DTC P0171, P0174 [MZI-3.7]

DTC P0171 DTC P0174	P0171: System too lean (RH) P0174: System too lean (LH)
DETECTION CONDITION	• P0171 indicates that the fuel/air ratio is too lean (RH). • P0174 indicates that the fuel/air ratio is too lean (LH). • The adaptive fuel strategy continuously monitors the fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit.
POSSIBLE CAUSE	Fuel System: • Damaged or leaking fuel pulse damper • Fuel filter plugged or dirty • Damaged or worn fuel pump • Leaking fuel pump check valve • Leaking/contaminated fuel injectors • Low fuel pressure or running out of fuel • Purge solenoid valve is leaking when the canister is clean • Fuel supply line restricted • Fuel rail pressure sensor bias Exhaust System: • Exhaust leaks in the exhaust manifold gasket or mating gaskets before or near the HO2S Intake Air System: • Air leaks after the MAF sensor • Vacuum leaks • PCV system is leaking or the valve is stuck open • Improperly seated engine oil dipstick • Damaged or contaminated MAF sensor

DTC P0171

DTC P0174

P0171: System too lean (RH)

P0174: System too lean (LH)

DETECTION CONDITION

• P0171 indicates that the fuel/air ratio is too lean (RH).

• P0174 indicates that the fuel/air ratio is too lean (LH).

• The adaptive fuel strategy continuously monitors the fuel delivery hardware. The test fails when the adaptive fuel tables reach a rich calibrated limit.

POSSIBLE CAUSE

Fuel System:

• Damaged or leaking fuel pulse damper

• Fuel filter plugged or dirty

• Damaged or worn fuel pump

• Leaking fuel pump check valve

• Leaking/contaminated fuel injectors

• Low fuel pressure or running out of fuel

• Purge solenoid valve is leaking when the canister is clean

• Fuel supply line restricted

• Fuel rail pressure sensor bias

Exhaust System:

• Exhaust leaks in the exhaust manifold gasket or mating gaskets before or near the HO2S

Intake Air System:

