DTC P0171, P0174

DTC P0171 : Rear Bank (Bank 1) Fuel System Too Lean DTC P0174 : Front Bank (Bank 2) Fuel System Too Lean

Special Tools Required

Vacuum Pump/Gauge, 0-30 inHg Snap-on YA4000A or equivalent, commercially available

NOTE:

Before you troubleshoot, review the general troubleshooting information - Refer to: How to Troubleshoot the Brake Booster Pressure Monitoring System (2016 2017 2018 2019 2020), or How to Troubleshoot the Fuel and Emissions Systems(With Auto Idle Stop System) (2016), or How to Troubleshoot the Fuel and Emissions Systems, or How to Troubleshoot the A/T System (2014 2015) .
Do not clear the DTC or reset the PCM before beginning this troubleshooting procedure until instructed to clear it. Clearing the DTC or resetting the PCM can delete valuable diagnostic information, even if the on-board snapshot and the freeze data are saved.
Inspect for aftermarket fuel system and engine components before beginning troubleshooting.
If any of the DTCs listed below are stored at the same time as DTC P0171 and/or P0174, troubleshoot those DTCs first, then recheck for P0171 and/or P0174.
P0101, P0102, P0103: MAF sensor

P0105 ('18-20model), P0107, P0108, P2073, P2074: MAP sensor

P0130 ('18-20model), P0133, P0150 ('18-20model), P0153, P1172 ('14-17models), P1174 ('14-17models), P2195, P2197, P2237, P2238, P2240, P2241, P2243, P2245, P2247, P2249, P2251, P2252, P2254, P2255, P2A00, P2A03: A/F sensor (Sensor 1)

P0134, P0135, P0154, P0155: A/F sensor (Sensor 1) heater

P0137, P0138, P0139, P0157, P0158, P0159, P2270, P2272, P2A01 ('18-20model), P2A04 ('18-20model): Secondary HO2S (Sensor 2)

P0141, P0161: Secondary HO2S (Sensor 2) heater

P0201, P0202, P0203, P0204, P0205, P0206: No. 1-No. 6 cylinder injector(s)

P0400, P0401, P0404, P0405 ('18-20model), P046C ('18-20model), P049D ('18-20model), P0406, P2413: EGR system

P0443: EVAP canister purge valve

P2279: Intake air leakage

P0521 ('18-20model), P2646, P2647, P2648, P2649, P2651, P2652, P2653, P2654, P2656, P2657, P2658, P2659: VTEC system

DTC Description	Confirmed DTC	Pending DTC	Freeze Frame
P0171 Rear Bank (Bank 1) Fuel System Too Lean
P0174 Front Bank (Bank 2) Fuel System Too Lean

DTC (PGM-FI)

Problem verification-Idle

-1. Start the engine, and let it idle without load (in P or N).

- 2. Check the parameter(s) below with the HDS.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold?

YES

The ST FUEL TRIM (B1, B2) is too high at idle. Go to step 3.

The ST FUEL TRIM (B1, B2) is OK at idle. Go to step 2.

Problem verification-On-board Snapshot

-1. Review the on-board snapshot and record the value of the parameters below when ST FUEL TRIM (B1, B2) was at its highest point.

ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

On-board Snapshot

- 2. Drive the vehicle according to the conditions determined above.

Signal	Current conditions
Signal	Values	Unit
ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

- 3. Check the parameter(s) below with the HDS while driving.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold under the on-board snapshot conditions?

YES

ST FUEL TRIM (B1, B2) is too high under the conditions above. Go to step 3.

ST FUEL TRIM (B1, B2) is OK under the same conditions as the on-board snapshot. The system is normal at this time.

Check for debris in MAF sensor
-1. Turn the vehicle to the OFF (LOCK) mode.

- 2. Remove the MAF sensor/IAT sensor.

- 3. Check for lint, bugs, and other foreign objects in the sensor.

Is there debris in the MAF sensor?

YES

Remove the debris from the MAF sensor and then use a vacuum cleaner to remove any debris from the air cleaner housing.

NO

Reinstall the MAF sensor/IAT sensor . Go to step 4.
Check for Disconnected or Damaged Components
-1. Visually inspect each of the components below for disconnected hoses, cracks, or damage:
- Crankcase breather hose (A)
- Air cleaner housing (B)
- Intake air duct (C)
- Oil cap (D) and dipstick (E)
Courtesy of HONDA, U.S.A., INC.

Courtesy of HONDA, U.S.A., INC.

Are any components disconnected or damaged?

YES

Repair as necessary.

NO

Go to step 5.

Check Brake Booster

-1. Start the engine, and let it idle.

- 2. Pinch the brake booster vacuum hose (A) connected to the intake manifold.

Courtesy of HONDA, U.S.A., INC.

- 3. Check the parameter(s) below with the HDS with the vacuum hose pinched.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold?

YES

The brake booster is not leaking. Go to step 6.

There is a leak in the brake booster or hoses. Check the brake booster vacuum hoses for leaks. If no leaks are found, replace the brake booster .

Check PCV System

-1. Continue to let the engine idle.

- 2. Pinch the PCV vacuum hose (A).

Courtesy of HONDA, U.S.A., INC.

- 3. Check the parameter(s) below with the HDS with the vacuum hose pinched.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold?

YES

The PCV system is not leaking. Go to step 7.

There is a leak in the PCV system. Check the vacuum hoses for leaks and repair as needed.

Check MAF Sensor Reading

-1. Continue to let the engine idle.

