New O2 Sensor -- Code 12 on DM -- Defective?
 

Car: 1995 E320 / M104.992

Had an O2 sensor installed a few weeks ago. Was an aftermarket,no specific brand purchased on eBay. Got a CEL today with a code 12 on the DM:

12 Heated oxygen sensor heater circuit open or circuit short

Is it a faulty sensor or something else?

Also -- does anyone know which generic Bosch 4-wire sensors are suitable replacement for an 95E320/M104.992 engine? I am doing emissions/safety this weekend and thus have to buy retail (and don't want to get dealer-gouged).

I tried a generic one time on my 91 300CE and the installation with the 4-wire splicing kit was a pain. I didn't do the best job splicing and the connection failed after being contaminated with ATF.

The cost of a generic vs. OEM Bosch seems to have narrowed so the OEM is well worth the small additional cost. It is the correct length and has the rubber stopper.

I am seeing Bosch direct fit/non-generic for about $118, only a $30 to $40 premium over the generics, so I agree it is worth the additional cost.

Word to the wise: There is no way a $50, no-brand, eBay sensor is a quality part. (Lesson learned.) :o

I see the Bosch OEM at $97 and the Bosch Universal 4 wire and (lame) splicing kit at $82.

From what I have read, oxygen sensors are sensitive to small variations in voltage, so if you use the universal, you have to make a very good splice.

BTDT with ebay parts. Can't count the hours I have wasted with ebay parts.

I definitely will not be splicing -- and am considering have it installed by a local shop for $50, since I do not want the "seized bolt" syndrome I always encounter with every DIY job. (However, because this sensor is only 3 weeks old, removing it should not be too troublesome, despite it not having anti-seize paste pre-installed.)

I checked the wires for corrosion or defects and found none, although the signal wire connector seemed loose and disconnected without being pulled. The connectors were also not replaced securely into the bracket by the installer but I don't know if that amounts to much.

Hopefully it is just a faulty sensor and nothing else.

Update
 

After refastening the the connectors the code 12 is gone, but I am still not trusting the sensor because the car feels wrong at idle and on acceleration -- as if the fan clutch is engaged, even though it is not.

Could the O2 sensor still be bad, even if not throwing a code?

Disregard previous message.

Last weekend I forgot to tighten the air intake pipe clamp behind the MAF and it was losing vacuum -- thus the stumbling idle, doggish acceleration, and loudness for the past days. Correcting that has restored idle smoothness, and trooper-titillating acceleration. :D

Still no more O2 sensor code(s) so hopefully I am OK with that.

OK -- Now I am getting a CEL and code 3 on the DM, which suggests something other than the O2 sensor. (I have never seen a code 3 on this car before.)

Last thing I did was spray electronics parts cleaner on the MAF sensor yesterday, and tighten down the pipe clamp at the crossover, and clear the codes at the DM. Car ran great thereafter. We had some very heavy rain today, just before this occurred -- and fuel economy dropped significantly.

The presumably-defective eBay O2 sensor is still on the car and I have a Bosch 13314 standing by for replacement. I checked the vacuum lines under the front cover and they are secure.

Since I threw an O2-related code 12 over the past few days I am assuming it is another fault with the sensor. (I am also a bit worried that I used electronics part cleaner rather than MAF cleaner on the sensor, even though that is probably not an issue.)

Any thoughts? Is it likely still just the O2 sensor?

Resolved
 

Problem solved.

Installed new OEM Bosch Premium Sensor (13314) and all is well -- engine is glass-smooth and dead silent at idle, and not straining on acceleration, even when hot with the fan clutch engaged.

WARNING: If you see an auction for a low-priced, no-name replacement O2 sensor on ebay, Don't Buy It. It would look something like this:
http://cgi.ebay.com/ebaymotor/ws/eBayISAPI.dll?ViewItem&item=390257576137
(I will neither confirm or deny that is where I got my failed sensor.)

The eBay sensor is promoted as Bosch, the photo is of a Bosch sensor, and Bosch is mentioned in the listing, but it is definitely not a Bosch part. Compared side by side, the eBay part was at least 1/3 smaller in diameter than OEM, and fit in the Bosch box with plenty of vertical and horizontal room to spare, whereas the Bosch sensor was jammed tight into its box. (The Bosch box could easily hold 2 of those eBay sensors.) The eBay sensor had a black cable, rather than the Bosch blue shown in the listing, and did not have the rubber grommet.

The eBay sensor may have been functional but clearly did not have the working capacity of the OEM. (Translation: It is junk.)

Once again: Lesson learned.

Good "stuff" Eric . . . and well reported! A lesson to us all . . . :(

Jim, thanks.

I just don't want anyone to have to repeat my experience. :)

Never heard of any problems with the Generic Bosch "Mustang" sensor that can be had at any AutoZone, CSK, NAPA, etc for around $40. I soldered the mustang sensor in over a year ago and no issues. Glad you figured it out. Generic parts on eBay scare me!

The part I had was definitely not Bosch, or any other known brand. In fact, it had no brand or markings at all. :eek: