Diagnostic Overview / Monitoring Function: Notes

(P0171, P0172, P0174, P0175) 

Sec (e) (6-6.4.6) (1968.2) 

The fuel system diagnosis uses two different monitors. The most important one is the evaluation of the percentage of the long term fuel control. The other one is the evaluation of the percentage of the physical limits of the short term fuel trim.

The physical limit controller is active during all engine states except of PUC (decal fuel cut-off). The evaluation of the long term fuel trim is active during its learning process and is not active during canister purge phases of the evaporative system. Because of this an additional learning process can be started in case of large deviations of the short term fuel trim and the evaluation can run. After the enable conditions are met different counters are started for both evaluations. If no condition is present the end diagnostic counter will decrement from a calibratable value to zero and a passing decision is made.

If a lean condition is present and total fuel control is above the calibratable threshold two timers are started. If the lean threshold counter exceeds the calibratible threshold before the reset timer has decrement from calibratible threshold to zero a lean error is set.

If a rich condition is present and total fuel control is below the calibratible threshold two timers are started. If the rich threshold counter exceeds the calibratible threshold before the reset timer has decremented from a calibratible threshold to zero a rich error is set.

The time counters are increased while "lambda controller" or "lambda adaptation" exceed minimum or maximum threshold.

The error is detected as soon as one of the time counters reaches its maximum value.