Circuit Description
- When the DTC P0171 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 is recorded, the actual air-fuel ratio is on the RICH side.
- If the vehicle runs out of fuel, the air-fuel ratio is LEAN and DTC P0171 is recorded. The malfunction indicator light (MIL) then comes on.
- If the total of the short-term fuel trim value and long-term fuel trim value is within +35.5 or -30% (engine coolant temperature is more than 176°F, 80°C), the system is functioning normally.
- The air-fuel ratio (A/F) sensor output voltage and the short-term fuel trim value can be read using the hand-held tester or the OBD II scan tool.
- The Engine Control Module (ECM) controls the voltage of terminals AF1A+ and AF1A- of the ECM to a fixed voltage. Therefore it is impossible to confirm the A/F sensor output voltage without the hand-held tester or the OBD II scan tool.
- The OBD II scan tool (excluding hand-held tester) displays the one fifth of the A/F sensor output voltage which is displayed on the hand-held tester.
The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim includes the short-term fuel trim and the long-term fuel trim.
The short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at stoichiometric air-fuel ratio. The signal from the A/F sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the stoichiometric air-fuel ratio. This variance triggers a reduction in the fuel volume if the air-fuel ratio is RICH and an increase in the fuel volume if it is LEAN.
The long-term fuel trim is the overall fuel compensation carried out in long-term to compensate for a continual deviation of the short-term fuel trim from the central value, due to individual engine differences, wear overtime and changes in the operating environment.
If both the short-term fuel trim and the long-term fuel trim are LEAN or RICH beyond a certain value, it is detected as a malfunction and the MIL is illuminated.