Type B Diagnostic Tests

There are special cases of type B diagnostic tests. These special cases are described in this article.

Unique to the misfire diagnostic, the Diagnostic Executive has the capability of alerting the vehicle operator to potentially damaging levels of a misfire. If a misfire condition exists that could potentially damage the catalytic converter as a result of high misfire levels, the Diagnostic Executive will command the Malfunction Indicator Lamp (MIL) to flash at a rate of once per second during the time that the catalyst damaging misfire condition is present.

The fuel trim and the misfire are special cases of type B diagnostics. Each time that a fuel trim or a misfire malfunction is detected, the engine load, the engine speed, and the engine coolant temperatures are recorded.

When the ignition is turned off, the last reported set of conditions remain stored. During the subsequent ignition cycles, the stored conditions are used as a reference for similar conditions. If a malfunction occurs during two consecutive trips, the Diagnostic Executive treats the failure as a normal type B diagnostic, and does not use the stored conditions. However, if a malfunction occurs on two non-consecutive trips, the stored conditions are compared with the current conditions. The MIL will then illuminate under the following conditions:

^ When the engine load conditions are within 10 percent of the previous test that failed.
^ The engine speed is within 375 RPM, of the previous test that failed.
^ The engine coolant temperature is in the same range as the previous test that failed.