DTC P0171

DTC P0171	System too lean
DETECTION CONDITION	The PCM monitors the short term fuel trim (SHRTFT) and long term fuel trim (LONGFT) when under closed loop fuel control. If the fuel trim is more than the specification, the PCM determines that the system is too lean. Diagnostic support note This is a continuous monitor (fuel system). The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM. PENDING CODE is available if the PCM detects the above malfunction condition during the 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 Excess air suction in intake-air system Leakage exhaust gas A/F sensor malfunction MAF sensor malfunction Fuel line pressure malfunction Leakage fuel Fuel pump unit malfunction Pressure regulator (built-in fuel pump unit) malfunction Ignition system malfunction High-tension lead malfunction incorrect power supply to ignition coil Ignition coil malfunction AIR system malfunction Insufficient compression Metering oil pump malfunction Engine oil condition malfunction Increased oil pressure Oil passage malfunction Engine malfunction Fuel injector malfunction PCM malfunction

System too lean

DETECTION CONDITION

The PCM monitors the short term fuel trim (SHRTFT) and long term fuel trim (LONGFT) when under closed loop fuel control. If the fuel trim is more than the specification, the PCM determines that the system is too lean.

Diagnostic support note

This is a continuous monitor (fuel system).
The MIL illuminates if the PCM detects the above malfunction condition in two consecutive drive cycles or in one drive cycle while the DTC for the same malfunction has been stored in the PCM.
PENDING CODE is available if the PCM detects the above malfunction condition during the 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
Excess air suction in intake-air system
Leakage exhaust gas
A/F sensor malfunction
MAF sensor malfunction
Fuel line pressure malfunction
- Leakage fuel
- Fuel pump unit malfunction
- Pressure regulator (built-in fuel pump unit) malfunction
Ignition system malfunction
- High-tension lead malfunction
- incorrect power supply to ignition coil
- Ignition coil malfunction
AIR system malfunction
Insufficient compression
- Metering oil pump malfunction
- Engine oil condition malfunction
- Increased oil pressure
- Oil passage malfunction
- Engine malfunction
Fuel injector malfunction
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 procedure 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 REPAIR INFORMATION AVAILABILITY Verify related Service Bulletins and/or on-line repair information availability. Is any related repair information available?	Yes	Perform repair or diagnosis according to the available repair information. If the vehicle is not repaired, go to the next step.
3		No	Go to the next step.
4	VERIFY RELATED PENDING CODE OR STORED DTC Turn the ignition switch off, then to the ON position (engine off). Verify the related PENDING CODE or stored DTCs. Are other DTCs present?	Yes	If misfire DTC is present, go to Step 9. If other DTC is present, go to the appropriate DTC inspection. (See DTC TABLE ).
4		No	If drive ability concern is present, go to Step 9. If other, go to the next step.
5	VERIFY CURRENT INPUT SIGNAL STATUS Access the following PIDs using the M-MDS: (See ON-BOARD DIAGNOSTIC TEST ). APP ECT MAF TP REL VSS Are the PIDs normal? (See PCM INSPECTION ).	Yes	Go to the next step.
5		No	Repair or replace the malfunctioning part according to the inspection results, then go to Step 24.
6	VERIFY CURRENT INPUT SIGNAL STATUS UNDER FREEZE FRAME DATA (MODE 2) CONDITION CAUTION: While performing this step with vehicle driving, 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 function and inspect later. Access the following PIDs using the M-MDS under the FREEZE FRAME DATA (Mode 2) condition: (See ON-BOARD DIAGNOSTIC TEST ). APP ECT MAF TP REL VSS Are the PIDs normal? (See PCM INSPECTION ).	Yes	Go to the next step.
6		No	Repair or replace the malfunctioning part according to the inspection results, then go to Step 24.
7	INSPECT A/F SENSOR Inspect the A/F sensor. (See AIR FUEL RATIO (A/F) SENSOR INSPECTION . Is there any malfunction?	Yes	Inspect for following: Air suction at following due to cracks, damages and loosing parts: From air cleaner to throttle body From throttle body to dynamic chamber From dynamic chamber and intake manifold Vacuum hoses NOTE: Engine speed may change when rust penetrating agent is sprayed on the air suction area. Loose installation of A/F sensor. If there is no malfunction detected, go to the next step. If there are malfunctions detected, repair or replace the malfunctioning part, then go to Step 24.
7		No	Go to Step 9.
8	INSPECT EXHAUST SYSTEM FOR EXHAUST GAS LEAKAGE Visually inspect exhaust gas leakage in the exhaust system. Is there exhaust gas leakage?	Yes	Repair or replace the malfunctioning part according to the inspection results, then go to Step 24.
8		No	Replace the A/F sensor, then go to Step 24. (See AIR FUEL RATIO (A/F) SENSOR REMOVAL/INSTALLATION ).
