What Is P0171?
P0171 means the ECU's fuel trim has hit its limit whilst trying to compensate for a lean air-fuel mixture on Bank 1 (the bank of cylinders that contains cylinder 1). "Lean" means there is too much air relative to fuel. The ECU uses long-term and short-term fuel trims to constantly adjust fuelling - when it runs out of adjustment range trying to add fuel, it stores P0171. This is a petrol engine code; diesel engines use a different fuelling strategy.
Common Symptoms
- Engine management light
- Rough or unstable idle
- Hesitation or stumble on acceleration
- Poor fuel economy (counterintuitively - the ECU is adding fuel to compensate)
- Occasionally stalling at idle
- Possible hissing sound from vacuum leak
Common Causes
How to Diagnose P0171
Every P0171 outcome, written out in full
The tool above asks a couple of questions and lands you on one of these 6 results. They are all set out here as well, so you can read them without clicking through, compare them against each other, or check one on a phone with no signal.
If you answered Rough or hunting idle, sometimes stalling
Drive carefully, book it in A lean code with a rough or hunting idle is the combination that points hardest at unmetered air. At idle the engine is drawing very little air on purpose, so even a small leak is a large percentage of the total and the fuelling cannot keep up with it.
- Vacuum leak after the air flow meter (Most likely) Split intake hoses, perished vacuum pipes, a leaking inlet manifold gasket or a brittle brake servo hose all admit air the ECU never measured. The idle hunts because the fuelling correction is chasing air the engine keeps finding for free. How to find a vacuum leak properly
- Failed crankcase breather diaphragm (Likely) The breather system is plumbed straight into the inlet, so a split diaphragm becomes a large permanent leak in the worst possible place. On a lot of VAG and BMW engines this is the first thing to check and it is frequently the whole answer.
- Dirty or sticking throttle body (Possible) Carbon around the throttle plate changes airflow at the very small openings used at idle, so the ECU loses its reference and the idle wanders. A clean and a throttle adaptation reset is cheap and occasionally all it takes.
What to do next: Find the leak before replacing anything. A smoke test is the quick and certain way and most garages will do one inside half an hour. Working at home, a careful listen around the inlet at idle plus pinching each breather hose to see whether the idle changes gets you a long way for nothing.
OBD Fault Code Plain English Guide
- 100 most common UK fault codes in plain English
- A severity rating for every code so you know what can wait
- Causes and next steps listed for each fault
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If you answered It only misbehaves for the first few minutes from cold
Drive carefully, book it in Symptoms that appear from cold and clear as the engine warms are a genuine diagnostic gift. Rubber shrinks when it is cold and swells when it is hot, so a leak that seals itself after a few minutes tells you a great deal about where to look.
- Intake or vacuum hose leaking only when cold (Most likely) A cracked hose opens up while the rubber is cold and stiff, then closes as underbonnet temperature rises. That is why so many of these faults cannot be found once the car has sat idling on a garage forecourt for twenty minutes.
- Air flow meter drifting at low temperature (Likely) A contaminated hot wire reads low while the intake air is cold and dense, which is exactly when the fuelling error is at its largest. The engine leans out until the closed loop corrections catch up as things warm through. P0101 mass air flow sensor range fault
- Fuel pressure regulator weak on cold start (Possible) A regulator that cannot hold pressure while everything is cold gives a genuinely lean mixture in the first few minutes. If cold starting also takes longer than it used to, that supports this rather than an air leak.
What to do next: Test it cold rather than warm. Do the smoke test first thing in the morning before the engine has run at all, and watch live fuel trims from the moment it starts. Trims that are badly positive at start and settle as it warms confirm what you are describing and stop anyone chasing the wrong thing.
If you answered Nothing at all, it drives perfectly
Safe to drive A lean code on a car with no symptoms at all is worth investigating but not worth worrying about. The engine management is correcting for it successfully, which is why you cannot feel anything, and nothing is being damaged at this stage.
- Small leak still inside the correction range (Most likely) Fuel trims can add a good deal of extra fuel before anything becomes noticeable from the driving seat. The code sets when the correction passes a threshold, and that happens long before the engine actually starts running badly.
- Air flow meter reading slightly low (Likely) A meter that under reads by a modest margin leaves the ECU adding fuel to compensate right across the range. There is no flat spot and no misfire, just a permanently positive trim and a stored code.
- Exhaust leak ahead of the oxygen sensor (Possible) A leaking manifold gasket or a cracked flexible pipe draws fresh air past the sensor between exhaust pulses, so it reports lean when the mixture is actually correct. Listen for a light tick from cold that changes with engine speed.
What to do next: Read your fuel trims rather than guessing. Long term trim under about ten percent is ordinary wear, ten to twenty means there is something real to find, and over twenty needs sorting soon. Worth remembering too that a lit engine light is an automatic MOT failure, so this cannot be left alone indefinitely.
If you answered P0174 is stored alongside it
Drive carefully, book it in Both bank codes stored together is genuinely useful information rather than twice the bad news. Two banks reporting lean at once rules out most of the individual bank components and points at something the two sides share.
- Leak on the common intake tract (Most likely) Anything between the air flow meter and the point where the inlet splits affects both banks equally, which is why a cracked intake pipe or a failed manifold gasket produces this pair. A single injector or one sensor simply cannot do it.
