What the car is doing
Start from what you feel behind the wheel, not from a code. Each sign here says what to look at in the live data — so that ten possible causes become two or three.
Do not drive
With this symptom you should not keep driving: the trip will cost more than a tow truck.
It stalls while drivingThe engine dies while the car is moving — at speed, in a corner or when lifting off. The steering and brakes go heavy at once.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.It cranks but will not startThe starter turns the engine over briskly, but it does not catch. Sometimes it catches and dies.It smells of petrolA fuel smell in the cabin or around the parked car. Sometimes with the Check Engine lamp on.
You can drive, but do not put it off
You can usually get home or to a garage, but every day of delay adds to the bill.
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 voltage jumps, the lights dimThe headlights and the dashboard change brightness while driving, sometimes the battery lamp flickers. The electronics behave oddly.The steering has gone heavyThe wheel turns noticeably harder than usual, especially when parking and at low speed. Sometimes the assistance returns after restarting the engine, sometimes it disappears entirely.Will not start in ParkIn P the starter is silent, but in N the car starts. Or the other way round. Sometimes every gear letter lights up at once; sometimes cruise control will not engage.The revs are capped and the car will not accelerateThe pedal is flat to the floor but the engine caps out around three thousand and goes no further. The car moves, but sluggishly, and overtaking is impossible. The check light is often on.The engine stumblesThe engine runs unevenly, the car shakes at idle, the exhaust note stutters. On the move the pull sags and consumption rises.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.The car has lost powerThe car moves, but sluggishly: flooring the throttle changes nothing, the revs climb reluctantly. Often the lamp is on as well.It stalls at idleAt the lights, or when you lift off, the engine dies. It usually restarts straight away.Smoke from the exhaustVisible smoke from the tailpipe. The colour is the clue: white, blue and black mean entirely different things.
Drivable — worth looking into
There is no danger, but the symptom will not go away by itself: it is the early stage of something that gets pricier.
The heater blows coldThe engine is warm but the heater blows cool or barely warm air. Sometimes one side is warmer than the other; sometimes it only gets warm while driving.The air conditioning will not come onThe button presses, the light comes on, but there is no cold air. Sometimes the compressor clutch does not click when switched on; sometimes the cold disappears only in heat or in traffic.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.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.Check engine came on after refuellingYou filled up, drove a few kilometres, and the light came on. The car drives exactly as before.The cooling fan runs all the timeThe fan roars right after starting and does not stop even on a cold engine or in frost.Cruise control stopped engagingThe button clicks but cruise never takes the speed — or it drops out after a few seconds.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.
If the lamp is already on, start from the dashboard symbols; if you already have a code, look it up in the reference.