By November 22, 20050 Comments Read More →

Check Engine Lights – Why Does This Dash Light Come On?

Reader Question

Hello Austin, I recently had the “service engine soon” or “check engine soon” light come on my ’94 Buick Century. Took it to my mechanic, and he said it was the EGR valve which he changed out for over $500. The light came on again within 24 hours and I took it back. He said he tightened the fittings, and said that the EGR computer may need to be replaced!??? Since then the “service engine light” has gone off and come back on several times. What is going on?


Thanks for your question Leo. I think you should ask for your money back. The onboard computer system in your 1994 Buick Century is fairly rudimentary compared to newer models and does not give the most in depth diagnostic information. The EGR valve is fairly expensive for your Buick, and the price sounds ok. However, if the problem is still not resolved you should ask to have your old valve re-installed (you did ask to keep your old parts and their original boxes didn’t you—?)

Tightening the fittings is a cop out, and is probably his way of saying “I don’t know how to fix it”. The SES (Service Engine Soon) light can be very tricky to turn off sometimes…it is best to diagnose the problem when the light is on. Every time you turn off the car and re-start the engine, the onboard computer does a sequential self check of all the sensors and components on the car. If the car has not been driven far or long enough, the system will not be able to complete its test. 

This explains why the Check Engine Light won’t always come on. So as you drive, the light might come on and stay on as the computer finds a fault during its routine check list. Also the faulty component might only malfunction at a certain speed or temperature which is VERY common.

My guess is that the EGR valve is NOT the problem, but your mechanic received an EGR trouble code from your computer system while running a diagnostic test. There are more components of the EGR (Exhaust gas Re-circulation) than the valve and any of those can cause an EGR trouble code. If you can’t get your money back, have him try his second diagnosis – a new computer (There is no such thing as an “EGR computer” on this
car) at HIS expense to see if the problem goes away.

 If you are a fairly regular customer at this shop, he will want to fix this problem for you and keep you happy. The price of an after-market computer should cost him about $100, and the installation is simple and quick. He will not be out as much money as you invested in his mis-diagnosis.

My gut says that this mechanic is not very computer savvy and this is probably over his head. The computer is usually the last thing to be replaced. Think of your home is usually a software problem that causes a glitch and not a problem with the hard drive or mother board.

This is what would have happened at my shop. This is how your mechanic should approach tracing and repairing an intermittent electrical problem.

  1. Read onboard car computer system trouble codes
  2. Re-set car computer memory and test drive the car at freeway speeds as the repair manual describes to see if the code re-appears
  3. If no code appears, give car back to customer to drive and monitor
  4. If a code does appear, retest all components involved with that code. If an EGR is determined to be faulty, replace with new, and KEEP old part IN ORIGINAL box for 30 days with customer name on it—or give old part to customer to hold.
  5. If light returns, re-test and see if same trouble code returns. If same code appears (scratch head) and try something else, such as inspection of EGR vacuum lines, until check engine light is out.
  6. Once light is out, re-install OLD EGR valve and give credit to customer. Bill customer for ALL time involved and only for parts that were used.
  7. Return all non-used parts to vendor.

We have rows of old parts on our shelves that we use as test parts in cases like this. Feel
free to ask your mechanic to try a good used part first, if he is not 100% sure that the part will fix the problem. The mechanic might feel better about the situation he is in if he knew you understood he is not totally certain about his diagnosis.

Be patient. Sometimes these silly check engine lights can be hard to track down even with the best diagnostic equipment and trained mechanic.

Incidentally the dash warning lights are color coded

A Yellow Warning light, like the Check Engine Light, means nothing terrible has happened, but have the car checked out at your convenience. 

A Red Warning light means pull over now, and turn off the engine.

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