Two-Trip Detection Logic

The diagnosis system operates in the 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 to prevent erroneous detection, and ensure a thorough malfunction detection. By switching the engine control module (ECM) to check mode when trouble shooting, the technician can cause the malfunction indicator light (MIL) to light up for a malfunction that is only detected once or momentarily (hand-held tester only). When a malfunction is first detected, the malfunction is temporarily stored in the ECM memory (1st trip). If the same malfunction is detected again during the second drive test, this second detection causes the MIL to light up (2nd trip). However, the ignition switch must be turned OFF between the 1st trip and 2nd trip.