P0171
Engine computer has reached its limit adding fuel on bank 1: the mixture is lean
Severity: MonitorRough idle with the check engine light on
Severity: Inspect soon
A rough idle with the light on points to misfire, unmetered air or a fuel-vapour fault that can lead to stalling and converter damage, so get it diagnosed soon.
It gets more serious if:
- Stop: Stop if the check engine light is flashing: that is misfire bad enough to damage the catalytic converter. Pull over safely away from anything flammable, switch off and let it cool. If it flashes again after a restart, have the car taken in or drive gently and slowly only to the nearest repair shop as the owner's manual directs.
How to tell what's causing it
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Lean codes (P0171 and/or P0174) are stored, and the idle smooths out at higher engine speed.
Air leaking in after the airflow sensor is not measured, and its effect is largest at idle, when the engine draws the least air.
Points to: P0171, P0174, Vacuum hoses and intake leaks
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A misfire code is stored and the idle shakes.
A cylinder that misfires at idle makes the engine stumble; a lean mixture can also cause misfire at idle.
Points to: P0300, Ignition coil
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An EVAP purge code (P0441, P0496) is stored, or the idle is rough and starting is hard right after refuelling.
A purge valve stuck open lets fuel vapour into the engine when it should not, which upsets the mixture at idle; on some Ford models a recall and a bulletin describe stalling or hard starting from this.
Points to: P0496, P0441, EVAP canister purge valve
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Camshaft timing codes (P0011, P0014, P0017) are stored with the rough idle.
MOTOR reports rough idle with camshaft timing codes caused by a defective cam actuator solenoid; it pairs the fix with an oil and filter change when the solenoid screens are clogged.
Points to: P0011, P0014, P0017, Variable valve timing (VVT) oil control solenoid
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The idle speed is too high rather than rough, and P0507 is stored.
The idle control test flags idle that is above the computer's target, which commonly points to extra air entering.
Points to: P0507
Codes that go with it
P0174
Engine computer has reached its limit adding fuel on bank 2: the mixture is lean
Severity: MonitorP0300
Random or multiple-cylinder misfire detected
Severity: Inspect soonP0441
EVAP purge flow incorrect
Severity: MonitorP0496
EVAP purge flow when purge should be off
Severity: MonitorP0011
Intake camshaft timing ahead of target or slow to respond (bank 1)
Severity: Inspect soonP0014
Exhaust camshaft timing ahead of target or slow to respond (bank 1)
Severity: Inspect soonP0017
Crankshaft and exhaust camshaft positions do not agree (bank 1, sensor B)
Severity: Inspect soonP0507
Idle speed higher than the computer is aiming for
Severity: Monitor
Sources and review
- Lean On Me
- Ford P0171 & P0174 Lean Codes (Larry Carley)
- Random Misfires (OBD II P0300 DTC)
- Vapor Canister Purge Valve
- Ford Safety Recall 18S32 Supplement #3 (canister purge valve)
- Ford technical service bulletin: hard start/no start or loss of rpm after refuelling (2013 Escape, 2013-2014 Fusion and MKZ)
- Service Slants: GM Inline Engine Cam Actuator Solenoid
- Final Regulation Order: 13 CCR §1968.2, Malfunction and Diagnostic System Requirements (OBD II), 2004 and subsequent model-year passenger cars, light-duty trucks and medium-duty vehicles (2015 amendments)
- OBD consumer brochure: Do you drive a 1996 or newer car or light truck? (EPA420-F-02-016)
- 2023 Integra Owner's Manual: Indicator, Coming On/Blinking
- 2020 Nissan Altima Owner's Manual
- Last reviewed
- Written by
- Saurabh K.
- Editorial review
- Pending
- Technician review
- Technician review pending
This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error