• Air leaks after the MAF sensor

• Vacuum leaks

• PCV system is leaking or the valve is stuck open

• Improperly seated engine oil dipstick

• Damaged or contaminated MAF sensor

STEP	INSPECTION	ACTION
1	VERIFY FREEZE FRAME DATA HAS BEEN RECORDED • Has FREEZE FRAME DATA been recorded?	Yes	Go to the next step.
No	Record FREEZE FRAME DATA on the repair order, then go to the next step.
2	VERIFY RELATED SERVICE INFORMATION AVAILABILITY • Verify related Service Information availability. • Is any related Service Information available?	Yes	Perform repair or diagnosis according to the available Service Information. • If the vehicle is not repaired, go to the next step.
No	Go to the next step.
3	VERIFY RELATED PENDING CODE OR STORED DTCS • Turn the ignition switch off then to the ON position (Engine off). • Verify related PENDING CODE or stored DTCs. • Are any DTCs present other than the following: ― P0171, P0174, P2195 or P2197?	Yes	Go to the appropriate DTC troubleshooting procedures. (See DTC TABLE [MZI-3.7].)
No	Go to the next step.
4	PERFORM VISUAL INSPECTION ON INTAKEAIR SYSTEM AND ALL VACUUM HOSES • Turn the ignition switch off. • Check the intake air system for leaks, obstructions, and damage. • Inspect the entire length of all the vacuum hoses for: ― Proper connections ― Damage or cracks ― Damaged or cracked vacuum hose joint • Verify the integrity of the positive crankcase ventilation system. • Verify that the PCV valve part number is correct. • Is a concern present?	Yes	Repair if necessary. Then go to Step 7.
No	Go to the next step.
5	INSPECT FOR PRESENCE OF VACUUM LEAK Note • Fuel trim values at idle are more sensitive to a vacuum leak. The vacuum leak (unmetered air) represents a larger portion of the total air flow at idle than at part throttle. • The BARO PID is not a commended PID to monitor when diagnosing a vacuum leak. BARO is calculated during high engine load, when the vacuum leak represents a small portion of the total air flow • When calculating the total fuel correction in the following steps, if LONGFT1 equals +13% and SHRTFT1 equals +23%, the total fuel correction for bank 1 equals +36%. If LONGFT2 equals +24% and SHRTFT2 equals -3% the total fuel correction for bank 2 equals +21%. • If the freeze frame ECT PID is available, stabilize the engine at the temperature recorded by the freeze frame ECT PID. If the freeze frame ECT PID is not available, maintain the engine coolant temperature between 82—101 °C {180—215 °F} and the intake air temperature at less than 46 °C {115 °F}. • Turn the ignition switch to the ON position (Engine running) • Access the PCM and monitor the ECT, CHT and IAT PIDs. • Access the PCM and monitor the LONGFT1, SHRTFT1, LONGFT2 and SHRTFT2 PIDs. • Allow the engine to stabilize at the temperature necessary to recreate the concern. • Add and record the LONGFT PID value to the SHRTFT PID value for each bank, correction at idle. • Increase the engine speed to 2,500 rpm for 10 s. Record the LONGFT1, SHRTFT1, LONGFT2, and system SHRTFT2 PID values. • Add and record the LONGFT PID value to the SHRTFT PID value for each bank, correction at 2,500 rpm. • Is the total fuel correction value difference between idle and 2,500 rpm less than 15%?	Yes	No vacuum leak is present. Go to Step 8.
No	Go to the next step.
6	LOCATE VACUUM LEAK Caution • Do not clamp or pinch a hard plastic hose. Use a vacuum cap or equivalent to restrict the hose. Note • Restricting the EVAP vapor hose while the EVAP emission canister is purging may shift the SHRTFT. Perform a visual inspection if necessary. • When monitoring for a decrease in the SHRTFT PIDs in the following steps, if SHRTFT1 equals +15% and the hose is restricted, SHRTFT1 decreases to -7%. The total decrease in the SHRTFT PIDs equals 22%. • Locate the vacuum hose joint for the intake air and PCV systems. • Access the PCM and monitor the SHRTFT1 and SHRTFT2 PIDs. • Restrict the vacuum lines one at a time for 30 s. If a vacuum leak in the intake is present, the SHRTFT PID values decrease as the hose is restricted. • Is the decrease in the SHRTFT PIDs more than 15% when one of the vacuum hoses is restricted?	Yes	Repair if necessary. Then go to the next step.
No	Inspect the intake air system for a vacuum leak in the intake manifold or intake gaskets. Repair if necessary. Then go to the next step.
