On-board diagnostics drive cycle vs. check engine light

Hey guys, I’m having a bit of confusion about my check engine light.

I just had my catalytic converter replaced. My check engine light came on, we checked what was wrong, and tried two cans of sea foam that didn’t help, so we had to replace it.

After that, another light came on, this time “P0446” or “Evaporative Emission Control System Vent Control Circuit Malfunction” right as I was about to leave my mechanic’s shop. My mechanic reset it, which also cleared all of the OBD drive cycle readings. Just a few miles later, not long enough to fill up all the drive cycle readings, the check engine light came back on.

My question is: assuming the light is on by mistake and there isn’t actually anything wrong, do I have to drive long enough for all of my drive cycle readings to be filled up again, before my check engine light will turn off? Or, after my mechanic reset the check engine light, should it have stayed off even if the drive cycle readings hadn’t been filled up again?

This is kindof a hypothetical question… I don’t really believe my check engine light is on by mistake. But my dad and I disagree on what my mechanic meant when he explained how the drive cycle counters work.

Thanks, I hope this made sense!

The code has nothing to do with the recent work you had done I do not believe. An p0446 is emission related, and will not effect engine operation as far as I know.

I’m not so much concerned about the code, just curious about when the light would go off if it was invalid. But what you said is very good to know, thank you!

It is doubtful the light will go off by itself. I have another year till I have to fix my P0440 before my next emissions test. If the car makes it till next feb I will have it looked at. Not saying it is the right solution, but it is my plan. A couple of years ago a new gas cap was my solution, something else going on now as a new gas cap did not help.

After the problem is corrected the check engine light will go out after two drive cycles are completed.

Since your mechanic has erased the fault and it returned there is very likely a problem with the evaporative emissions system.

Inspect the purge line to see if it was damaged or disconnected during the exhaust repair.

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When you reset the computer or disconnect the battery the engine light will stay OFF… UNLESS there is a hard error like something disconnected under the hood…if that condition exists it will immediately post a code for that item. However there is some variability among some vehicle makes and models in how and when they post or do not post codes…it is all up to the computer programmer of that make and model. Most will post an error along the way regardless of how many monitors have been fulfilled, but in the end the error code will post…the timing of this is up to the programmer.

The quick diagnostic procedure you listed prior to replacing your catalytic convertor really did not sum up the proper way to rule out something like a Cat. Cats very rarely go bad in my experience however OBD2 codes often point to the Cat as being the culprit for some engine running conditions such as a faulty O2 sensor. O2 sensor malfunctions as well as exhaust leaks tend to produce codes that suggest Cat replacement is needed, when in reality, it does NOT need replacement at all.

I have seen many “mechanics” follow OBD codes as if they were the be all, end all, discussion regarding problem resolution on vehicles…this is probably the WORST thing you could do when diagnosing a vehicle or troubleshooting an issue. The codes are not meant to be interpreted as a Silver Bullet type of thing…especially when those codes point to something that is only one part in an engine subsystem. They are more akin to a suggestion…or a possibility and should be handled as such. There are exceptions to this of course, when a code points to something that is by itself for instance…it is usually that item…but when it is one cog in a multi cog system…you cannot operate thata way with the codes.


After a code reset, any new codes are valid. Some tests take time and certain conditions must be met before they will set a code. These are called onboard module or systems tests. Until these tests are complete, there will be a trouble code stored, but will not turn on your check engine light.


Yes that sounds correct… I forgot to mention the “pending” category of codes… Oops

Generally if the CEL is on, then it may take some time (or drive cycles) to go out after a repair is effected; but if it is out then comes back on again the computer has detected a problem and it won’t go out again until the problem is fixed or goes away by itself.

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