CEL coming on but no Code

Long time follower on here but brand new member. This community has helped me solve so many issue and for that. Thank you all!!! Now hopefully I can get some good advice on my most recent issue. Yes I have scoured the threads for over an hour couldn’t find one.

Truck: 2003 4x4 V6 180k Miles

Issue: Originall muffler finally rusted through just after Cat. It was replaced. CEL kept comming on for Evap. Replaced gas cap. Still comes on. Replaced downstream O2 sensor. Still came on(seal on o2 sensor wasn’t tight). Fixed that issue. Here I am trying to get it to pass NYS Emissions test. Hasn’t ran complete drive cycle. I go for a drive. After 30 Miles CEL comes on. I take it to Autozone to get read. They plug in and can not find a code being throwen. I start truck. CEL is off now. I continue to drive to complete drive cycle to clear emissions. 35 Miles later. CEL is back on. Take back to Autozone. Same thing. No code. Start truck. CEL is off. Any advice or help would be much appreciated before I take it to a shop and spend $$$$$$. Thanks!!

 

Why not look at something like a Bluedriver? You plug it into your OBD2 like any other scanner but it pairs with your phone. If the code comes up while your driving, pull over and read the code or codes on the spot.

That way you can see what you're running into.

Maybe it's a new guy at autozone clearing the codes on accident?

 

a number of things come to mind. The first is that the reason for the CEL is not a major issue (soft code) as it clears itself. The second is next time the CEL is on dont shut it off and take it and get it scanned. I hope that the counter guy who is doing the testing is familiar with the test equipement. He needs to look at freeze frame data and pending codes.

Freeze frame data is what was the PCM was "seeing" at the exact instant the CEL was set.

pending codes are issues that havent risen to the point of setting a CEL off. A certain number of misfires have to happen before a CEL is set, a certain set of O2 sensor readings have to be meet before a CEL is set etc. etc. If it wasnt set up this way and every issue was flagged immediatly you would get tired of dealing with the constant CEL's.

 

I agree with what Glamisman wrote.

If possible, get a CEL readout from a different store (like O'Reilly Auto, or Pep Boys, etc.