Monitors, Trips, Drive Cycles and Readiness Codes

2 TRIP DETECTION LOGIC:
Certain diagnostic trouble codes (DTC) (21, 25, 26, 27 and 71), can be triggered by driving conditions without a malfunction present. To improve self diagnostic reliability, "2nd trip" detect logic is used.
When any a fault related to any of the above DTC's is detected, the engine control module (ECM) retains a temporary record of the fault in memory. An actual code is set only if the same fault is recorded during the next ignition/driving cycle.
The malfunction indicator light ("CHECK" engine light) is only illuminated after the "2nd trip."
Because the ECM must see the fault for 2 consecutive trips, the chances of a false code are greatly reduced.

DIAGNOSIS

Two Trip Detection Logic - Driving Pattern:

A specific driving pattern must be followed to recreate a fault code.
^ Drive the vehicle in the same manner the customer has indicated caused the fault.
^ Stop vehicle and turn ignition OFF.
^ Restart vehicle and repeat test-drive.