Sis's car needs a new pre-cat O2 sensor, according to her mechanic. This is something I'd like to tackle myself, but she told me she found out somewhere that if I replace the pre-cat sensor, I also need to replace the entire wiring harness?

Her car is a '97 T5. I would really appreciate any tips and hints for this job.

Thanks

 

go to fcpgroton.com where you can find it cheaper with the wiring harness. Dont go the cheap route. too many ppl have had problems with the cheap O2 sensors.

 

What's the technical explanation for having to replace an O2 sensor instead of cleaning it?

I've read that for most cars they clean the O2 sensors really well and they work just fine. It's probably just dirt or oil accumulated on the sensor that is not allowing it to work properly.

It's just some zirconia ceramic with a couple of wires. If the ceramic is not damaged then cleaning it should restore it.

Am I being too optimistic?

 

I don't know. I'd like an answer to that question as well...

as for the parts...so I need to order the sensor, the wiring harness and the strap (as FCP calls it)?

 

Stick as close to OEM as you can or you will have problems.

 

I plan on an OEM sensor. I just need to know if I need to replace the wiring harness with it. FCP has me hopelessly confused, as there is an adapter cable and a wiring harness in addition to the strap, and I don't know which one to get...all that site says is order "the two items below, wiring and strap, with the sensor" but does not give part numbers -- and the adapter cable and wiring harness are different part numbers...but both are "wiring." Ugh!

 

The sensor should come with the "adapter harness". From Volvo anyway, I don't know about other suppliers.

Just for knowledge, the "adapter" is installed to correct a problem with the engine harness. All 02 sensors use "reference air" to be able to read the oxygen content in the exhaust. Reference air is just current sample of the outside air.

Normally there are vent holes around the 02 sensor, but not on these. It is actually drawn into the sensor through the wiring harness, between the wire and the insulation!

The problem is that the engine harness insulation emitted some king of contaminant that plugs up the venting of the sensor, and causes the sensor to fail. That is why they don't want you putting die-electric grease on any of their 02 sensor connectors. If you look on the sensor harness, you'll see the vent holes near the terminals.

Make sure you always install the adapter, and install it according to the instructions!

B

 

Ok, so that explains that I'll need a new wiring harness with a new sensor, right? Is that for both sensors? I still don't understand what to order. If somebody can just tell me, straight up, what to order it would help a lot. I'm still confused since I only need to replace the pre-cat sensor...does the wiring harness go to both sensors? Do I need a new adaptor if I'm using an OEM sensor? >_<

 

I bought my sensor at the dealer and I did not not need any extra wiring harness. It came just like the one I was taking off. I did have to buy a special tool to get it off. It's like a socket with a split in it for the wire to go through. AutoZone will rent you one for free. Basically you rent it for the cost of buying it then take it back.

 

Ok....thanks JimKW. Do/did you have any problems doing it that way(just replacing the sensor)?

How much did your sensor cost at the dealer, and was this the pre-cat sensor? Sorry for all the questions.

 

You need the front O2 sensor, the wiring harness, and a regular tie strap. (zip tie). I would go with the Bosch O2 sensor. It seems pretty straight forward on the Groton site.

 

OK thanks. Then why does the FCP site say "required" in bold print after the adapter cable?

 

I would say because you are supposed to install it with the new sensor.

Jim Im not sure why yours didn't come with the harness.
All the o2 sensors we buy from the dealer have it all in the same box as the sensor.

The Contaminant is oil from the insulation of the wiring harness.
When you replace the sensor look closely at the harness plug and I will bet that you will see oil in the plug.

The harness does fix the problem.

 

I guess that's what I meant was I just bought the O2 sensor from the dealer and it was everything I needed. I didn't have to go buy anything else separate. It's been a while since I did it, and my short term memory isn't that great. If I had done it 40 years ago, I would remember exactly what I did, but since it was only about three years ago.

 

miatamon, you can also email or call fcp and the guy there is extremely knowledgeable and can explain what you need and why. I've only exchanged emails with him once but the other guys here can atest to his CS and technical skills!

 

I emailed him and he sent me a list. He told me to get everything! Including the rear sensor! I only need to replace the front sensor...do I really have to do both? Here's the list he sent....

http://www.fcpgroton.com/volvo850oxygensensors.htm#1995

15062 x 1 (rear sensor)
15097 x 1(front sensor)
9202714 x 1 (Adapter cable)
9202715 x 1 (wiring harness)
983750 x 4 (tie straps)

 

I would replace the front one only and get the adapter and tie straps, unless you have some. The rear one may go out soon. What most ppl don't understand is that when there is something wrong with the engine, it can spew gunk downstream and messes up the O2 sensors. I learned this when i added aftermarket headers to my Tahoe.

There was enough silicone gasket INSIDE the exhause chamber that when I drove the truck and it burned off, it blew into the exhaust pipes, past the O2 sensors and messed them up.

If you have the money, replace both. If you dont, you pay extra in shipping when you have to get the rear ones...


with that said, if it was me, id just get the fronts...

 

I would just do the front unless you have the extra $$ then do both.

 

I only did the front when I did mine and that was close to three years ago.