- Air flow meter under reading (Likely) There is one meter feeding the whole engine, so a meter reading low starves both banks by exactly the same proportion. Comparing calculated load against what the engine should be producing at a given throttle exposes it quickly.
- Fuel supply falling short across the engine (Possible) A tired pump, a blocked filter or a restricted feed leans both banks under demand. Fuel pressure measured under load rather than at idle is what separates this from an air leak. P0087 low fuel rail pressure
What to do next: Concentrate on what the two banks share and ignore anything that only affects one side. Smoke test the common intake first, then check the air flow meter reading against expected values, then measure fuel pressure under load. Working in that order stops you buying injectors for a problem that is upstream of them.
If you answered A flat spot or hesitation when I accelerate, then Worse the harder I push it, uphill or overtaking
Drive carefully, book it in Hesitation that gets worse the harder you push points at fuel delivery rather than at an air leak. A vacuum leak matters least when the throttle is wide open, so a fault that appears exactly then is the opposite pattern.
- Fuel pump losing pressure under demand (Most likely) A worn pump holds pressure perfectly at idle and cruise and then falls short when the engine actually wants fuel. It has to be measured while driving under load, because a static test on the drive will look completely acceptable.
- Blocked fuel filter or restricted feed (Likely) A partly blocked filter behaves exactly like a failing pump and costs a fraction of one to change. On any car with an unknown service history this is the first thing to do rather than the last.
- Injectors no longer flowing enough (Possible) Deposits build on the injector tips and reduce delivery at high demand while leaving the idle perfectly acceptable. A flow test or a proper clean sorts it, and it is worth ruling out the two cheaper causes above first. P0300 random misfire diagnosis
What to do next: Get a fuel pressure gauge on it and take it out on the road with the gauge where you can see it, rather than testing on the drive. Pressure that holds at idle and drops away when you load the engine is your answer. Change the filter first if there is no record of one, because it is cheap and it is often the cause.
If you answered A flat spot or hesitation when I accelerate, then Worse at low revs and light throttle, clears if I press on
Drive carefully, book it in Trouble at light throttle that clears when you press on is the signature of unmetered air. Manifold vacuum is at its highest under light load, so that is exactly when a leak draws in the most air and upsets the mixture most.
- Split hose or gasket leaking under high vacuum (Most likely) The suction at light throttle pulls air through cracks that seal themselves again once you open the throttle and vacuum falls. That is precisely why the problem goes away when you press on, and why it can be so hard to pin down.
- Brake servo or its hose leaking (Likely) The servo is the largest single vacuum consumer on the car and a perished hose or a failed diaphragm becomes a substantial leak. A brake pedal that feels harder than it used to alongside this code makes it the first place to look.
- Evaporative purge valve stuck open (Possible) The purge valve should only open under specific conditions, and one stuck open bleeds fuel vapour and air into the inlet continuously. Unplug it and see whether the idle and the trims change straight away.
What to do next: Smoke test the inlet with the engine off and the intake blanked, and watch where it seeps out. Failing that, run the engine and spray a little carburettor cleaner around the manifold gasket and hose joints, listening for a change in idle speed. Keep it well away from anything hot and have a fire extinguisher within reach.
Check Fuel Trim Values
Check long-term fuel trim (LTFT) values. LTFT values above +10% to +15% on Bank 1 confirm a lean condition. Values above +25% indicate a significant problem.
Check for Vacuum Leaks
Use a can of brake cleaner or carburettor cleaner around the inlet manifold, throttle body gasket, and all vacuum hoses. A change in idle RPM reveals the leak.
Inspect MAF Sensor and Air Intake
Look for splits in the air intake duct between the MAF sensor and throttle body.
Check Fuel Pressure
Low fuel pressure will cause lean conditions across all cylinders.
Test Upstream O2 Sensor
It should switch rapidly between rich and lean at idle. A slow-responding sensor cannot provide accurate feedback.
Inspect Injectors
Listen to each injector with a stethoscope. A dead injector sounds different. Live injector data on a scanner can also reveal delivery issues.
Mechanic's Corner - P0171 on UK Cars
On UK cars - particularly Ford Focus, Fiesta, and Mondeo with the 1.0 EcoBoost or Duratec engines, and VW/Audi 1.4 and 1.8 TSI engines - the most overlooked cause of P0171 is a split intake hose that only opens under load. The crack is invisible at idle because vacuum holds the hose together. Take the vehicle on a test drive, rev to 3,000 rpm under load, then immediately stop and open the bonnet. Feel around all rubber intake sections between the air filter and throttle body - any air escaping is immediately felt as a sharp draught on your fingertips.
On Ford EcoBoost engines specifically, a failed PCV (positive crankcase ventilation) valve or cracked oil separator hose causes a significant unmeasured air leak into the intake. This is a common cause of persistent P0171 that survives MAF sensor and injector cleaning. The oil separator hose runs from the cylinder head cover to the intake manifold and cracks at approximately 80,000 miles.
Verdict
P0171 is usually caused by a vacuum leak or MAF sensor fault - both DIY-diagnosable with an OBD scanner and basic tools. Fuel trim data is the key piece of information; without it, diagnosis is guesswork.
