DTC Advanced Diagnostics: P0171
DTC P0171:Fuel System Too Lean

General Description

The powertrain control module (PCM) detects the oxygen content in the exhaust gas from the air/fuel ratio (A/F) sensor (sensor 1) signal voltage, and it uses fuel feedback control to maintain the optimal air/fuel ratio. The air/fuel ratio coefficient for correcting the amount of injected fuel is the short term fuel trim. The PCM varies short term fuel trim continuously to keep the air/fuel ratio close to the stoichiometric ratio for all driving conditions. Long term fuel trim is computed from short term fuel trim and is used to regulate long term deviation from the stoichiometric air/fuel ratio, which occurs when fuel metering components deteriorate with age or system failures occur. In addition, long term fuel trim is stored in the PCM memory and is used to determine when fuel metering components malfunction. When long term fuel trim is higher than normal, which is about 1.0 (0 %), the amount of injected fuel must be increased, and when lower than normal, it must be decreased. If long term fuel trim is normal (too lean) or more, the PCM detects a malfunction in the fuel metering components and stores a DTC.

Monitor Execution, Sequence, Duration, DTC Type
Execution
Once per driving cycle*
Sequence
None
Duration
Every 3.0 seconds
DTC Type
Two drive cycles, MIL on
*: The malfunction judgment is cleared when it is judged as normal under the same driving conditions in which the malfunction is detected.

Enable Conditions
Condition
Minimum
Maximum
Engine coolant temperature
[ECT SENSOR 1]
156 °F (69 °C)
–
Intake air temperature
[IAT Sensor (1)]
18 °F (-7 °C)
–
Engine speed
[ENGINE SPEED]*1
530 rpm
4,000 rpm
Engine speed
[ENGINE SPEED]*2
620 rpm
4,000 rpm
MAP value [MAP SENSOR]
16 kPa (120 mmHg, 4.8 inHg)
–
Fuel feedback
Closed loop
Monitoring priority
P0133, P0420, P219C, P219D, P219E, P219F
*1: CVT
*2: M/T
[ ]: HDS Parameter

Malfunction Threshold
Long term fuel trim is 1.33 (+33 %) or more.

Possible Cause
NOTE: The causes shown may not be a complete list of all potential problems, and it is possible that there may be other causes.
Fuel pump failure
Fuel injector failure
Fuel pressure regulator failure
Fuel line failure
Fuel supply system failure
Mass airflow (MAF) sensor range/performance problem
Manifold absolute pressure (MAP) sensor range/performance problem
A/F sensor (sensor 1) failure
Secondary HO2S (sensor 2) failure
Vacuum hose misinstalled
Throttle body air leak
Intake manifold air leak
EVAP canister purge valve failure

Confirmation Procedure
Operating Condition
1.
Start the engine. Hold the engine speed [ENGINE SPEED] at 3,000 rpm without load (CVT in P or N, M/T in neutral) until the radiator fan comes on.
2.
Drive at a steady speed between 15 - 75 mph (25 - 120 km/h) for at least 15 minutes, and watch the long term fuel trim. If the long term fuel trim stays at about 1.0, the vehicle is OK or it is a very minor problem. If a significant fault is still present, the long term fuel trim will move up or down while driving.
When freeze data is stored, drive the vehicle under those conditions instead of Driving Pattern step 2.
After clearing the DTC by disconnecting the 12 volt battery or using the scan tool, drive at steady speed between 15 - 55 mph (25 - 88 km/h) for at least 40 minutes to allow time for long term fuel trim to recover.
Drive the vehicle in this manner only if the traffic regulations and ambient conditions allow.
With the HDS
None.

Diagnosis Details
Conditions for setting the DTC
When a malfunction is detected during the first drive cycle, a Pending DTC is stored in the PCM memory. If the malfunction returns in the next (second) drive cycle, the MIL comes on and a Confirmed DTC and the freeze data are stored.

Conditions for clearing the DTC
The MIL is cleared if the malfunction does not return in three consecutive drive cycles in which the engine conditions are similar to the first time the malfunction was detected. The MIL, the Pending DTC, the Confirmed DTC, and the freeze data can be cleared with the scan tool Clear command or by disconnecting the 12 volt battery.