PID 06%

Short term fuel trim B1

Short term fuel trim, bank 1. The unit watches the oxygen sensor and adds or takes away fuel on the move: plus means the mixture was lean, minus means rich. The figure swings all the time and that is normal; what matters is the average and the direction it drifts. Long term trim (PID 07) sits next to it: it remembers what the short term one keeps repeating.

What is normal

Stays near zero. A steady plus over 10 % means extra air or not enough fuel.

These figures hold for most cars, but the maker has the last word: some engines run hotter or idle higher by design.

Codes about this parameter

P0171System too lean (bank 1) — air leak or not enough fuel P0172System too rich (bank 1)

We have a detailed piece on thisLean and rich mixture: P2177, P2187, P2195 vs P0171

Step by stepHow to find an air leak using fuel trims

Codes behind this reading

If the value is out of range, the unit usually says so with one of these codes.

P0171System too lean (bank 1) — air leak or not enough fuel P0172System too rich (bank 1)

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.The temperature gauge is climbingThe needle goes above its usual place, there is a hot smell from under the bonnet, and the heater may blow cold on a hot engine.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.Slow to warm up, the heater blows coldThe temperature needle sits low for ages, the cabin stays cold, and on the motorway it gets worse. Consumption rises noticeably.It smells of petrolA fuel smell in the cabin or around the parked car. Sometimes with the Check Engine lamp on.Smoke from the exhaustVisible smoke from the tailpipe. The colour is the clue: white, blue and black mean entirely different things.Check engine came on after refuellingYou filled up, drove a few kilometres, and the light came on. The car drives exactly as before.Check engine came on after a wash or heavy rainYou washed the car or drove through a downpour — and the light came on. It often clears itself in a day or two.

How to see it

In CarScan: the Live data tab, «Start polling». The app asks the car several times per second, draws a graph and remembers the highest and lowest value of the trip.

What the app does