DTC P0171: Fuel System 1/1 Lean Or DTC P0174: Fuel System 2/1 Lean: Testing

NOTE: Check for contaminants that may have damaged the O2 Sensor such as contaminated fuel, unapproved silicone, oil and coolant.

Turn ignition on, engine not running. With the DRBIII®, read DTCs and record the related Freeze Frame data. Is the Good Trip counter displayed and equal to zero? If yes, go to next step. If no, go to step 16.

WARNING: The fuel system is under a constant pressure (even with the engine off). Before testing or servicing any fuel system hose, fitting or line, the fuel system pressure must be released. See FUEL SYSTEM PRESSURE RELEASE .

Install a fuel pressure gauge to the fuel rail. Turn ignition on, engine not running. With the DRBIII®, actuate the ASD Fuel System test and observe the fuel pressure gauge. Fuel pressure should be 53-63 psi (365-434 kPa). Turn the ignition off. If fuel pressure is below specification, stop all actuations and go to step 13. If fuel pressure is within specification, stop all actuations and go to next step. If fuel pressure is above specification, stop all actuations. Replace the fuel filter/pressure regulator. See FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION article.
Start the engine. Allow the engine to reach normal operating temperature. With the DRBIII®, read the O2 Sensor voltage. Is the voltage switching between 2.5 and 3.4 volts? If yes, go to next step. If no, go to step 10.
Turn ignition on, engine not running. With the DRBIII®, monitor the O2 Sensor voltage. Is the O2 Sensor voltage above 4.5 volts? If yes, replace the O2 Sensor. If no, go to next step.

NOTE: Throttle linkage must be moved very slowly while looking for a smooth change in voltage in this step.

Turn ignition on, engine not running. With the DRBIII®, read TP Sensor voltage. The throttle must be against the stop. Is the voltage 0.92 volt or less with the Throttle closed? If yes, go to next step. If no, check for a binding throttle condition. Repair as necessary. If okay, replace the Throttle Position Sensor.
Turn ignition on, engine not running. With the DRBIII®, read the TP Sensor voltage. While monitoring the DRBIII®, slowly open and close the throttle. Does the voltage increase and decrease smoothly? If yes, go to next step. If no, replace the Throttle Position Sensor.
Turn the ignition off. Connect a Vacuum Gauge to a Manifold Vacuum source. Start the engine. Allow the engine to idle. If engine will not idle, maintain a constant RPM above idle. With the DRBIII® in Sensors, read the MAP Sensor vacuum value. Is the DRBIII® reading within 1 in. Hg of the Vacuum Gauge reading? If yes, go to next step. If no, replace the MAP Sensor.

NOTE: For this test to be valid, the thermostat must be operating correctly. This test works best if performed on a cold engine (cold soak).

Turn ignition on, engine not running. With the DRBIII®, read the Engine Coolant Temperature Sensor value. If the engine was allowed to sit overnight (cold soak), the temperature value should be a sensible value that is somewhere close to the ambient temperature. If engine coolant temperature is above 180°F (82°C), allow the engine to cool until 150°F (65°C) is reached. Start the Engine. During engine warm-up, monitor the Engine Coolant Temperature value. The temperature value change should be a smooth transition from start up to normal operating temperature. The value should reach at least 180°F (82°C). Did the Engine Coolant Temperature increase smoothly and did it reach at least 180°F (82°C)? If yes, go to next step. If no, replace the Engine Coolant Temperature Sensor.
Check for any of the following conditions/mechanical problems:
- Air induction system must be free from leaks.
- Engine vacuum must be at least 13 in. Hg in Neutral.
- Engine valve timing must be within specification.
- Engine compression must be within specification.
- Engine exhaust system must be free of restrictions or leaks.
- Engine Positive Crankcase Ventilation (PCV) system must flow freely.
- Torque converter stall speed must be within specification.
- Power brake booster must have no internal leaks.
- Fuel must be free of contamination.
- Fuel injectors must be free of restrictions.
Are there any engine mechanical problems? If yes, repair as necessary. If no, test is complete.
Turn ignition on, engine not running. Disconnect the O2 Sensor harness connector. With the DRBIII®, monitor the O2 Sensor voltage. O2 Sensor voltage should read 5.0 volts on the DRBIII® with the O2 Sensor harness connector disconnected. Using a jumper wire, jumper the O2 Sensor Signal circuit to the O2 Sensor Return circuit at the O2 Sensor harness connector. The voltage should drop from 5.0 volts to 2.5 volts with the jumper wire in place. Did the O2 Sensor volts change from 5.0 volts to 2.5 volts? If yes, replace the O2 Sensor. If no, go to next step.
With the DRBIII®, read the O2 Sensor voltage. Is the voltage above 4.8 volts? If yes, go to next step. If no, check the O2 Sensor Signal circuit for a short to ground, open, or short to voltage. Inspect the O2 Sensor connector and the PCM harness connectors. Repair connectors as necessary. If connectors are okay, replace PCM. Program the new PCM. See PROGRAMMING
Ignition on, engine not running. Measure the voltage on the O2 Sensor Return circuit in the O2 Sensor harness connector. Is the voltage 2.5 volts? If yes, test is complete. If no, check the O2 Sensor Return circuit for a short to ground, open, or short to voltage. Inspect the O2 Sensor connector and the PCM harness connectors. Repair connectors as necessary. If connectors are okay, replace PCM. Program the new PCM. See PROGRAMMING

Raise vehicle on hoist, and disconnect the fuel pressure line at the fuel pump module. Connect 5/16" Fuel Line Adapter (6539) or 3/8" Fuel Line Adapter (6631) between disconnected fuel line and the fuel pump module. Attach a fuel pressure test gauge to the "T" fitting on fuel line adapter hose. Turn ignition on, engine not running. With the DRBIII®, actuate the ASD Fuel System test and observe the fuel pressure gauge. Fuel pressure should be 53-63 psi (365-434 kPa). Is the fuel pressure within specification? If yes, stop all actuations and repair or replace fuel supply line as necessary. If no, stop all actuations and go to next step.
Turn the ignition off. Release fuel pressure. Remove the Fuel Pump Module and inspect the Fuel Inlet Strainer. See FUEL SYSTEMS in REMOVAL, OVERHAUL & INSTALLATION article. Is the Fuel Inlet Strainer plugged? If yes, replace the Fuel Pump Inlet Strainer. If no, go to next step.
There are no possible causes remaining, replace the Fuel Pump Module.

NOTE: Check for contaminants that may have damaged the O2 Sensor such as contaminated fuel, unapproved silicone, oil and coolant.

WARNING: When the engine is operating, do not stand in a direct line with the fan. Do not put your hands near the pulleys, belts or fan. Do not wear loose clothing.

The conditions that set the DTC are not present at this time. The following may help in identifying the intermittent condition. With the engine running at normal operating temperature, monitor the DRBIII® parameters related to the DTC while wiggling the wiring harness. Look for parameter values to change and/or a DTC to set. Review the DRBIII® Freeze Frame information. If possible, try to duplicate the conditions under which the DTC was set. Refer to any Technical Service Bulletins (TSB) that may apply. Visually inspect the related wiring harness. Look for any chafed, pierced, pinched, or partially broken wires. Visually inspect the related wiring harness connectors. Look for broken, bent, pushed out, or corroded terminals. Were any of the above conditions present? If yes, repair as necessary. If no, test is complete.