Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
DTC P0171: Fuel System Too Lean; DTC P0172: Fuel System Too Rich
- Before you troubleshoot, record all freeze data and any on-board snapshot, and review the general troubleshooting information (see GENERAL TROUBLESHOOTING INFORMATION ).
- If some of the DTCs listed below are stored at the same time as DTC P0171 and/or P0172, troubleshoot those DTCs first, then recheck for DTC P0171 and/or P0172.
P0107, P0108, P1128, P1129: Manifold absolute pressure (MAP) sensor
P0133, P1157, P1172, P2195, P2238, P2252, P2A00: Air fuel ratio (A/F) sensor (Sensor 1)
P0134, P0135: Air fuel ratio (A/F) sensor (Sensor 1) heater
P0137, P0138, P0139, P2270, P2271: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) heater
P2646, P2647, P2648, P2649: VTEC system
P0401, P0404, P0406, P2413: Exhaust gas recirculation (EGR) system
P0443, P0496: EVAP canister purge valve
P2279: Intake air leakage
- Check the fuel pressure (see
FUEL PRESSURE TEST
).
Is the fuel pressure OK?
YES - Check the engine valve clearances, and adjust if necessary (see VALVE CLEARANCE ADJUSTMENT ). If the valve clearances are OK, replace the injectors (see INJECTOR REPLACEMENT ), then go to step 2.
NO - Check these items:
- If the pressure is too high, replace the fuel pressure regulator (see FUEL PRESSURE REGULATOR REPLACEMENT ), then go to step 2.
- If the pressure is too low, check the fuel pump and the fuel feed line, then go to step 2.
- Turn the ignition switch ON (II).
- Reset the ECM/PCM with the HDS.
- Do the ECM/PCM idle learn procedure (see ECM/PCM IDLE LEARN PROCEDURE ).
- Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral), until the radiator fan comes on, then let it idle.
- Test-drive under these conditions:
- Engine coolant temperature (ECT SENSOR 1) above 158°F (70°C)
- A/T in D position (M/T in 5th gear)
- Drive at a steady speed between 15-75 mph (24-120 km/h)
NOTE: DTC P0171 and/or P0172 may take up to 80 minutes of test driving to set. Using the HDS, monitor the long term fuel trim (LT FUEL TRIM) or the air fuel feed back average (AF FB AVE). If the long term fuel trim/air fuel feed back average stays within 0.80-1.25, there is no problem at this time. - Check for Temporary DTCs or DTCs with the HDS.
Is DTC P0171 or P0172 indicated?
YES - Go to step 1 and recheck.
NO - Troubleshooting is complete. If any other Temporary DTCs or DTCs are indicated, go to the indicated DTCs troubleshooting.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
Disclosure: Verified tool recommendations may earn an affiliate commission at no cost to you.
When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.
Rather have a pro handle this?
A certified mobile mechanic comes to your home or office. They bring the tools, you stay comfortable. No tow truck, no waiting room, no surprises.
No obligation. See pricing before you book.