A strange "check engine" light issue....?

Question

Car is a '97 Mazda 626, electrical everything.

While giving the car a tune-up ready for its smog inspection, I connected my laptop to the diagnostic port on the car to see the fuel trims etc. I was surprised to see that the ECU was reporting the MIL as 'on', when it isn't. My first thought, of course, was that the bulb was blown, and that I've just not been noticing that it doesn't light up at engine on. I turned the engine off then on, and the "Check Engine" light comes on for a few seconds, then off, just as it normally should. The ECU, however, was still reporting that it should be on, with an EGR code. 

I cleaned the EGR ports and reset the code, and the ECU showed it as being off. I then disconnected a sensor to force a code, and the ECU dutifully reported the code, and that the MIL was "on" - but the light doesn't come on.

The car passed smog just fine once I reset the code, so it's not a huge issue for the next two years... but can anyone think of a reason to explain why a "check engine" should illuminate briefly at start-up, as it should, but then not be on when it should be?

Richard

Stpaulguy · Accepted Answer

Yup, bad software in your laptop. Those programs are ok most of the time, but I've heard of this problem with laptop software before. I bet if you hook up a real pro scan tool you won't see the same results.

Anonymous · Answer

there are 2 types of codes set in the ecu on a car,  
the easiest way of explaning this is 
Hard codes,  
and soft codes... 
 hard codes usually indicate a constang, reoccuring, fault situation, where a sensor is reading out of range,  or not sending a signal  at all  
soft codes usually indicate a sensor reading within normal values,  but some times out of range,  
a soft code may not always give you an MIL  or  be stored in memory, so every time you cycle your key, it will be flashed from memory, and no longer appear.  Now if this condition persists , in OBDII  and newer models the computer will eventually store this soft code into memory, and you will get an MIL,  
If this is not the case,  you may have a malfunctioning ECU, and it may be unable to comunicate with your light properly, it may not seem reasonable to replace the ECU because it seems to function properly, BUUUUUUUT  prolly a good idea to replace it, some trouble codes indicate issues that over prolonged periods of time can cause serious mechanical issues with your engine..   imagine if you had no idea that somthing hurts..  (ohh the burns I'd have)  thats your cars "pain light"
id fix it...go to a wrecker and dont forget your vin,  used ECU's are usually just fine as long as the are kept dry...   anywho  best of luck

James · Answer

had the same problem with a isuzu   change the egr valve, cleaning will not fix the problem, then change your octane gas from 87 to 89 more expensive but will cure the problem, with 87 octane the carbon builds up in the bottom of the egr won't burn hot enough to get rid of all the excess gases causing the check light to come on

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A strange "check engine" light issue....?

3 Answers