What to Do if You See a Check Engine Light After a Knocking Noise

by G.D. Palmer

When the "check engine" light turns on in your car, you might be conflicted about what to do. After all, engine problems are nothing to ignore. However, this light doesn't always mean there's a problem with the engine itself. That can make it hard to decide whether your car needs to go to the shop.

Engine Knock

Your engine might produce a knocking noise for several reasons. Knocking or tapping from a cold start may indicate that a car hasn't been properly maintained. It also may indicate internal mechanical issues such as a broken ring, a broken piston, a filing rod bearing or wrist pin, or a blown head gasket. These knocks often sound metallic and increase as the engine speed increases. They may increase as the engine load rises as well.

Engines can knock when the engine timing is wrong, because the cooling system is not working correctly or because the fuel mixture is wrong. The sound may be a true knock--a hollow sound--or a rattling noise. Engines with cooling-system issues or uneven combustion may develop a pinging sound. No matter what it sounds like, engine knock indicates a real problem. You should never ignore a loud engine.

Check Engine Light

The "check engine" light may be the most misunderstood light on the dashboard. It's capable of sensing problems with the engine, but it can't tell you where they come from. Your "check engine" light could turn on for something as small as a loose gas cap, to something as big as an engine that's misfiring. Although that light doesn't usually mean you'll need a tow, it does serve as a heads-up, letting you know that something is definitely wrong.


Owners of knocking cars should never ignore their "check engine" lights. Take a little time and check the basics--reseat the gas cap, check coolant levels and allow the car to "rest" before starting it again. Reduce speed and load, as well. If your car wasn't built to tow, you may see the "check engine" light.

If these steps don't correct the problem, there could be something seriously wrong. Take your car to a mechanic you trust as soon as you can. The repairs may seem expensive now, but you'll save money in the long run. A car with a poorly running engine is more polluting and less fuel efficient. Engine knock could also indicate a problem that won't get any better. Your mechanic can help you find the problem and fix it.

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