OBD2 codes.
A code identifies a signal or system that needs to be checked, not a part you should replace.
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P0171Mixture too lean (bank 1)Urgency medium
System Too Lean (Bank 1)
The engine is getting more air or less fuel than it should: this is called a “lean mixture”. The engine computer tries to make up for it by adding fuel and, when it reaches its limit, it stores this code. “Bank 1” is the side of the engine with cylinder 1; an inline four-cylinder engine has only one bank.
What the car has detected
Bank 1 fuel trim (the amount the engine computer adds to or takes away from the planned fuel quantity) has gone past its programmed limit while the oxygen sensor in the exhaust still reads a lean mixture.
It is not usually dangerous in the short term, but the engine may hesitate, stall at idle or misfire. Get it sorted soon.
Symptoms
- Engine warning light on
- Rough idle or stalling
- Hesitation or lack of power when accelerating
- Possible misfires at idle
Possible causes
- Air leak: split intake hose, intake manifold gasket or loose vacuum line (air that gets in without being measured)
- Faulty crankcase ventilation (PCV) valve or a purge valve stuck open
- Mass air flow sensor (the sensor that measures incoming air) dirty or reading wrong
- Not enough fuel: weak pump, clogged filter, pressure regulator or dirty injectors
- Exhaust leak before the oxygen sensor, making it read lean
- Faulty oxygen sensor, less common than people tend to think
Checks for the garage
- Save the codes, the freeze frame data and the fuel trims before clearing anything
- Confirm with live data: short- and long-term fuel trims above +10%
- Check hoses, intake gaskets and the PCV valve, and run a smoke test
- Check the mass air flow sensor with live data and clean it with a dedicated cleaner
- Measure fuel pressure under the conditions in which the fault appears
- Look for exhaust leaks before the sensor and check its response before replacing it
Next step
Check for split or loose intake hoses and ask for a smoke leak test, a mass air flow sensor check and a fuel pressure test.
Go to a garage if you can't find an obvious air leak, if the engine stalls or if misfire codes appear as well. A smoke test and a fuel pressure test need workshop equipment.