I'll start from the beginning. I have a stock 1996 Z24 Cavalier. a few weeks ago the engine light came on so I took it to a Midas near my job. They swapped out both O2 sensors (it needed that anyway, 125K miles and never been done yet). Afterwards all was good until about 2 days later. The engine light came back on and the code was P0141, Sensor Heater Circuit Malfunction. I figured that maybe they didn't install it correctly. I took it back a week later and they swapped out the rear sensor. Before I left they told me that it was either the O2 sensor that was a dud or my computer is bad. They said that if the engine light comes on again to give it a few days before bringing it back in. Well sure enough two days later the engine light is back on and staying on. I stopped by Advance Auto to get a free scan and, once again, P0141 code again.

So my question is, could it really be the computer or is Midas yanking me and/or doing something wrong? I've tried the battery unplug reset to no avail. I guess it won't be horrendous to get a new computer, but I don't really have the money to throw away on that if it isn't the problem.

I would take a look at the wiring harnesses for both O2 sensors, The smallest break or short in the wiring could cause alot of problems, Also give you check engine light on.

Good Luck.

Does the rear sensor even have a heater? I thought only the first one did.

You could try a resistance check on the O2 sensor. On Bosch sensors, the two white wires go to the heater. Make sure there is continuity between them. If they're open, then your ECU was right after all. Some other Z24 guys around here may be able to check their sensors and tell you what Ohm readings to expect. The ECU is the LAST thing I'd look at since it seems to be managing the rest of your car alright. Watch out for shops that try to throw random parts at your car, they've got no problem throwing your money down the drain.

2002 Cavalier 2200 5spd

The computer is probably not bad. A heated O2 sensor has a 12volt line that is ran to it for the heater. The heater will ground threw the housing or another wire. You will need a meter and a wiring diagram to figure this out. I would check your fuses first, when I say check your fuses I mean with a test light not pulling all of them out. But is sounds like you may have a open circuit some were in the system.

good luck

Eh yeah I work for a living as a mechanic. Like everyone else said, it is most likely a wiring problem. A bad ECU and it is only causing one problem, and one code, nope thats not how they fail. How they fail is the other day for example, car had no spark, no injector pulse, and the OBD II port wouldnt talk to my scanner. Turns out that was a dead ECU.




- 2004 Cavalier - 124k, owned since new

I get it too.... I dont know why... also sometimes I get the po420 too. I just keep clearing them each time the light comes on with my scanner. Till I find the problm at least.

Thanks everybody. I'm thinking I just need to take it to a different shop, but somewhere I know is more reliable. I never had a good feeling from these guys at this Midas anyway. Its time to ask my co-workers for advice about where to take it. I'm (somewhat) new to the area that I live so I don't know where the good and bad shops are. I live on the far northside of Chicago (near Foster and Damen) if anyone on here can make any suggestions. Otherwise if I had a garage I would just check it myself. Thanks

real quick, can someone tell me where the rear O2 sensor plugs in to? i have a chiltons book but it doesn't give me a whole lot of detail.

Can someone post a photo of where the O2 sensor harnesses are supposed to be in the engine compartment? I looked under the hood and it looks like the mechanic hastily just clamped the harnesses to the first place he could find, and did not place everything like it was before. I checked the 4-wire O2 harness and from what i could tell everything looked good, but that was just eyeballing it. I've been trying for the third and last time to get it back into Midas where all the work has been done already, but they seem to be giving me every excuse to not get me in, which really unnerves me. This last Friday I unplugged the O2 sensor, checked the harness, and plugged it back in. I disconnected the battery to reset. My check engine light was off until this morning. Again its still running just fine, I'm just getting sick of seeing that taunting little check engine light on!!!

Do you know what brand sensors they put in?



FU Tuning

check ur fuse box it should say something like 02h check that fuse i had that light for awhile and i checked it. it was blown out so i replace it and the light went away

John Higgins wrote:Do you know what brand sensors they put in?

I don't remember the name but I do remember that it wasn't Bosch or AC Delco.

Dan Houser wrote:check ur fuse box it should say something like 02h check that fuse i had that light for awhile and i checked it. it was blown out so i replace it and the light went away

OMG!!! That was it!! I didn't have a 10 amp fuse readily available so i just yanked my fog lamp fuse and replaced the blown O2 sensor fuse (I don't use my fog lamps right now anyway since one bulb is blown. waiting for warmer weather before i change it). What really pisses me off is that Midas didn't even catch this. Thank god I didn't have to pay anything the second time I took my car in. I mean come on, I'm getting a code specifically saying that the O2 sensor HEATER is not working correctly. Thanks for all of your help!! It is much appreciated.

floyd wrote:

Dan Houser wrote:check ur fuse box it should say something like 02h check that fuse i had that light for awhile and i checked it. it was blown out so i replace it and the light went away

OMG!!! That was it!! I didn't have a 10 amp fuse readily available so i just yanked my fog lamp fuse and replaced the blown O2 sensor fuse (I don't use my fog lamps right now anyway since one bulb is blown. waiting for warmer weather before i change it). What really pisses me off is that Midas didn't even catch this. Thank god I didn't have to pay anything the second time I took my car in. I mean come on, I'm getting a code specifically saying that the O2 sensor HEATER is not working correctly. Thanks for all of your help!! It is much appreciated.

hey did'nt I tell you that a week ago?

I had the same problem, took it to firestone and they replaced the two oxygen sensors and the light came back on same day, then i found this posting a check the fuse and it was blown so I replaced it and light is gone.... thanks

P.s i hate that i spent almost $300 bucks for a easy 50cent fix

Well I have an annoying p0141 code that won't go away. I have a new ACDELCO sensor installed, heater fuse is good, ground and power wires at the harness (ecm side) checks out. I suspect the computer because the heater wires from the ecm harness to the harness at the o2 connector has conductivity but signs of any power when probe tested. For those who have a solid idea what's really going on and/or solution, please reply asap. Thanks JBody fam!