DTC P0171: System Too Lean (Bank 1); DTC P0172: System Too Rich (Bank 1); DTC P0174: System Too Lean (Bank 2); DTC P0175: System Too Rich (Bank 2): Description
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 the stoichiometric air-fuel ratio. The signal from the heated oxygen 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 short-term fuel trim varies from the central value due to individual engine differences, wear over time and changes in the operating environment. The long-term fuel trim, which controls overall fuel compensation, steadies long-term deviations of the short-term fuel trim from the central value.
If both the short-term fuel trim and the long-term fuel trim are lean or rich beyond a certain value, the ECM determines this to be a malfunction, illuminates the MIL and sets a DTC.
| DTC No. | DTC Detection Condition | Trouble Area |
|---|---|---|
| P0171 P0174 |
When air-fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on lean side (2 trip detection logic) |
|
| P0172 P0175 |
When air-fuel ratio feedback is stable after warming up engine, fuel trim is considerably in error on rich side (2 trip detection logic) |
|
HINT:
- When DTC P0171 or P0174 is recorded, the actual air-fuel ratio is on the lean side. When DTC P0172 or P0175 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 or P0174 may be recorded. The MIL is then illuminated.
- If the total of the short-term fuel trim value and long-term fuel trim value is within the malfunction threshold (and ECT is more than 75°C (167°F)), the system is functioning normally.