Check Engine Light Disappears when Scan Tool Connected?

[uslspct]

I have a check engine light on my other car. I've gone twice to Autozone for them to check with their scan tool what code it is. Both times as soon as they plug it in and we turn the car on the light goes off and they tell me there's nothing there. WTF? I did a search and couldn't find any answers.

I can disconnect the battery and it goes away as well, but the light will come back on eventually.

I used to get codes p420/430 last summer then I changed the O2 sensors and it went away. Don't know if this problem really is the cat or even related to that at all.

[sonikaccord]

Ol school member!!

Maybe they have a faulty scan tool? Maybe if you sweet talk the techs and the dealership, they'll pull it for you.

[Raf99]

Without connecting the scan tool and turning on the car does the light go away?

I had this happen with a O2 sensor and is known to occur with a O2 sensor issue. Just a though.

[cp[mike]]

Previously being under the impression that OBD-2 required the use of the scan tool, I was surprised to discover my OBD-2 CRV still retained the ability to read codes by counting the flashing lights, just like OBD-1 does. I don't know about other cars but I would assume they would probably be the same way too. Maybe you can try doing a flashing-code readout directly instead of hooking up a scanner, see if you can get it out that way.

[uslspct]

No, there's no way to jump terminals and get the flashing codes.

I disconnected the battery yesterday and I got 25 miles before the CEL came back on. That's the normal part, but not being able to read it with a scan tool doesn't make sense. I guess I'll try another place and see what they say.

The car drives without any noticeable problem just like before when they were able to read my codes.

[Raf99]

You didn't answer my question

[uslspct]

Without connecting the scan tool and turning on the car does the light go away?

I had this happen with a O2 sensor and is known to occur with a O2 sensor issue. Just a though.

Yes, the light goes away, that's the point of this thread. And the 2 downstream sensors were replaced around September.

[GhostAccord]

What make model and year of vehicle are we talking here? What brand O2 sensor did you buy?

[uslspct]

It's a 10 year old Toyota. I bought oem Denso's.

[GhostAccord]

Well you mentioned that "the 2 downstream sensors were replaced" So I was wondering if this was a V6 or I4. Also whether it is OBD1 or OBDII makes a difference on how the CEL lights and diagnostic computers act!

['93CB7Ex]

if you just say "a 10 year old toyota" how do you expect us to really help you? if you get p0420/430 your cats are fucked. there wasnt really any need to replace o2 sensors. they were working and doing their job.