Is it normal for the Check Engine (service interval) reminder to turn off?

[Steve Theodore]

My own 996TT is ready for its 45k service and the CEL light came on with the "Take car to workshop" message at about 44997 miles. I've made my plans, called the dealer, etc. to have the 45k done but today noticed that the message has gone away, and there is no CEL anymore. I always thought the dealer would be required to reset this light (or other scan/diagnostic tool). Is this behavior normal and expected? Thanks in advance.

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[wross996TT]

I am unaware of a service interval CEL for any mileage....so you actually got a CEL and need to get the codes read.

[Steve Theodore]

I disabled the rear wing several hundred miles ago, so obviously that could have something to do with it. This happened so close to 45k miles that I assumed it was the service light, now it's gone and everything is back to normal. Thanks for the clarity concerning the lack of service interval CEL, and it sounds like I'll need to have this scanned to see what is going on.

[Steve Theodore]

Come to think of it (more), I believe others have said that when the rear wing throws a CEL, it's something more along the lines of "wing malfunction" or something like that. My error seemed more generic than that, and I assumed it was related to the mileage and service interval based on the timing.

However, is it normal for CEL's to clear themselves if the problem is no longer detected? I never reset anything, nor disconnected the battery. It just went away again after a few more miles of driving.

[Porsche Tech]

996 TT and earlier not equiped with service interval reminder. Just coincidental that the MIL came on around the same point.

Every time the car is started, the DME tests all of the emissions control devices to make sure each system passes. The DME has "fault healing" where, if the DME sees that a system does not pass, it will set a fault and turn on the MIL and then start a count down timer. After x numbers if ignitions if the fault has not been verified as present again, it will heal the fault and shut off the MIL. The soft set fault code will typically remain with milage and event data (temp, load rpm etc etc) for future review.

Most common situations where a fault can be present and then disapear are fuel and evap emission- fuel cap not clicked all the way will register too much tank leak down. Once reseated it will test again and if passed, shut the light off. Fuel tank purge and vent valves sticking occasionally, same deal. Also, if you had a drastic temp/humidity change, if a catalytic converter were starting to fail, you can get a cat converter inneficiency fault that when tested again may well pass. There are certainly other situations but those are pretty common ones.

[Steve Theodore]

I couldn't have asked for a better response. Thanks so much Porsche Tech, as it does help me to understand the way the car works better with regard to the MIL/CEL process.

I did fill up the car at least twice during this timeline so can suspect that, but obviously will have my dealer scan/check when I take the car in for the 45k service. Thanks again for your valuable insight.

[ard]

There is no service indicator.

An CEL **indcator** light will 'clear' when the original error is no longer presented to the OBDC system (after a number of starts)... HOWEVER, the underlying code/fault will still be sitting in the computer.

This is one reason why when people "pull codes" there can be a whole mess of codes, some current and some old- some related to the new problem and old red herrings... (this with simple code readers that don't also pull the time the code was set)

Get them read. Better yet, read them yourself.

A

Edit: Beat to the answer...typing too slow!

[wross996TT]

BTW, not all codes will reset (with ignition cycles)...there are some that will and others can be reset with battery disconnect...still others that need PIWIS, PST2 or Durametric to clear. Of course if the fault still exists it will come back....

[Steve Theodore]

Thanks a lot to ard and wross996TT for their follow up advice and inputs...it is much appreciated.