Diagnostic Aids

• 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.

Check for poor HO2S1 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 HO2S1 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.