Drive Cycle and CEL

[nickb21]

This is a half generic, half Subaru related question..

 

I did some internet searching a few weeks back to better understand how the CEL gets turned on and off. I assumed that it was the same process for any code that could be set in any OBD-II make/model.

 

I didn't dig very deep, but I found that it takes a fault to be present for 3 drive cycles before the CEL is turned on. I also saw mention that it took 40 drive cycles to turn off the CEL. The definition of a drive cycle was from cold-start to full operating temperature.

 

My reason for writing this, is that I'm questioning the "generic" information that I found.

 

My CEL came on yesterday (haven't check what it's for yet). And went out just this afternoon. I've only driven the car 3 or 4 times in between, and didn't stop for gas at all.

 

How is it possible for the light to go on and off in such a short time? Are there different drive cycle/thresholds for different error codes?

[porcupine73]

It is different for each code. Some codes illuminate the CEL/MIL immediately at fault recognition. Some require two or three or more consecutive trips to make throw a code. Until then it is a 'pending code' which will not illuminate the CEL/MIL.

 

Right and then if it doesn't see the problem for a certain number of trips or however that code is setup then the CEL/MIL may go out by itself. Some codes get stored in history, so even if the CEL/MIL isn't on now, it may be possible to get it with a code reader by going to 'code history'.

[nipper]

There is also no set standard for what a drive sysle is defined as, so each mfg will have a different definition of what it is.

 

nipper

[nickb21]

Thanks for the information, that definitely make sense.

 

Turns out my short-lived code was P1507 (IAC). I've seen this before, it's time for a cleaning.

[zstalker]

40 drive cycles is for the ECU to ERASE the code, not turn off the MIL. Depending on the code, it can turn on immediately, or after 2 or 3 occurranced, and the ECU will turn the light off after reaching the same circumstance that set the code 3 or so times. The code will still be retained in memory as a "history" code for future refefence until erased (either ECU cleared or the aforementioned 40 cycles).

~Erik~

[nipper]

I just ran into this with inspection. They put 30 miles on the car and they still couldnt get it inspected, so they said to use the car and bring it back tuesday.

 

The ECU wasnt ready for inspection.

 

grrr

 

 

nipper

[porcupine73]

Yes I had that problem before too. It takes a number of trips to get all the readiness I/M monitors to set after ECU reset or battery disconnection (or every time you turn the car off if you had a '96).

 

The evap seems to take the longest, probably because of the fair number of conditions that have to be met for it to run that test. Not sure if the scanguage will show the I/M readiness status, but most code readers will.

[nickb21]

40 drive cycles is for the ECU to ERASE the code, not turn off the MIL. Depending on the code, it can turn on immediately, or after 2 or 3 occurranced, and the ECU will turn the light off after reaching the same circumstance that set the code 3 or so times. The code will still be retained in memory as a "history" code for future refefence until erased (either ECU cleared or the aforementioned 40 cycles).

~Erik~

 

I gotcha, very cool information. I didn't realize that the ECU would respond that quickly (relatively speaking) to an error condition (excluding critical stuff like misfire, etc).

 

Thanks!

[Cougar]

I just ran into this with inspection. They put 30 miles on the car and they still couldnt get it inspected, so they said to use the car and bring it back tuesday.

 

The ECU wasnt ready for inspection.

 

grrr

 

nipper

 

I know the frustration Nipper. I went through this same thing when I had to renew my registration some time back. I wasn't driving the car enough and the battery was dead so the ECU was reset. It took me 3 trips to the shop to get enough monitors set so I could pass the test.

 

During this incident the IM tech told me each model car has a procedure to follow in order to get all the monitors to set fairly quickly. He showed me the procedure for my '01 OB in a manual he had and I copied it down. I left the shop trying to follow the procedure as best I could. There were about 11 steps I had to do that included a number of speed changes along with stops and other manuvers. By the time I got back from my 20 mile trip I had set just enough moitors to pass the test. I also learned that when you clear the ECU error codes with a scan tool that it includes the monitors also. I didn't know that then and though it was just the error codes that were being erased. After learning that I am more cautious now when thinking about resetting the ECU.