A fuel mixture fault usually needs a check soon to avoid poor running and damage.
Trouble codes · P0171
P0171 meaning
System Too Lean Bank 1
The same first verdict Car Assist shows from this code alone, before a diagnosis. The code does not tell the app whether the warning light is flashing.
What this code doesn’t tell you
A code says where the car noticed a problem. It does not prove which part failed, whether the fault is still present, or what repair this car needs. The engine, other codes, freeze frame and live readings matter.
Usual causes, most likely first
The order is a starting point across cars; the exact engine and its readings can change it.
- Intake or vacuum leak
- Air flow sensor under-reading
- Low fuel delivery or an exhaust leak
What you can check
With the engine off and cold, look for split intake ducts and disconnected small hoses. Keep the freeze frame and note whether rough running is worse at idle.
Words the car uses
- Stored codes
- Faults the car has confirmed and saved. They help explain a check-engine light, but a code alone does not prove which part needs replacing.
- Freeze frame
- A snapshot of the engine's readings when a fault was recorded. It helps show the conditions that caused the fault, even if the car now seems fine.
- Readiness monitors
- The car's own checks of its emissions systems. An unfinished check means the car has not tested that system yet, so a clean code scan may miss a fault.
- Fuel trims (STFT and LTFT)
- How much the engine computer adds to or takes away from its planned fuel amount. STFT is the immediate adjustment and LTFT is the learned adjustment; B1 and B2 identify groups of cylinders, and large adjustments can point to an air or fuel problem.
- Air flow (MAF)
- The amount of air flowing into the engine. The computer uses it to choose how much fuel to add, so a wrong reading can make the engine run poorly.
- Air and fuel balance
- How the engine balances air and fuel; lambda describes that balance. A requested value is the computer's target, rather than a measurement.
Find out what’s causing yours
Car Assist reads your car through an OBD-II adapter and your iPhone. Its assistant uses the codes, freeze frame and live readings to work through the likely cause with you.