On this page
The check engine light comes on with no warning and no explanation, and the car usually feels exactly the same as it did a minute ago. That is what makes it hard to judge. It can mean a fuel cap that was not tightened, and it can mean a fault that will destroy an expensive part within a few miles. This guide is the order I would go through it in, starting with the one thing you can tell from the driver's seat.
Flashing or steady: the first thing to look at
Look at how the light behaves before you do anything else, because this one detail decides how urgent the rest is.
- Flashing. The engine computer has detected a misfire severe enough to send unburned fuel into the exhaust. That fuel overheats the catalytic converter, and the damage happens while you drive. Slow down, avoid hard acceleration and get the car stopped somewhere safe. Treat it as a reason to stop, not as something to look at this weekend.
- Steady. The computer has stored a fault, but it does not consider it an immediate threat to the catalytic converter. You normally have time to find out what it is, as long as the car drives normally.
The same goes for how the car feels. A steady light with a loss of power, rough running, a strong smell or smoke is not a calm situation, whatever the lamp is doing.
Other lights are a different matter altogether. A red oil pressure or coolant temperature warning means stop now and turn the engine off, because running on can wreck the engine in very little time. The check engine light is amber for a reason: it asks for attention, and the red ones demand it.
What to check before you worry
Some checks take a minute and need no tools.
- The fuel cap. Take it off and put it back on until it clicks. A loose or worn cap lets vapour escape, and the evapourative system reports that as a leak. The light will not go off straight away: the computer has to run its check again, which can take a few days of normal driving.
- The temperature gauge and the oil level. If either one looks wrong, deal with that first.
- Anything that changed recently. A fill-up at an unusual station, a battery replaced, a workshop visit or a disconnected sensor are all useful clues. Write them down, because the mechanic will ask.
Do not add a bottle of additive and hope for the best. If the fault is real you will have spent money on nothing, and you will still not know what the fault was.
Read the code instead of guessing
The light itself says nothing. The information is in the code the computer stored, and getting it is easier than most people think. Every petrol car sold in the United States since 1996 and in Europe since 2001, and every diesel in Europe since 2004, has a standard diagnostic socket, usually under the dashboard near the steering column. An inexpensive reader or a Bluetooth adapter with a phone app can read it, and many parts shops and workshops will do it quickly.
A code looks like P0420: a letter for the system and four characters for the fault. You can look up any generic code in our list of OBD-II fault codes, which says what it means, what usually causes it, what to check first and whether it is safe to keep driving. These are some of the ones drivers meet most:
- P0300 to P0316: misfires. Often spark plugs or coils, and the one case where a flashing light means stop.
- P0420 and P0430: catalytic converter efficiency. The converter is often not the first part to replace.
- P0171 and P0174: a lean mixture, frequently an air leak or a dirty air flow sensor.
- P0442 and P0455: an evapourative leak, the code a loose fuel cap can cause.
- P0128: the coolant stays below temperature, usually a thermostat.
Remember that a code names a symptom the computer measured, not the part that has failed. An oxygen sensor code does not prove the sensor is bad. Replacing the part named in the description without checking anything is the most common way to spend money and keep the light.
Why clearing the code is not a repair
A cheap reader can also erase the code, and the light goes off. That feels like progress and it is not one. The fault is still there, and the light will come back when the computer detects it again. In the meantime you have thrown away the stored data about the conditions when the fault happened, which is exactly what a mechanic would have used.
Clearing also resets the car's readiness monitors, the self-tests an emissions test looks at. A car with its monitors freshly reset can fail or be refused at the test until it has been driven enough to complete them. So write the code down before you clear anything, and clear it only after a repair, to confirm that the repair worked.
When to go to a workshop
Go straight away, or have the car recovered, if the light is flashing, if the car has lost power or is running roughly, if you can smell fuel or burning, or if a red warning has come on as well. Book a visit soon if the light is steady, the car drives normally and the code is one you do not understand or cannot fix yourself.
When you go, you will get a better answer if you bring facts. Say which code came up, when the light first appeared, whether it stays on or comes and goes, and what the car was doing at the time. A good workshop will test before it replaces, and it is fair to ask what the test showed.
These pages describe the generic meaning every manufacturer shares. Each brand adds its own detail, so your handbook, the manufacturer's service information and a qualified mechanic have the last word.
Keep the code with the car's history
A fault that comes back after six months is much easier to solve when somebody knows what was found last time. Most people have forgotten by then, and the receipt is gone.
That is what a service log is for. In OdoKeep you can record the scan with its codes, then the repair and what it cost, on the car it belongs to, at the odometer reading it happened. The app does not read codes from the car, so you still need a reader or a workshop for that part. What it does is make sure the answer is still there the next time the light comes on, and when you sell the car. If you are starting from scratch, we also wrote about what a useful service history holds.
