present when the code was originally set.

2. The MIL will be turned on constantly if misfire occurs within the next 80 driving cycles.

3. Once on, it takes three consecutive driving cycles, with similar operating condition, without misfire to turn the light off.

4. The DTC will be automatically erased if the MIL is off and there has been no misfire for 40 warm-up cycles.

Low Fuel DTC
Once again, always remember to check for a low fuel DTC anytime you find a misfire DTC. If you find a low fuel code don't jump to conclusions, there may still be a real problem. The benefit is that you will know enough to ask a few more questions. With a little luck the customer might remember that the MIL came on after his son borrowed the car and ran out of gas.