DTC P0171 & P0172: Fuel Supply System

WARNING: This page is about a different car, the 2000 Honda Civic. However, it is still accessible from the selected car via links, so may be relevant.

NOTE: If DTCs P0106, P0107, P0108, P0135, P0137, P0138, P0141, P0401 (D16Y5 engine), P1128, P1129, P1259 (B16A2, D16Y5 or D16Y8 engine), P1491 (D16Y5 engine), and/or P1498 (D16Y5 engine) are displayed, trouble shoot DTCs P0171 and/or P0172 last.

NOTE: DTC P0171 and/or P0172 is set and MIL illuminated when malfunction has been detected in long term fuel trim, during 2 consecutive trips.

If DTC P0171 (system too lean) is set, check for:
- Insufficient fuel pump flow or pressure.
- Clogged or leaking fuel feed line.
- Fuel pressure regulator stuck open.
- Clogged fuel filter.
- Clogged or air blocked fuel injector.
- Fuel octane level less than specification.
- Deteriorated primary HO2S-1.
- Excessive EGR system flow (D16Y5 engine).
- Valves incorrectly adjusted.
- Exhaust system leak.
Repair as necessary. If no problems are found, go to step 3.
If DTC P0172 (system too rich) is set, check for:
- Fuel pressure regulator clogged or stuck closed.
- Clogged fuel return line.
- Leaking fuel injector.
- Fuel octane level less than specification.
- Deteriorated primary HO2S-1.
- Insufficient EGR system flow (D16Y5 engine).
- Leaking or stuck open EVAP purge control solenoid valve.
- Valves incorrectly adjusted.
Repair as necessary. If no problems are found, go to next step.
Check fuel pressure. See FUEL SYSTEM in BASIC DIAGNOSTIC PROCEDURES article. If fuel pressure is okay, go to next step. If fuel pressure is too high, check fuel pressure regulator and fuel return pipe. Repair as necessary. If fuel pressure is too low, check fuel filter, fuel pump, fuel feed pipe, and fuel pressure regulator. Repair as necessary.
On all models except D16Y5 engine with M/T, start engine and run at 3000 RPM with no loads and transmission in Park or Neutral until radiator fan comes on. Using scan tool, check primary HO2S-1 output voltage. If voltage does not stay at less than 0.3 volt or more than 0.6 volt, go to next step. If voltage stays at less than 0.3 volt or more than 0.6 volt, replace HO2S-1.
On all models, using a vacuum pump, apply vacuum to EVAP purge control solenoid valve from intake manifold side. On D16Y7 engine, EVAP purge control solenoid valve is located at right side of engine, below air leaner-to-air tube connection. On B16A2, D16Y5 and D16Y8, EVAP purge control solenoid valve is located below fuel injector rail cover. On all models, if vacuum does not hold, replace EVAP purge control solenoid valve. If vacuum holds, check valve adjustment. On D16Y5 engine, check EGR system. On all models, if no problems are found, replace fuel injectors.