P0171 code: system too lean (Bank 1)

What each part of the code means

P: Powertrain
0: Generic code, defined by SAE
1: Fuel and air metering
71: The specific fault

P0171 and the related codes

P0171: System Too Lean (Bank 1)
P0174: System Too Lean (Bank 2)
P0172: System Too Rich (Bank 1)
P0175: System Too Rich (Bank 2)

How serious is it?

DeepDiag rates a stored P0171, P0172, P0174 or P0175 as major, and a pending one as pending. The rating is a starting point, not a safety check.

Possible causes

Lean means the mixture has too much air for the fuel. The engine computer aims for an air-to-fuel ratio of about 14.7 to 1, and corrects the mixture with fuel trim, an adjustment to its base fuel amount. A positive trim means it is adding fuel. P0171 sets when that adjustment reaches the limit the maker allows and Bank 1 (whichever side of the engine has cylinder 1) still reads lean.

In no particular order: which one is likeliest depends on your car and its data.

Air leaks after the MAF sensor: cracked vacuum hoses, intake gaskets, a split intake boot, the brake booster or a purge valve stuck open. Fuel trims that are high at idle and improve by about 2,500 rpm point this way.
The PCV system: a failed valve or hose in the crankcase ventilation system can also let in unmeasured air.
The MAF sensor: a dirty or faulty sensor misreads the air coming in. Trims that are high under load or at higher rpm point toward the MAF sensor or fuel delivery.
Fuel delivery: a weak fuel pump, a clogged filter or a faulty pressure regulator. A weak pump may only fall short when demand is high, so check fuel volume as well as pressure.
Fuel injectors: dirty or restricted injectors that deliver too little fuel.
An exhaust leak: it can make the oxygen sensor's reading look lean when the mixture isn't.
Other parts: an EGR valve, a MAP sensor, or engine software that the maker has since updated.

What to check in the data

Fuel trims: live short-term and long-term fuel trim for each bank, where the car supports them.
Fuel system monitor tests: results for each bank, where the car reports them.
Emissions readiness: whether the fuel system monitor has completed its self-test since codes were last cleared.
Freeze frame: the sensor snapshot from the moment the fault was recorded.
A recorded drive: live sensor data that you can record on a drive for the AI to review, including a guided test drive.
Other codes: a failure in one component can set a code in another, so read every stored and pending code.

Before you clear it or buy parts

Clearing erases stored and pending codes and resets the emissions readiness checks, so the car may fail an emissions test until it has been driven enough for them to complete again. If a stored code comes straight back, that points to an active fault.

Rank the possible causes by likelihood, then run the simplest, cheapest test that would confirm the cause before you buy a part. Read what to do before you clear codes.

Don't start with the oxygen sensor: the code doesn't prove the sensor has failed. It may be correctly reporting a real lean mixture, and a new sensor won't fix a leak.

How DeepDiag helps

DeepDiag reads P0171 and the data above, then explains in plain language what the data shows, what the AI suspects and how confident it is. It also suggests checks to run before you buy parts, and says whether it's a job you can do yourself.

DeepDiag gives an AI-assisted reading of the data your car reports. It isn't a professional inspection, a safety check or a guarantee, and the AI can be wrong. Every feature needs an active Premium subscription.

Coming soon

Get told when it's ready See a sample report

Sources

The possible causes, definitions and repair tips on this page come from these sources, checked in November 2026. What DeepDiag reads and how it rates a code come from the app itself.

P0171: system too lean (Bank 1).