7	VACUUM LEAK REPAIR VERIFICATION Note • If the freeze frame ECT PID is available, stabilize the engine at the temperature recorded by the freeze frame ECT PID. If the freeze frame ECT PID is not available, maintain the engine coolant temperature between 82—101 °C {180—213 °F} and the intake air temperature less than 46 °C {115 °F}. • Access the PCM and monitor the SHRTFT1 and SHRTFT2 PIDs. • Allow the engine to stabilize at the temperature necessary to recreate the concern. • Record the SHRTFT1 and SHRTFT2 PID values. • Turn the ignition switch off. • Repair the vacuum leak. • Turn the ignition switch to the ON position (Engine running) • Allow the engine to stabilize at the temperature necessary to recreate the concern. • Access the PCM and monitor the SHRTFT1 and SHRTFT2 PIDs. • Compare the recorded SHRTFT PID values, prior to the no vacuum leak repair, to the current SHRTFT PID values. • Is the decrease in the SHRTFT PIDs more than 15%?	Yes	Reset the keep alive memory. Then go to Step 13.
No	A vacuum leak is still present. Return to Step 6.
8	INSPECT FUEL PRESSURE Warning • Fuel line spills and leakage are dangerous. Fuel can ignite and cause serious injuries or death. Fuel can also irritate skin and eyes. To prevent this, always complete the “BEFORE REPAIR PROCEDURE”. Note • For vehicle specific fuel pressure ranges, refer to the “FUEL LINE PRESSURE INSPECTION”. • Remove the jumper wire(s). • Connect the HO2S connector. • Relieve the fuel pressure. • Connect the mechanical fuel pressure gauge. • Pressurize the fuel system. • Turn the ignition switch to the ON position (Engine running) • Allow the fuel pressure to stabilize. • Turn the ignition switch off. • Access the PCM and control the FP PID. • Run the fuel pump to obtain maximum fuel pressure. • Is the fuel pressure within the range for the vehicle being diagnosed?	Yes	Go to Step 10.
No	Go to the next step.
9	INSPECT FUEL LINE FROM FUEL PUMP TO FUEL DELIVERY PIPE • Visually inspect fuel line for any leakage. • Is fuel leakage found?	Yes	Replace suspected fuel line. Then go to Step 13.
No	Inspect the low-pressure side fuel filter for the following: • Foreign materials or stain inside fuel filter. Perform the following actions as a result. • If foreign material or staining is found inside the fuel filter (low-pressure side), clean the fuel tank and filter. • If normal, replace fuel pump unit. Then go to Step 13.
10	INSPECT FUEL INJECTOR OPERATION • Remove fuel injector. • Inspect the injector operation. • Is the fuel injector normal?	Yes	Go to the next step.
No	Replace the fuel injector, then go to Step 13.
11	INSPECT FRONT HO2S OUTPUT VOLTAGE • Visually inspect the HO2S circuit for exposed wiring, contamination, corrosion and correct assembly. Repair if necessary. • Measure the voltage between HO2S terminal B and D on the component side. • Increase the engine speed to 2,000 rpm for 3 min. • Turn the ignition switch to the ON position (Engine running) • Monitor the signal voltage. • Is the voltage more than 0.5 V at any time during the engine running?	Yes	Go to the next step.
No	Replace the HO2S. Then go to Step 13.
12	PERFORM THOROUGH WIGGLE TEST ON HO2S WIRING HARNESS • Turn the ignition switch off. • Connect the HO2S connector. • Turn the ignition switch to the ON position (Engine running) • Verify that the engine is at normal operating temperature. • Access the PCM and monitor the HO2S signal PID. • Wiggle, shake, and bend small sections of the wiring harness while working from the sensor to the PCM. • While monitoring the HO2S PID, does the HO2S stop switching?	Yes	Isolate the concern and repair if necessary. Then go to the next step.
No	Go to the next step.
13	VERIFY TROUBLESHOOTING OF DTC P0171, P0174 HAS BEEN COMPLETED • Verify that all disconnected connectors are reconnected. • Turn the ignition switch to the ON position (Engine off). • Clear the DTC from the PCM memory using the M-MDS. • Start the engine and run it at idle. • Retrieve DTCs using the M-MDS. • Is the PENDING CODE for this DTC present?	Yes	Replace PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION [MZI-3.7].)
No	Go to the next step.
14	VERIFY AFTER REPAIR PROCEDURE • Perform “AFTER REPAIR PROCEDURE”. (See AFTER REPAIR PROCEDURE [MZI-3.7].) • Are any DTC present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE [MZI-3.7].)
No	Troubleshooting completed.

