The amazing DISAPPEARING check engine light

About 300 miles into my recent road trip, I was surprised by the appearance of my CEL. Oil and water were ok, the car was running very well, and there was nothing evident on visual inspection.

After another 300 +/- miles, I was relieved that the light vanished. 1500 miles later, it came back. It disappeared again about 300 miles from home and has never re-appeared in the two weeks I’ve been home.

My mechanic checked the codes and found something suggesting - unfortunately - the catalytic converter, as well as a sensor that I think he said was an air flow sensor, but I may be wrong. He compared that to the oxygen sensor, but with some additional technology. He also cautioned me that there are false readings at times, but that a cat code usually comes back.

He cleared the codes and advised me to come back if the light came on again.

What would explain this intermittent light?

My only clue is that the light came on as I approached the LA basin (a bad air day), vanished after I’d left San Diego, then came back near Tuscon, after I’d been in remote areas for a week. Could localized air pollution have any role in this?

The car’s got a lot of miles on it, surely over 200k, but it’s all a guess since the odometer was non-functional when I bought it.

Until you provide us with the actual DTC’s, it’s anybody’s guess.


I’m not trying to figure out what’s wrong, but curious about why the light appears, vanishes, appears, then vanishes again.

Your mechanic has that information…

More than likely it’s a problem with an O2 sensor.


Tester’s first reply is spot on.

Unless we know what the DTC codes were, we can’t give much of any input as to why it appears and disappears.