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DATE: January, 2009
MODEL: All (exc. Raider)


DIAGNOSING FUEL TRIM CODES: P0171 “BANK 1 LEAN” AND P0174 “BANK 2 LEAN” — All Models (exc. Raider). DTCs P0171 and P0174 result in many Techline calls, often after a 30k service doesn’t resolve the problem. Using a logical diagnostic approach to determine which bank is affected and what RPM range the engine is operating at when the DTC is set will help you Fix it Right the FIRST TIME!


Start your diagnosis by looking at the long term fuel trim values in the freeze frame data for both banks. Typically, GOOD long term fuel trim values are numbers; near zero or within a few percent. Bad (lean) fuel trim values are at 12.5 (which turns on the check engine light). Fuel trim values close to 12.5 are suspicious and should be considered evidence that there is something not right on that bank. If you see one bank is at 12.5 and the other at 10.9, concentrate your diagnosis on conditions which affect BOTH banks.

Using the scan tool:

Monitor live MFI data at idle for two minutes and make a note of the long term fuel trims, air flow sensor and O2 readings for each bank.

Increase engine speed to 2500 rpm and hold it for two minutes. Note the fuel trims, air flow and O2 readings again, then let the engine speed return to idle.

Examine your data, and ask yourself two questions:

1) Is the vehicle severely lean at idle, at 2500 RPM, or both?

2) Is it severely lean on one bank or both?

If the fuel trim values indicate the engine is lean at idle only (high vacuum), there is a good chance of a vacuum leak (which is minimized at higher RPM).

While monitoring the data list, pinch off vacuum supply hoses to the brake booster, PCV valve, fuel pressure regulator, and 4WD solenoids at the source.

If the fuel trims read lean at higher RPM only, suspect fuel delivery (clogged filter, weak pump output), or fuel quality.

If the data shows that the engine is lean on only one bank, check for clogged injectors, a faulty O2 sensor, leaking plenum gasket, or an exhaust leak. If both banks read lean, suspect air flow sensor readings, inadequate fuel delivery, fuel quality, or a jumped timing belt.

Remember as RPM and airflow increase, so do your air flow sensor readings. Depending on altitude and vehicle model, typical air flow sensor values are 4 grams/second at idle, 8 at 1500 RPM, and 13 at 2500 RPM. Before condemning any component, compare your readings to the Data List Reference Table in the service manual and test components using the procedures described in the service manual. If you’re still unsure, monitor the data list for “typical readings” of a like vehicle. Once you have made any repairs based on your findings, confirm your repairs to verify that fuel trims have returned to near-zero values indicating the vehicle has been successfully repaired.

This diagnostic approach (Which bank? What RPM?) can also be used to identify the cause of P0172 and P0175 (rich) codes. Just change your thinking to diagnose for those conditions that would make the mixture rich.


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