STEP

INSPECTION

ACTION

VERIFY FREEZE FRAME DATA HAS BEEN RECORDED

• Has FREEZE FRAME DATA been recorded?

Yes

Go to the next step.

Record FREEZE FRAME DATA on the repair order, then go to the next step.

VERIFY RELATED SERVICE INFORMATION AVAILABILITY

• Verify related Service Information availability.

• Is any related Service Information available?

Yes

Perform repair or diagnosis according to the available Service Information.

• If the vehicle is not repaired, go to the next step.

Go to the next step.

VERIFY RELATED PENDING CODE OR STORED DTCS

• Turn the ignition switch off then to the ON position (Engine off).

• Verify related PENDING CODE or stored DTCs.

• Are any DTCs present other than the following:

― P0171, P0174, P2195 or P2197?

Yes

Go to the appropriate DTC troubleshooting procedures.

(See DTC TABLE [MZI-3.7].)

Go to the next step.

PERFORM VISUAL INSPECTION ON INTAKEAIR SYSTEM AND ALL VACUUM HOSES

• Turn the ignition switch off.

• Check the intake air system for leaks, obstructions, and damage.

• Inspect the entire length of all the vacuum hoses for:

― Proper connections

― Damage or cracks

― Damaged or cracked vacuum hose joint

• Verify the integrity of the positive crankcase ventilation system.

• Verify that the PCV valve part number is correct.

• Is a concern present?

Yes

Repair if necessary. Then go to Step 7.

Go to the next step.

INSPECT FOR PRESENCE OF VACUUM LEAK

Note

• Fuel trim values at idle are more sensitive to a vacuum leak. The vacuum leak (unmetered air) represents a larger portion of the total air flow at idle than at part throttle.

• The BARO PID is not a commended PID to monitor when diagnosing a vacuum leak. BARO is calculated during high engine load, when the vacuum leak represents a small portion of the total air flow

• When calculating the total fuel correction in the following steps, if LONGFT1 equals +13% and SHRTFT1 equals +23%, the total fuel correction for bank 1 equals +36%. If LONGFT2 equals +24% and SHRTFT2 equals -3% the total fuel correction for bank 2 equals +21%.

• If the freeze frame ECT PID is available, stabilize the engine at the temperature recorded by the freeze frame ECT PID. If the freeze frame ECT PID is not available, maintain the engine coolant temperature between 82—101 °C {180—215 °F} and the intake air temperature at less than 46 °C {115 °F}.

• Turn the ignition switch to the ON position (Engine running)

• Access the PCM and monitor the ECT, CHT and IAT PIDs.

• Access the PCM and monitor the LONGFT1, SHRTFT1, LONGFT2 and SHRTFT2 PIDs.

• Allow the engine to stabilize at the temperature necessary to recreate the concern.

• Add and record the LONGFT PID value to the SHRTFT PID value for each bank, correction at idle.

• Increase the engine speed to 2,500 rpm for 10 s. Record the LONGFT1, SHRTFT1, LONGFT2, and system SHRTFT2 PID values.

• Add and record the LONGFT PID value to the SHRTFT PID value for each bank, correction at 2,500 rpm.

• Is the total fuel correction value difference between idle and 2,500 rpm less than 15%?

Yes

No vacuum leak is present. Go to Step 8.

Go to the next step.

LOCATE VACUUM LEAK

Caution

• Do not clamp or pinch a hard plastic hose. Use a vacuum cap or equivalent to restrict the hose.

Note

• Restricting the EVAP vapor hose while the EVAP emission canister is purging may shift the SHRTFT. Perform a visual inspection if necessary.

• When monitoring for a decrease in the SHRTFT PIDs in the following steps, if SHRTFT1 equals +15% and the hose is restricted, SHRTFT1 decreases to -7%. The total decrease in the SHRTFT PIDs equals 22%.

• Locate the vacuum hose joint for the intake air and PCV systems.

• Access the PCM and monitor the SHRTFT1 and SHRTFT2 PIDs.

• Restrict the vacuum lines one at a time for 30 s. If a vacuum leak in the intake is present, the SHRTFT PID values decrease as the hose is restricted.

• Is the decrease in the SHRTFT PIDs more than 15% when one of the vacuum hoses is restricted?

Yes

Repair if necessary. Then go to the next step.

Inspect the intake air system for a vacuum leak in the intake manifold or intake gaskets. Repair if necessary. Then go to the next step.

VACUUM LEAK REPAIR VERIFICATION

Note

• If the freeze frame ECT PID is available, stabilize the engine at the temperature recorded by the freeze frame ECT PID. If the freeze frame ECT PID is not available, maintain the engine coolant temperature between 82—101 °C {180—213 °F} and the intake air temperature less than 46 °C {115 °F}.

• Access the PCM and monitor the SHRTFT1 and SHRTFT2 PIDs.

• Allow the engine to stabilize at the temperature necessary to recreate the concern.

• Record the SHRTFT1 and SHRTFT2 PID values.

• Turn the ignition switch off.

• Repair the vacuum leak.

• Turn the ignition switch to the ON position (Engine running)

• Allow the engine to stabilize at the temperature necessary to recreate the concern.

