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
Special Tools Required
Vacuum pump/gauge, 0-30 in.Hg, Snap-on YA4000A or equivalent, commercially available
- 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 P0171 and/or P0172.
P0106, P0107, P0108, P1128, P1129: MAP sensor
P1162, P1163, P1164, P1165: A/F sensor (Sensor 1)*1
P1166, P1167, P1168, P1169: A/F sensor (Sensor 1) heater*1
P1149, P1157, P1158, P1159, P1162, P1163, P1164: A/F sensor (Sensor 1)*2
P0134, P1166, P1167: A/F sensor (Sensor 1) heater*2
P0133, P1157, P2238, P2252, P2A00: A/F sensor (Sensor 1)*3
P0134, P0135: A/F sensor (Sensor 1) heater*3
P0137, P0138: Secondary HO2S (Sensor 2)
P0141: Secondary HO2S (Sensor 2) heater
P1259 (P2646)*4 (P2646, P2647)*5 : VTEC system
*1: 2000-2001 M/T models
*2: 2002-2003 models
*3: 2004-2006 models
*4: 2004 model
*5: 2005-2006 models
- Before you troubleshoot, record all freeze data and review the general troubleshooting information (see GENERAL TROUBLESHOOTING INFORMATION ).
- Check the fuel pressure.
- 2000-2003 M/T models (see 2000-2003 M/T MODELS )
- 2001-2003 CVT models (see 2001-2003 CVT MODELS )
- 2004-2005 M/T models (see 2001-2003 CVT MODELS )
- 2004-2005 CVT models (see 2004-2005 CVT MODELS )
- 2006 model (see 2006 MODEL )
Is fuel pressure OK?
- YES - Go to step 2.
- NO
- Check these items:
- If the pressure is too high, check the fuel pressure regulator and the fuel return line.
- If the pressure is too low, check the fuel level, the fuel pump, the fuel feed line, the fuel filter, and the fuel pressure regulator.
- Start the engine. Hold the engine speed at 3,000 rpm without load (in Park or neutral) until the radiator fan comes on.
- Check the A/F sensor (Sensor 1) output with the HDS.
Does it stay at less than 0.3 V or more than 1.0 V?
- YES - Replace the A/F sensor (Sensor 1) (see A/F SENSOR REPLACEMENT .
- NO - Go to step 4.
- Turn the ignition switch OFF.
- Disconnect the vacuum hose from the EVAP canister purge valve (A), and connect a vacuum pump/gauge, 0-30 in.Hg, to the hose.
Use the Manual With the Right Hardware
Pair factory procedures with proven DIY tools so the instructions are easier to execute.
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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.
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