What does code P0171 mean?

P0171 — System too lean, Bank 1 — engine is running out of fuel margin; ECM has been compensating but has hit its correction limit (LTFT > +25%). Belongs to the powertrain.fuel-trim system. Severity: warning.

What are the common causes of P0171?

Vacuum leak on the intake — most common: cracked intercooler boot, torn PCV hose, brittle vacuum lines; Contaminated MAF sensor — spider webs, dust, oil from aftermarket filters; clean with MAF-safe cleaner (not WD-40!); Low fuel pressure — clogged filter, weak pump, failing regulator; One or more clogged fuel injectors — common on GDI engines (TSI, FSI, Skyactiv); Broken or corroded Bank 1 upstream O2 sensor connector

Can I drive with code P0171?

Limited: reaching the nearest workshop is fine, but avoid long trips. May progress into a more serious fault.

How do I diagnose P0171?

Read the code and freeze-frame with a scanner, check related-system errors, inspect the components listed (wiring, connectors, sensors). See the "How to diagnose" section above for the full step-by-step.

How do I clear code P0171?

The code can be cleared via an OBD-II scanner (e.g. ELM327 + the Car Diagnostics app): plug the adapter into the OBD port, go to Errors → Clear. If the underlying cause isn't fixed, the code will return within a few drive cycles.

OBD-II Code P0171 — Car Diagnostics Platform

What it means

Common causes

Vacuum leak on the intake — most common: cracked intercooler boot, torn PCV hose, brittle vacuum lines
Contaminated MAF sensor — spider webs, dust, oil from aftermarket filters; clean with MAF-safe cleaner (not WD-40!)
Low fuel pressure — clogged filter, weak pump, failing regulator
One or more clogged fuel injectors — common on GDI engines (TSI, FSI, Skyactiv)
Broken or corroded Bank 1 upstream O2 sensor connector
Cracked intake manifold or leaking manifold gasket
Clogged in-tank fuel pump strainer — can't flow enough fuel during acceleration
Faulty MAP sensor — readings don't match actual load

How to diagnose

01
Read all stored and pending codes with an OBD-II adapter — get the full picture, not just the one code.
02
Capture freeze-frame data (RPM, coolant temperature, engine load) at the exact moment the code triggered.
03
Inspect the named subsystem with our app's AI explanation — get vehicle-specific causes for your make, model and year.

Decode P0171 on your actual car

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