P0171

DTC P0171: Fuel Trim System Lean Bank 1

System Description
If the adaptation value threshold is permanently exceeded, the deviation of the adaptive terms enables to detect a slow default coming out. 2 time counters (one for the rich side and another one for the lean side) are increased while the lambda controller exceeds the adaptation thresholds. As soon as one of the time counters reaches its maximum value, the error is detected.

The aim of this test is to simulate a failure that would result in exceeding the adaptive terms. 2 kinds of failure must be created.
- A lean side deviation: P0171, P0174
- A rich side deviation: P0172, P0175

It is thus necessary to determine, for each kind of failure, the limit good and the limit bad. For a given failure, measure the emission threshold until the legal emission thresholds are exceeded. Note that the problem is due to the emission thresholds required, it is not simple to disturb the system so that the emission thresholds will be exceeded. The tuning has been made thanks to a dedicated calibration but, as such a procedure is not permitted by the regulation, it is necessary to create some material malfunction (fuel pressure regulator, fuel injector, air leakage...).

Conditions for Setting the DTC
- System is in Closed Loop.
- Canister is not too much loaded.
- Engine speed is greater than 608 rpm.
- Mass air flow is greater than 49 mg/stk.
- Engine coolant temperature is greater than 75 degrees C (167 degrees F).
- Barometric Pressure is greater than 749.9 kPa.
- Intake air temperature is greater than -10.5 degrees C (13.1 degrees F).
- No fuel system, TP, ECT, IAT, MAF, CMP, Misfire, CKP, MAP, ignition coil or injector faults exist.
- The average short term fuel trim value is higher than expected.

Action Taken When the DTC Sets
- The Malfunction Indicator Lamp (MIL) will illuminate.
- The ECM will record operating conditions at the time the diagnostic fail. This information will be stored in the Freeze Frame and Failure Records buffers.
- A history DTC is stored.

Conditions for Clearing the MIL/DTC
- The MIL will turn OFF at the end of 3 consecutive validation cycles in which the diagnostic runs without a fault.
- A history DTC will clear after 40 warm up cycles without a fault.
- DTC(s) can be cleared by using the scan tool.

Diagnostic Aids

CAUTION: After repairs, use the scan tool Fuel Trim Reset function to reset the long-term fuel trim to 128 (0%).

- Fuel pressure - The system will be lean if the pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm.
- MAP sensor - An output that causes the ECM to sense a lower than normal manifold pressure (high vacuum) can cause the system to go lean. Disconnecting the MAP sensor will allow the ECM to substitute a fixed (default) value for the MAP sensor. If the lean condition is gone when the sensor is disconnected, substitute a known good sensor and recheck.
- Fuel contamination - Water, in even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water causes a lean exhaust and can set DTC P0171, P0174.

Check for poor O2S or MAP sensor connection at the ECM. Inspect the harness connectors for the following conditions:
- Backed-out terminals
- Improper mating
- Broken locks
- Improperly formed
- Damaged terminals
- Poor terminal-to-wire connection

Inspect the wiring harness for damage. If the harness appears to be OK, observe the O2S display on the scan tool while moving the connectors and the wiring harness related to the engine harness. A change in the display will indicate the location of the fault. Check the brake power booster check valve for possible leaks.

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DTC Troubleshooting