9	VERIFY CURRENT INPUT SIGNAL STATUS OF MAF SENSOR Start the engine. Access the MAF PID using the M-MDS. (See ON-BOARD DIAGNOSTIC TEST ). Verify that the MAF PID changes quickly according to RPM when the engine is raced. Is the PID normal? (See PCM INSPECTION ).	Yes	Go to Step 11.
9		No	Go to the next step.
10	INSPECT INTAKE-AIR SYSTEM FOR EXCESSIVE AIR SUCTION Visually inspect for looseness, cracks or damaged hoses in the intake-air system. Is there any malfunction?	Yes	Repair or replace the malfunctioning part according to the inspection results, then go to Step 24.
10		No	Replace the MAF/IAT sensor, then go to Step 24. (See MASS AIR FLOW (MAF)/INTAKE AIR TEMPERATURE (IAT) SENSOR REMOVAL/INSTALLATION ).
11	INSPECT FUEL LINE PRESSURE Perform the "FUEL LINE PRESSURE INSPECTION". (See FUEL LINE PRESSURE INSPECTION ). Is there any malfunction?	Yes	If the fuel line pressure is too low, go to the next step. If the fuel line pressure is too high, replace the fuel pump unit, then go to Step 24. (See FUEL PUMP UNIT REMOVAL/INSTALLATION ).
11		No	Go to Step 13.
12	INSPECT FUEL SYSTEM FOR FUEL LEAKAGE Visually inspect for fuel leakage in the fuel system. Is there fuel leakage?	Yes	Repair or replace the malfunctioning part according to the inspection results, then go to Step 24.
12		No	Replace the fuel pump unit then go to Step 24. (See FUEL PUMP UNIT REMOVAL/INSTALLATION ).
13	INSPECT IGNITION COIL OPERATION AND HIGH-TENSION LEAD WITH TIMING LIGHT Inspect the blinking condition on each high-tension lead using timing light at idle. Do alt high-tension leads show blinking condition?	Yes	Go to Step 17.
13		No	Go to the next step.
14	INSPECT HIGH-TENSION LEAD OF NON BLINKING HIGH-TENSION LEAD Inspect the high-tension leads. (See HIGH-TENSION LEAD INSPECTION ). Is there any malfunction?	Yes	Replace the malfunctioning high-tension lead, then go to Step 24. (See HIGH-TENSION LEAD REMOVAL/INSTALLATION ).
14		No	Go to the next step.
15	INSPECT IGNITION COIL POWER CIRCUIT FOR OPEN CIRCUIT OR SHORT TO GROUND Turn the ignition switch to the ON position (engine off). Measure the voltage between ignition coil terminal C (wiring harness-side) and body ground. Is the voltage B+?	Yes	Go to the next step.
15		No	Repair or replace the wiring harness for a possible open circuit or short to ground, then go to Step 24.
16	INSPECT IGNITION COIL Inspect the ignition coil. (See IGNITION COIL INSPECTION ). Is there any malfunction?	Yes	Replace the ignition coil, then go to Step 24. (See IGNITION COIL REMOVAL/INSTALLATION ).
16		No	Inspect for open circuit in wiring harness between ignition coil terminal B and ground. Repair or replace the wiring harness for a possible open circuit, then go to Step 24.
17	INSPECT AIR SYSTEM OPERATION Perform the Secondary Air Injection (AIR) System Inspection. (See ENGINE CONTROL SYSTEM OPERATION INSPECTION ). Does the AIR system operate properly?	Yes	Go to the next step.
17		No	Repair or replace the malfunctioning part according to the inspection results, then go to Step 24.
18	INSPECT ENGINE COMPRESSION Inspect the engine compression. (See COMPRESSION INSPECTION ). Is there any malfunction?	Yes	Go to the next step.
18		No	Go to Step 23.
19	INSPECT METERING OIL PUMP Inspect the metering oil pump. (See METERING OIL PUMP INSPECTION ). Is there any malfunction?	Yes	Repair or replace the malfunctioning part according to the inspection results. Overhaul or replace the engine, then go to Step 24.
19		No	Go to the next step.
20	INSPECT ENGINE OIL CONDITION Inspect the engine oil condition. Is the engine oil condition normal?	Yes	Go to the next step.
20		No	Replace the engine oil. Inspect the ECT sensor and related harnesses. (See ENGINE COOLANT TEMPERATURE (ECT) SENSOR INSPECTION ). Overhaul or replace the engine, then go to Step 24.
21	INSPECT OIL PRESSURE Inspect the oil pressure. (See OIL PRESSURE INSPECTION ). Is there any malfunction?	Yes	Repair or replace the malfunctioning part according to the inspection results. Overhaul or replace the engine, then go to Step 24.
21		No	Go to the next step.
22	INSPECT OIL PASSAGE Inspect the oil pipe between metering oil pump and metering oil nozzle. Is there any malfunction?	Yes	Inspect for leakage or clogging in the engine oil passage. Repair or replace the malfunctioning part according to the inspection results. Overhaul or replace the engine, then go to Step 24.
22		No	Overhaul or replace the engine, then go to Step 24.
23	INSPECT FUEL INJECTOR Inspect the fuel injector. (See FUEL INJECTOR INSPECTION ). Is there any malfunction?	Yes	Replace the fuel injector, then go to the next step. (See FUEL INJECTOR REMOVAL/INSTALLATION ).
23		No	Go to the next step.
24	VERIFY TROUBLESHOOTING OF DTC P0171 COMPLETED Make sure to reconnect all disconnected connectors. Clear the DTC from the PCM memory using the M-MDS. (See AFTER REPAIR PROCEDURE ). Perform the KOER self test. (See KOEO/KOER SELF TEST ). Is the PENDING CODE same as the DTC present?	Yes	Replace the PCM, then go to the next step. (See PCM REMOVAL/INSTALLATION ).
24		No	Go to the next step.
25	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 ).
25		No	DTC troubleshooting completed.