DTC P0171

Fuel trim system too lean

DETECTION CONDITION

The PCM monitors short term fuel trim (SHRTFT) and long term fuel trim (LONGFT) values when closed loop fuel control. If the LONGFT and the sum total of these fuel trims or the SHRTFT exceed preprogrammed criteria, the PCM determines that the fuel system is too lean.

Diagnostic support note

This is a continuous monitor (fuel system).
The MIL illuminates if PCM detects above malfunction condition in two consecutive drive cycles or in one drive cycle while DTC for the same malfunction has been stored in PCM.
PENDING CODE is available if PCM detects the above malfunction condition during first drive cycle.
FREEZE FRAME DATA (Mode 2/Mode 12) is available.
The DTC is stored in the PCM memory.

POSSIBLE CAUSE

Erratic signal to PCM
- APP sensor signal malfunction
- ECT sensor signal malfunction
- MAF sensor signal malfunction
- TP sensor signal malfunction
- VSS signal malfunction
A/F sensor heater malfunction
A/F sensor malfunction
- A/F sensor looseness
- Leakage exhaust system
- A/F sensor deterioration
Air suction in intake air system
MAF sensor malfunction
Improper operation of purge solenoid valve
- Purge solenoid valve malfunction (stuck open)
- Purge solenoid hoses improper connection
Improper operation of variable valve timing control system
Fuel supply system malfunction or improper fuel line pressure
- Pressure regulator (built-in fuel pump unit) malfunction
- Fuel pump unit malfunction
- Fuel filter clogged or restricted
- Fuel leakage on fuel line from fuel distributor and fuel pump
Ignition system malfunction
- Ignition coil related wiring harness malfunction
- Ignition coil malfunction
- Spark plug malfunction
Improper operation of fuel injector
- Fuel injector malfunction
- Fuel injector related wiring harness malfunction
Insufficient engine compression
Misfire
PCM malfunction

