Two-Trip Detection Logic

The diagnosis system operates in normal mode during normal vehicle use. It also has a check mode for technicians to simulate malfunction symptoms and trouble shoot. Most DTCs use 2 trip detection logic (when a malfunction is first detected, malfunction is temporarily stored in ECM memory, 1st trip) to prevent erroneous detection, which ensures malfunction detection. By switching ECM to check mode when trouble shooting, technician can cause MIL to light up for a malfunction that is only detected once or momentarily (hand-held tester only). See NORMAL MODE CODE RETRIEVAL  under RETRIEVING DIAGNOSTIC TROUBLE CODES.

If same malfunction is detected again during second drive test, this second detection causes MIL to light up (2nd trip). However, ignition switch must be turned OFF between 1st trip and 2nd trip.