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On a recent business trip where I was, unfortunately, driving my wife's 2006 Toyota Camry, the Check Engine Light (CEL) came on after about 90 minutes of driving. I had no tools with me, but stopped within a few minutes at a garage and had them pull the codes -- and there were none! I then restarted the car and the light was off and remained so for the rest of the trip that day (about another 60 minutes of driving) and the entire return trip a few days later. There were no other symptoms other than the light.

I'd just write it off to gremlins, but my wife reports that the same thing had happened to her a few months earlier in the same car on a similar trip to the same city.

My questions:

  1. how can the CEL come on but there are no ODBII codes?
  2. is there any hope of troubleshooting this or should I just wait and see if it happens again?
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  • 1. short term errors are not stored in memory, ignore it until it sets a code. 2. See answer 1.
    – Moab
    Apr 4, 2018 at 22:33

2 Answers 2

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The CEL can be triggered for a number of reasons, depending on what the manufacturer wanted. Only some of those reasons are emissions-related. Everything in the OBDII code list is, in some way, related to emissions.

I'd guess that something happened which triggered the light, but the condition didn't persist. So, no codes when it was scanned.

However, all error codes are stored, so you can look at historical data, even with a $10 bluetooth OBDII dongle, and see what the code was before. These things are kept by the ECM / ECU basically forever, or until it is reset, cleared or runs out of space.

Either get a cheap scanner and pull the historical and freeze-frame data, or wait until it happens again. Some issues start out as intermittent problems, but become more persistent as time goes on.

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  • I had thought that the garage used the same ODBII code scan that I have at home and I watched as they came up with nothing at all. But, based on your advice, I read the codes with my scanner and lo and behold, it had a P0420 code stored. I'll check the O2 sensors this weekend. Thanks for the tip -- I should have thought of that!
    – Edward
    Apr 4, 2018 at 23:55
  • @Edward - Fixed! :o)
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Apr 5, 2018 at 14:18
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The ECM runs checks on diagnostic data which is what the light is based off of. If there’s an prevalent issue within a given drive cycle, a light will sound off. For instance, if the engine’s dirty, and a cylinder misfires, the computer will take note, and a code will be lit. If it was a single occurrence, and later stopped misfiring, then the computer will analyze the data from the drive cycle, and if needed, turn off the light. It’s an issue when the light is actively on, and no codes are pulling. But this just sounds like a cylinder misfire and resolved itself just by driving it. If that’s a frequent occurrence, I’d suggest getting the oil changed sooner, treating your fuel system, and ensuring the spark (or glow depending) plugs are in good order, as these are common issues to causing cylinder misfires. Good luck!

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