DIAGNOSTIC PROCEDURE

STEP	INSPECTION		ACTION
1	IDENTIFY TRIGGER DTC FOR FREEZE FRAME DATA (MODE 2) Is the DTC P0171 on FREEZE FRAME DATA (Mode 2)?	Yes	Go to the next step.
1		No	Go to the troubleshooting procedures for DTC on FREEZE FRAME DATA (Mode 2). (See DTC TABLE ).
2	VERIFY FREEZE FRAME DATA (MODE 12) AND DIAGNOSTIC MONITORING TEST RESULTS HAVE BEEN RECORDED Have the FREEZE FRAME DATA (Mode 12) and DIAGNOSTIC MONITORING TEST RESULTS (fuel system related) been recorded?	Yes	Go to the next step.
2		No	Record the FREEZE FRAME DATA (Mode 12) and DIAGNOSTIC MONITORING TEST RESULTS on the repair order, then go to the next step.
3	VERIFY RELATED SERVICE BULLETINS AVAILABILITY Verify related Service Bulletins availability. Is any related Service Bulletins available?	Yes	Perform the repair or diagnosis according to the available Service Bulletins. If the vehicle is not repaired, go to the next step.
3		No	Go to the next step.
4	VERIFY RELATED PENDING CODE AND STORED DTC Turn the ignition switch off, then to the ON position (Engine off). Verify related PENDING CODE and stored DTC. Are any DTCs present?	Yes	If misfire DTC is present: Go to Step 11. If other DTC is present Go to the applicable DTC inspection. (See DTC TABLE ).
4		No	If drive ability concern is present Go to Step 11. If not: Go to the next step.
5	VERIFY CURRENT INPUT SIGNAL STATUS (KEY TO ON/IDLE) Access the following PIDs using the M-MDS: APP1 APP2 ECT MAF TP REL VSS Is there any signal that is far out of specification when the ignition switch is turned to the ON position and the engine runs? (See PCM INSPECTION ).	Yes	Inspect the suspected circuit. If the inspection results are normal: Go to the next step. If the inspection results are not normal: Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
5		No	Go to the next step.
6	VERIFY CURRENT INPUT SIGNAL STATUS UNDER FREEZE FRAME DATA (MODE 2) CONDITION CAUTION: While performing this step, always operate the vehicle in a safe and lawful manner. When the M-MDS is used to observe monitor system status while driving, be sure to have another technician with you, or record the data in the M-MDS using the PID/DATA MONITOR AND RECORD capturing function and inspect later. Inspect the same PIDs as in Step 5 while simulating FREEZE FRAME DATA (Mode 2) condition. Is there any signal which causes drastic changes?	Yes	Inspect the suspected circuit. If the inspection results are normal: Go to the next step. If the inspection results are not normal: Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
6		No	Go to the next step.
7	INSPECT A/F SENSOR HEATER Inspect the A/F sensor heater. (See AIR FUEL RATIO (A/F) SENSOR INSPECTION ). Is there any malfunction?	Yes	Replace the A/F sensor, then go to Step 20. (See AIR FUEL RATIO (A/F) SENSOR REMOVAL/INSTALLATION ).
7		No	Go to the next step.
8	INSPECT A/F SENSOR Inspect the A/F sensor. (See AIR FUEL RATIO (A/F) SENSOR INSPECTION ). Is there any malfunction?	Yes	Go to the next step.
8		No	Go to Step 11.
9	INSPECT INSTALLATION OF A/F SENSOR Inspect if the A/F sensor is loosely installed. Is the A/F sensor installed securely?	Yes	Go to the next step.
9		No	Retighten the A/F sensor, then go to Step 20. (See AIR FUEL RATIO (A/F) SENSOR REMOVAL/INSTALLATION ).
10	INSPECT GAS LEAKAGE FROM EXHAUST SYSTEM Visually inspect if there is any gas leakage between the exhaust manifold and A/F sensor. Is there gas leakage?	Yes	Repair or replace any malfunctioning exhaust part, then go to Step 20.
10		No	Replace the A/F sensor, then go to Step 20. (See AIR FUEL RATIO (A/F) SENSOR REMOVAL/INSTALLATION ).
11	INSPECT MAF PID Start the engine. Access the MAF PID using the M-MDS. Race the engine and verify that the MAF PID changes quickly according to change in the engine speed. Is the MAF PID response normal?	Yes	Go to Step 13.
11		No	Go to the next step.
12	INSPECT EXCESSIVE AIR SUCTION IN INTAKE AIR SYSTEM Visually inspect for loosen, cracks or damages hoses on intake air system. NOTE: Engine speed may change when rust penetrating agent is stayed on the air suction area. Is there air leakage?	Yes	Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
12		No	Replace the MAF/IAT sensor, then go to Step 20. (See MASS AIR FLOW (MAF)/INTAKE AIR TEMPERATURE (IAT) SENSOR REMOVAL/INSTALLATION ).
13	INSPECT PURGE SOLENOID VALVE OPERATION Carry out the Purge Control System Inspection. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION ). Does the purge control system work properly?	Yes	Go to the next step.
13		No	Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
14	INSPECT VARIABLE VALVE TIMING CONTROL SYSTEM OPERATION Carry out the Variable Valve Timing Control System Operation Inspection. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION ). Does variable valve timing control system work properly?	Yes	Go to the next step.
14		No	Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
15	INSPECT FUEL LINE PRESSURE Turn the ignition switch off. NOTE: If engine won't start, inspect fuel line pressure with ignition switch ON. Inspect the fuel line pressure while the engine running. (See FUEL LINE PRESSURE INSPECTION ). Is fuel line pressure normal?	Yes	Go to Step 17.
15		No	If the fuel line pressure is too low: Go to the next step. If the fuel line pressure is excess high: Replace the fuel pump unit, then go to Step 20. (See FUEL PUMP UNIT REMOVAL/INSTALLATION ).
16	INSPECT FUEL LINE FROM FUEL PUMP TO FUEL DISTRIBUTOR Visually inspect the fuel line for fuel leakage. Is there any fuel leakage?	Yes	Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
16		No	Inspect fuel filters for the foreign materials or stain inside fuel filter (low-pressure side). Perform the following actions according to the inspection results: If the foreign materials or stain is found inside fuel filter (low-pressure side): Clean the fuel tank and filter (low-pressure side). If normal: Replace the fuel pump unit. (See FUEL PUMP UNIT REMOVAL/INSTALLATION ). Go to Step 20.
17	INSPECT IGNITION SYSTEM OPERATION Carry out the Spark Test. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION ). Is the strong blue spark visible at each cylinder?	Yes	Go to the next step.
17		No	Repair or replace the malfunctioning part according to the inspection results, then go to Step 20.
18	INSPECT ENGINE COMPRESSION Inspect the engine compression. (See COMPRESSION INSPECTION ). Is there any malfunction?	Yes	Perform the engine overhaul for repairs, then go to Step 20.
18		No	Go to the next step.
19	INSPECT FUEL INJECTOR OPERATION Perform the Fuel injector Operation Inspection. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION ). Do the fuel injectors operate normally?	Yes	Go to the next step.
19		No	Repair or replace the malfunctioning part according to the inspection results, then go to the next step.
20	VERIFY TROUBLESHOOTING OF DTC P0171 COMPLETED Make sure to reconnect all disconnected connectors. Clear the DTC from the PCM memory using the M-MDS. Perform the Drive Mode 03 (PCM Adaptive Memory Production, EGR System, A/F Sensor Heater, HO2S Heater, A/F Sensor, HO2S, and TWC Repair Verification Drive Mode). (See OBD-II DRIVE MODE ). Stop the vehicle and access the ON BOARD READINESS TEST to inspect DRIVE MODE completion status. Verify the FUEL_EVAL PID changes to yes. If not, perform the Drive Mode 03 (PCM Adaptive Memory Production, EGR System, A/F Sensor Heater, HO2S Heater, A/F Sensor, HO2S, and TWC Repair Verification Drive Mode) again. (See OBD-II DRIVE MODE ). Is the PENDING CODE for this DTC present?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION ).
20		No	Go to the next step.
21	VERIFY AFTER REPAIR PROCEDURE Perform the "AFTER REPAIR PROCEDURE". (See AFTER REPAIR PROCEDURE ). Are any DTCs present?	Yes	Go to the applicable DTC inspection. (See DTC TABLE ).
21		No	DTC troubleshooting completed.