P0171Powertrain
System too lean (bank 1) — air leak or not enough fuel
Where to start
Most often this is an air leak: cracks in the intake hose after the air filter, a hose that slipped off, a tired gasket. Less often — a dirty mass air flow sensor or a weak fuel pump. Start by looking at the hoses, that costs nothing.
What to watch in live data
A code is not a diagnosis. It says which check failed, not which part to replace. Clearing it without repair only means the car will report it again in a few trips.
How to put together a report a mechanic will actually read
We have a detailed piece on thisFault code P0171: where to start
Step by stepHow to find an air leak using fuel trims
How the code is built
Letter — the systemPowertrain: engine and transmission
Common or the maker's ownGeneric code: the same on every car
Which part of the engineFuel and air metering
Number of the faultThe serial number inside that group — it does not say how serious the fault is
All codes in «Mixture: lean and rich»
How it feels behind the wheel
The same fault shows itself in different ways. Each sign below says what to look at in the live data.
Hard to start when hotIt starts instantly from cold, but after a drive it cranks for a long time and catches reluctantly. Pressing the throttle sometimes helps.Knocking under loadUnder acceleration or on a hill the engine makes a metallic rattle, as if small change were being shaken. It goes away at steady throttle.The engine stumblesThe engine runs unevenly, the car shakes at idle, the exhaust note stutters. On the move the pull sags and consumption rises.Idle speed huntsThe tachometer needle wanders up and down with the car standing still: 1200, then 600. Sometimes the engine nearly stalls and picks up again.Fuel consumption has gone upYou refuel noticeably more often for the same driving. The lamp is usually off — the unit thinks it is coping.It jerks under accelerationAt steady throttle everything is fine, but under acceleration the car jerks or hesitates, as if the pedal was released for a moment.It stalls at idleAt the lights, or when you lift off, the engine dies. It usually restarts straight away.Check engine came on after refuellingYou filled up, drove a few kilometres, and the light came on. The car drives exactly as before.
How the app helps
CarScan reads this code from your car through an OBD-II adapter, shows whether it is active now or only stored, names the module that reported it, and puts it into a plain-text report for your mechanic.