- 2. Make sure the following conditions are met:

ECT Sensor 1 above 158 °F (70 °C)
Transmission in P or N
All engine and electrical loads turned off

Signal	Current conditions
Signal	Values	Unit
ECT SENSOR 1

- 3. Monitor the ENGINE SPEED in the DATA LIST with the HDS, then hold the engine speed at 2, 500 rpm with the accelerator pedal held steady.

Signal	Current conditions
Signal	Values	Unit
ENGINE SPEED

- 4. While holding the engine speed at 2, 500 rpm for 30 seconds, check the parameter(s) below with the HDS.

'14model

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
MAF SENSOR	9.8-13.8	g/s

'15-20models

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
MAF SENSOR	10.2-14.4	g/s

Do the current condition(s) match the threshold?

YES

The MAF sensor is OK. Go to step 8.

The MAF sensor is reading incorrectly. Inspect for a cracked, damaged, or incorrectly installed air cleaner assembly. If OK, replace the MAF sensor/IAT sensor .

Check Fuel Pressure

-1. Check the fuel pressure under the conditions when ST FUEL TRIM (B1, B2) is 1.33 or more.

Signal	Current conditions
Signal	Values	Unit
ST FUEL TRIM B1
ST FUEL TRIM B2

Is the fuel pressure between 390-440 kPa (4.0-4.5 kgf/cm₂ , 57-64 psi)?

YES

The fuel pressure is OK. Go to step 9.

Check the correct power supply and ground for the fuel pump and if both are OK, replace the fuel tank unit .

Replace Fuel:

NOTE: There is an increased likelihood of a fuel related issue if the lean condition appeared shortly after refueling. The MIL DIST (MIL Distance) value in the DATA LIST in combination with the fuel level may help you make this determination.

-1. Turn the vehicle to the OFF (LOCK) mode.

- 2. Drain the fuel from the fuel tank, then refill the fuel tank with known-good fuel.

- 3. Operate the vehicle under the same conditions where ST FUEL TRIM (B1, B2) was 1.33 or more.

ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

On-board Snapshot

Signal	Current conditions
Signal	Values	Unit
ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

- 4. Start the engine.

- 5. Check the parameter(s) below with the HDS while driving.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold under the on-board snapshot conditions?

YES

The fuel is not the cause of the high ST FUEL TRIM (B1, B2) value. Go to step 10.

The fuel was causing the high ST FUEL TRIM (B1, B2) value.

Check for Intake Manifold Vacuum Leaks
-1. Disconnect the intake air duct from the air cleaner housing.

- 2. Connect a large intake manifold adapter cone into the intake air duct from the OTC LeakMaster evaporative emissions system tester kit.

NOTE: If a large intake manifold adapter cone is not available, leave the intake air duct installed, and remove the brake booster vacuum hose from the brake booster, and inject smoke into the hose.

- 3. Use the OTC LeakMaster evaporative emissions system tester to direct smoke into the intake manifold.

- 4. Inspect each of the areas below for signs of leaks:
- Intake manifold and gaskets
- Throttle body and gasket
- Air intake ducting
- All vacuum and air hoses
Are there any leaks?

YES

Repair the vacuum leak.

NO

Reconnect any disconnected components, then go to step 11.

Replace Injectors

-1. Turn the vehicle to the OFF (LOCK) mode.

- 2. Replace the injectors with known-good injectors.

- 3. Start the engine.

- 4. Operate the vehicle under the same conditions where ST FUEL TRIM (B1, B2) was 1.33 or more.

ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

On-board Snapshot

Signal	Current conditions
Signal	Values	Unit
ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

- 5. Check the parameter(s) below with the HDS while driving.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold under the on-board snapshot conditions?

YES

The injectors are not the cause of the high ST FUEL TRIM (B1, B2) value. Go to step 12.

The injectors were causing the high ST FUEL TRIM (B1, B2) value. Replace the original injectors .

Replace A/F Sensor (Sensor 1)

-1. Turn the vehicle to the OFF (LOCK) mode.

- 2. Replace the A/F sensor (Sensor 1) with a known-good sensor.

- 3. Start the engine.

- 4. Operate the vehicle under the same conditions where ST FUEL TRIM (B1, B2) was 1.33 or more.

ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

On-board Snapshot

Signal	Current conditions
Signal	Values	Unit
ENGINE SPEED
VEHICLE SPEED
REL TP SENSOR
APP SENSOR

- 5. Check the parameter(s) below with the HDS while driving.

Signal	Threshold		Current conditions
Signal	Values	Unit	Values	Unit
ST FUEL TRIM B1	More than 1.33
ST FUEL TRIM B2	More than 1.33

Do the current condition(s) match the threshold under the on-board snapshot conditions?

YES

The A/F sensor (Sensor 1) is not the cause of the high ST FUEL TRIM (B1, B2) value, reinstall the original A/F sensor. Go to step 13.

The A/F sensor (Sensor 1) was causing the high ST FUEL TRIM (B1, B2) value. Replace the original A/F sensor (Sensor 1) .

EVAP canister purge valve check
-1. Turn the vehicle to the OFF (LOCK) mode.

- 2. Disconnect the following connector.

EVAP canister purge valve 2P connector

- 3. Disconnect the vacuum hose (A) from the EVAP canister purge valve (B) in the engine compartment, and connect the vacuum pump/gauge, 0-30 inHg, to the purge line.

Courtesy of HONDA, U.S.A., INC.

- 4. Start the engine.

Does the vacuum gauge indicate vacuum?

YES

Replace the EVAP canister purge valve .

NO

The EVAP canister purge valve is OK. Substitute a known-good PCM - Refer to: PCM Removal and Installation (2016), or PCM Removal and Installation (2014 2015) .