P0171 System too Lean (Bank 1) - Automotive Repair Guides & Equipment

GENERAL DESCRIPTION

In order to provide the best possible combination of drivability, fuel economy and emission control, the PCM uses a closed loop air/fuel metering system. The PCM monitors the HO2S signal voltage and adjusts fuel delivery based it in closed loop fuel control. Changes in fuel delivery will be indicated by the long-term and the short-term fuel trim values. The ideal fuel trim value is around 0%. The PCM will add fuel when the HO2S signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0%. The PCM will reduce fuel when the HO2S signal is indicating a rich condition. Reduction in fuel is indicated by fuel trim values that are below 0%. The DTC relevant to fuel trim will be set when the amount reaches excessive levels because of a lean or rich condition.

DTC DESCRIPTION

Checking the fuel trime value under detecting condition, if its average exceeds the limit over certain period, PCM sets P0171.

DTC DETECTING CONDITION

Item

Detecting Condition

Possible Cause

DTC Strategy

Monitor the fuel trim value

1.	Air leakage

2.	Improper fuel pressure

3.	PCV valve stuck

4.	Clogging of injector

Enable Conditions

Engine warm-up sufficiently

Drive at a steady speed over 5 minutes ( Vehicle speed ≤ 130km/h)

60°C(140°F) ≤ Engine coolant temperature ≤ 115°C(239°F)

No disabling faults (DTCs related to HO2S, purge valve,catalyst )

Threshold value

Average of short term fuel trim > 0.8 and Average of long term fuel trim > 1.2

Diagnosis Time

Continuous

MIL On Condition

3 Driving Cycles

MONITOR GDS DATA

1.	Monitor DTCs related to MAFS, MAPS, ECTS, Injector or CVVT items with scantool.

Are there the DTCs above-mentioned?

	▶ Do all repairs associated with those codes before proceeding with this procedure.
	▶ Go to “System Inspection” procedure

SYSTEM INSPECTION

■ Check air leakage

Visually/physically inspect the air leakage in intake/exhaust system for following items.

▶ Vacuum hoses for splits, kinks and improper connections.

▶ Throttle body gasket

▶ Gasket between intake manifold and cylinder head

▶ Seals between intake manifold and fuel injectors

▶ Exhaust system between HO2S and three way catalyst for air leakage

Has a problem found in this procedure?

	▶ Repair or replace it which has a problem, and go to “Verification of Vehicle Repair” procedure.
	▶ Go to “Check the fuel line” as follows.

■ Check the fuel line

1.	Check the fuel line for following items ▶ Connector connection state ▶ Damage/ connection state for vacuum hoses connected to fuel line ▶ Bent/ pressed/ twisted fuel line or fuel leakage

Has a problem found in this procedure?

	▶ Repair or replace it which has a problem, and go to ” Verification of Vehicle Repair” procedure.
	▶ Go to “Check fuel pressure” as follows.

■ Check fuel pressure

Be cautious that Fuel is explosive and an empty fuel tank can still contain explosive gases. When working on fuel system make sure to supply adequate ventilation to the work area. Do not smoke, and keep sparks and open flames away.

The fuel system remains under pressure when the engine is not running. Release fuel system pressure before disconnecting any fuel line to reduce the chance of presonal injury or fire damage to vehicle components.

1.	IG “OFF” and disconnect Fuel Pump Relay in Junction Box.

2.	Start-up and wait until it stops itself.

3.	IG “OFF” and connect Fuel Pump Relay.

4.	Install the fuel pressure gauge to the delivery pipe with the fuel pressure gauge adaptor.

5.	Activate the fuel pump, and with fuel pressure applied, check that there is no fuel leakage from the pressure gauge or connection part.

Measure the fuel pressure at idle.

379.5kPa (3.87kgf/cm², 55.0psi)

Is the measured fuel pressure within specifications ?

	▶ Go to “Component Inspection” procedure.
	▶ Repair or replace according to the below table. And then, go to ” Verification of Vehicle Repair” procedure.

Condition	Possible Cause
Fuel Pressure is too low	Fuel filter, fuel pressure regulator, in-tank fuel hose or the fuel pump
Fuel Pressure is too high	Fuel pressure regulator, hose or pipe

COMPONENT INSPECTION

■ Check PCV

1.	IG “OFF” and remove PCV valve from rocker cover.

2.	Insert a thin stick into the PCV valve from the threaded side and verify that the plunger is moving.

Is the PCV valve normally moving?

	▶ Go to “Check injector” as follows.
	▶ Replace it, and go to “Verification of Vehicle Repair” procedure.

■ Check injector for normal operation

1.	Start engine.

2.	Check its RPM decrease when doing the injector’s actuation test.

Is there any cylinder with no change in RPM or only a small change in RPM?

	▶ Replace injector, and go to “Verification of Vehicle Repair” procedure.
	▶ Clear DTC and Test-drive under enable conditions above-mentioned. After the test, If this DTC is set, go to “Inspection & Repair” procedure. If not, troubleshooting is completed.

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