Find the cause of a ‘check engine’ light immediately

By Tom Torbjornsen

Dear Tom,
I own a 2000 VW Jetta, which runs very well. The “check engine” light has been on for a few weeks now and I don’t know what it means? Since the car is running great, could it be a malfunction in the dashboard lights? What should I do?
John from Spokane, WA

The self-diagnostics has identified a fault in the system that must be checked. Get it into a shop immediately for on-board computer diagnostics to track down the fault and repair it before the domino theory takes place and other problems ensue. Don’t ignore this light because it could cost you a lot of money down the road.

John brings up a popular question, one that I’ve handled hundreds of times over the years.

Why is the “check engine” light lit? Today’s cars have performance systems that constantly self-monitor and make adjustments as you’re driving down the road. When a “check engine” light stays lit, the system has sensed a glitch. Something is wrong.

How do you track the problem down? The only way to investigate the cause of the problem is to scan the car’s computer system with a diagnostic scan tool. This tool interfaces with your car’s computer and pulls any stored diagnostic codes, which helps the tech diagnosis the drivability problem.

The light is lit but the car seems to run fine. Why? Engine management systems mask problems in what is called “limp mode” (or limp home mode). The car will seem to run relatively well when, in fact, it is not.

Common causes of lit “check engine” lights:
• Faulty sensor, for example O2, MAP, Coolant Temperature, Mass Airflow.
• Broken wire or connection.
• Mechanical malady, for example intake manifold vacuum leak, vacuum line leak, burnt valve, blown head gasket, bad timing belt or chain, excessive carbon buildup, worn piston rings.
• Fuel delivery system malfunction.

These are just the more common causes. There are many more possibilities. The key to solving the mystery of a lit “check engine” light is effective diagnostics. Period.

‘Til next time … keep rollin’.

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