Introduction

SYSTEM DESCRIPTION
The Engine Control Module (ECM) (ECCS control module) has an on-board diagnostic system, which detects engine system malfunctions related to sensors or actuators. The malfunction indicator lamp (MIL) on the instrument panel lights up when a malfunction is detected twice (two trip detection logic).

Two Trip Detection Logic
When a malfunction is detected for the first time, the malfunction (DTC and freeze frame data) is stored in the ECM memory. <1st trip> The malfunction indicator lamp will not light up at this stage. If the same malfunction is detected again during next drive, this second detection causes the malfunction indicator lamp to light up. <2nd trip>

The "trip" in the "Two Trip Detection Logic" means performing of the "DTC Detecting Condition". Only the following on-board diagnostic items will light up or blink the Malfunction Indicator Lamp (MIL) even in the 1st trip.

- "Misfire (Criterion which will damage three way catalyst)" - DTC: 65-71 (P0304-P0300)
- "Three way catalyst function" - DTC: 72 (P0420)
- "Closed loop control" - DTC: 37 (P0130)

FREEZE FRAME DATA
The ECM has a memory function, which stores the driving condition such as fuel system status, calculated load value, engine coolant temperature, short fuel trim, long fuel trim, engine speed, vehicle speed at the moment the ECM detects a malfunction. Stored data is called Freeze Frame Data.

The data is useful for tracking down conditions at the time of the malfunction. Such conditions include whether vehicle was running or stopped, engine warm up, air-fuel ratio, etc. This data can be utilized to duplicate the malfunction and to diagnose the trouble.

The data will be erased along with the diagnostic trouble code by the above-mentioned method. The data can be stored only at the 1st trip. It can not be renewed even at the 2nd trip. The freeze frame data can be stored for only one item. Therefore, the ECM has the following priorities to update the data.




For example, the Exhaust Gas Recirculation (EGR) malfunction was detected and the freeze frame data was stored in the 1st trip. After that when the misfire is detected in another trip, the freeze frame data will be updated from the EGR malfunction for the misfire.