Check Engine light is ON

Tiny
LIVIA MITCHELL
  • MEMBER
  • 2002 MERCEDES BENZ C320
  • V6
  • 2WD
  • AUTOMATIC
  • 92,000 MILES
After putting regular gas the check engine light came on and it has been there since. I have always put the highest octane. This happened about 4-5 months ago. The car has been driving OK, no knocking, or anything. I have read that it could be faulty oxygen sensors. What would be the estimated repair cost.
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Wednesday, April 6th, 2016 AT 2:29 PM

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Tiny
CARADIODOC
  • EXPERT
Slow down. There are well over 2000 potential fault codes and only about half of them relate to things that could adversely affect emissions. Those are the codes that will turn on the Check Engine light.

The first thing you must do is have the fault code(s) read and recorded. That will indicate the circuit or system that needs further diagnosis or the unacceptable operating condition. That's how your mechanic knows where to start looking. There's over a dozen fault codes related to the oxygen sensors but no code ever says to replace parts or that one is bad. There's no way to know at this point what is needed, and we don't get involved with costs here because there's way too many variables. Your mechanic will prepare an estimate once he has had time to diagnose the cause of the problem.

You must also be aware that there is always a long list of conditions that must be met to set a fault code, and one of those is certain other codes can't already be set. This is especially true when two different signals or conditions are compared by the Engine Computer to determine when something is wrong. For example, the computer knows that when the engine has been off for at least six hours, the coolant temperature sensor and the intake air temperature sensor had better be reading the same temperature. When one fault code is set, whatever it pertains to can't be relied on as a reference for other things so some of those tests will be suspended. One problem that can cause that can be serious is a relatively minor problem can go undetected due to the suspended test, and some of those can turn expensive if they're ignored. You could be driving around right now with one of those potentially serious problems and not even know it. Even if a second problem is detected, you won't know it because the Check Engine light is already on.

The second problem is once this first problem is fixed, the suspended self-tests will resume, and that is when some other ongoing problem will be detected, set a new diagnostic fault code, and turn on the Check Engine light again. That might not occur until you're a mile or two from the repair shop. Naturally you assume, incorrectly, that your mechanic is incompetent or didn't fix your car properly, ... Or, ... During the repair he has to come out and tell you more parts or services are needed than what he was originally aware of. We hate having to do that and that makes customers frustrated too.
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Wednesday, April 6th, 2016 AT 3:10 PM
Tiny
LIVIA MITCHELL
  • MEMBER
This is so complicated! Thank you for your prompt reply.
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Thursday, April 7th, 2016 AT 7:56 AM
Tiny
CARADIODOC
  • EXPERT
Unnecessarily so. Welcome to our world.
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Thursday, April 7th, 2016 AT 9:34 PM

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