Hi guys,

I have a 1991 J-Spec 850i. Recently, I've been getting the CATALYST OVERHEAT error on the OBC/MID, complete with tone/beep, immediately after starting the car. Happens regardless of cold/hot, and this is not the quick 2 second one that J-Spec owners see that disappears right away.

The Euro/JSpec cars have after-cat temperature sensors, part numbers:
11 78 1 722 763, or 11 78 1 738 347. Don't want to throw parts at this, otherwise I would have replaced the sensors by now in hopes of it fixing it, but I would rather try and troubleshoot/diagnose this to make sure it's the sensors and not something else.

Anyone ever deal with this?

Info: 1991 J-Spec 850i, Automatic
-Just passed emissions testing recently with great results
-Air filters and sparkplugs are new, will be doing caps+rotors soon, but since the error comes on instantly, I do not believe this to be an ignition/rich issue
-Behaves the exact same way with both sensors plugged in, both unplugged, 1 unplugged, other unplugged.

thanks!
-Andrew

Was it giving you this error message before you changed plugs? What plugs did you put in?

Hey Tom,

It was giving the message before I put the plugs in.
I used Bosch F8LCR as recommended by all.

It did not start appearing in synch with any maintenance I did.
Air filters had been done for a while before it started poppin' up.

Realized I had no CHECK ENGINE bulb, so I thought it might be in conjunction with an engine error I hadn't been seeing yet, but I put a bulb in and checked for codes, just 444 and the CHECK ENGINE doesn't come on, just the CHECK.

-Andrew

And the Catalyst Overheat error message on the OBC stays on?
I guess my next step would be to put new temp sensors in and see what that does. Dont think I have any here tho. Pretty sure they are on my #1 car and not connected. Not removing them tho so dont even go there. 8-)
See you tomorrow at WrenchFest in the Rain?

The Catalyst Overheat message stays on after the bong/beep/tone...if I clear it by hitting CHECK, it returns after a few seconds. Will definitely see you tomorrow. Can I take a look under yours to see if they are in fact disconnected? Is your #1 Euro/J?

Nope my #1 (FrankenBimmer) is a hybrid - a Canadian Spec car with a J-spec engine, transmission, cats and engine controllers and a hodgepodge of other stuff.

Very happy to report this has been dealt with.
At Wuffer's mini-wrenchfest , I was very fortunate to run into a fellow named Wayne who had had the same problem. By shorting the wires(proper term?) before the sensor, the therm sensor is effectively disabled, not reporting any error to the MID/OBC. Wayne and I were in agreement that these sensors don't seem to do a darn thing other than warn you about temperature...ie., the car still runs great when one of these sensors is toast.
Thanks to chromeowner for pointing me in the right direction initially, Wuffer for bringing the 8-guys together, and Wayne for sharing the simple fix.

Staehling, can you talk about the 'shorting the wires' fix and how you go about doing it. Thanks

Staehling, can you talk about the 'shorting the wires' fix and how you go about doing it. Thanks


Interesting. I was told by a BMW mechanic that this method does not fix the issue. This would be a much easier solution that I thought given how EGT thermocouples generally work.

Hey guys, it definitely eliminated my persistent error.

It's a two-wire system, so I snipped the wire between the connector and the sensor itself, opened the sheath up, and connected the two wires together and tucked them away...not great at describing this so I'll try to get a picture next time I'm under the car.

Hope this helps.

Hey guys, it definitely eliminated my persistent error.

It's a two-wire system, so I snipped the wire between the connector and the sensor itself, opened the sheath up, and connected the two wires together and tucked them away...not great at describing this so I'll try to get a picture next time I'm under the car.

Hope this helps.

good to know. I understand completely. On an E34 there is only one and it is very easy to see right after the cat. In fact there is a connector on the box frame where you can easily short the wires.

I learned something new. So much for BMW mechanics???

good to know. I understand completely. On an E34 there is only one and it is very easy to see right after the cat. In fact there is a connector on the box frame where you can easily short the wires.

I learned something new. So much for BMW mechanics???


:thumbup:

Thanks a lot for the assistance fellas, will get under there and give this a looky. Cheers

If you need a pic let me know

Yeah, if you could post something, that'd be great. Thanks again

I second that, i would like to see what you did before i begin snipping. Im getting the same cat overheat. I think i know what to snip/short out, but better to be certain first.

I'm an ass for not posting this sooner, sorry to any who were waiting to see the pic.

http://www.staehling.org/e31/catoverheatdefeat.jpg