• Access the PCM and monitor the SHRTFT1 and SHRTFT2 PIDs.

• Compare the recorded SHRTFT PID values, prior to the no vacuum leak repair, to the current SHRTFT PID values.

• Is the decrease in the SHRTFT PIDs more than 15%?

Yes

Reset the keep alive memory. Then go to Step 13.

A vacuum leak is still present. Return to Step 6.

INSPECT FUEL PRESSURE

Warning

• Fuel line spills and leakage are dangerous. Fuel can ignite and cause serious injuries or death. Fuel can also irritate skin and eyes. To prevent this, always complete the “BEFORE REPAIR PROCEDURE”.

Note

• For vehicle specific fuel pressure ranges, refer to the “FUEL LINE PRESSURE INSPECTION”.

• Remove the jumper wire(s).

• Connect the HO2S connector.

• Relieve the fuel pressure.

• Connect the mechanical fuel pressure gauge.

• Pressurize the fuel system.

• Turn the ignition switch to the ON position (Engine running)

• Allow the fuel pressure to stabilize.

• Turn the ignition switch off.

• Access the PCM and control the FP PID.

• Run the fuel pump to obtain maximum fuel pressure.

• Is the fuel pressure within the range for the vehicle being diagnosed?

Yes

Go to Step 10.

Go to the next step.

INSPECT FUEL LINE FROM FUEL PUMP TO FUEL DELIVERY PIPE

• Visually inspect fuel line for any leakage.

• Is fuel leakage found?

Yes

Replace suspected fuel line. Then go to Step 13.

Inspect the low-pressure side fuel filter for the following:

• Foreign materials or stain inside fuel filter. Perform the following actions as a result.

• If foreign material or staining is found inside the fuel filter (low-pressure side), clean the fuel tank and filter.

• If normal, replace fuel pump unit.

Then go to Step 13.

INSPECT FUEL INJECTOR OPERATION

• Remove fuel injector.

• Inspect the injector operation.

• Is the fuel injector normal?

Yes

Go to the next step.

Replace the fuel injector, then go to Step 13.

INSPECT FRONT HO2S OUTPUT VOLTAGE

• Visually inspect the HO2S circuit for exposed wiring, contamination, corrosion and correct assembly. Repair if necessary.

• Measure the voltage between HO2S terminal B and D on the component side.

• Increase the engine speed to 2,000 rpm for 3 min.

• Turn the ignition switch to the ON position (Engine running)

• Monitor the signal voltage.

• Is the voltage more than 0.5 V at any time during the engine running?

Yes

Go to the next step.

Replace the HO2S. Then go to Step 13.

PERFORM THOROUGH WIGGLE TEST ON HO2S WIRING HARNESS

• Turn the ignition switch off.

• Connect the HO2S connector.

• Turn the ignition switch to the ON position (Engine running)

• Verify that the engine is at normal operating temperature.

• Access the PCM and monitor the HO2S signal PID.

• Wiggle, shake, and bend small sections of the wiring harness while working from the sensor to the PCM.

• While monitoring the HO2S PID, does the HO2S stop switching?

Yes

Isolate the concern and repair if necessary. Then go to the next step.

Go to the next step.

VERIFY TROUBLESHOOTING OF DTC P0171, P0174 HAS BEEN COMPLETED

• Verify that all disconnected connectors are reconnected.

• Turn the ignition switch to the ON position (Engine off).

• Clear the DTC from the PCM memory using the M-MDS.

• Start the engine and run it at idle.

• Retrieve DTCs using the M-MDS.

• Is the PENDING CODE for this DTC present?

Yes

Replace PCM, then go to the next step.

(See PCM REMOVAL/INSTALLATION [MZI-3.7].)

Go to the next step.

VERIFY AFTER REPAIR PROCEDURE

• Perform “AFTER REPAIR PROCEDURE”.

(See AFTER REPAIR PROCEDURE [MZI-3.7].)

• Are any DTC present?

Yes

Go to the applicable DTC inspection.

(See DTC TABLE [MZI-3.7].)

Troubleshooting completed.