Change Rear Oxygen (O2) Sensor, 850 T5 Tutorial

Post by **matthew1** » 05 Jan 2012, 12:23

I've had an annoying P0422 code for months (catalytic converter efficiency). I reset it and it comes back on between 40 and 150 miles later. No degrade in fuel economy.

So I got the Bosch sensor for my '97 850 T5 (T=turbo) from one of my sponsors, Bosch part #15062. It was around $90. I didn't want to fiddle with the Bosch universal sensor, although some here have used that with success and thus saved money.

A sticker on the box says "Oxy Sensor R 850T CSV70 NA98". Take that to mean what you will. In other words, this Bosch part might fit many/most of the gen 1 FWD Volvos.

Why rear O2 sensor and not a new cat. convertor? Oh because a new cat and labor are about 6x as expensive, and with 110k miles on the rear sensor there are posts here pointing that direction. Also, the cat does not rattle when tapping it, further evidence it's not the cat.

The sensor came with the wiring, wiring shielding, and the harness. Even the clips positioned at various points were in the right spots. Made it easy.

Jack up the Volvo safely. I just did the left side and left right side wheels on the ground.

Remove the belly pan. Four easy 12mm bolts. I jacked it up, sprayed those with liquid Wrench, waited 10 minutes, the easily took off the pan. Then I blasted the O2 sensor threads with Liquid Wrench and waited a few hours. It still took a ton of force, but nothing more than a 3/8ths inch ratchet.

Looking toward the front of the car. Notice the belly pan is a clip location for the wire (still attached).

Here's a shot of my fancy new 7/8ths inch oxygen sensor socket. Less than $10. Didn't want to just cut the wires, you never know if a) the O2 sensor is indeed failed and b) if it's not, if you'll need one again.

There's just barely enough room to slide a 3/8ths inch socket wrench into the socket. Just push it on.

Looking down into the engine bay, driver's side. These are the rear (L) and front (R) O2 sensor harnesses. The rear harness is open. This is the hardest part in my opinion. Hard to get to from underneath, hard to get to from up top. Slide a flat blade screwdriver under the red tab and twist to open it.

New (top), and old.

It's an easy job, really. No exotic tools necessary. I did my front sensor years ago without the fancy socket, and it went ok. Be safe under the car.

Post by **Volvo4251** » 13 Mar 2012, 10:12

My 850 tripped that code recently too. It's a funny thing that code, as it really doesn't affect the drivability of the car at all. Dealer wanted $517 for the job!! Looks like you did it for MUCH less!

byeboy · Post by **byeboy** » 13 Mar 2012, 12:26

Did that clear your codes?

Any effect on MPG?

Nice write-up. Thanks!

Post by **matthew1** » 13 Mar 2012, 13:12

I cleared the code after this, but it came back a few times. I like things to be cut-n-dried, but this was not. Oh well.

Looking back, it was probably a vacuum line or elbow that was causing the code. Or a gunked up PTC, which I cleaned.

Ironically, it could have been all the above plus something I didn't mean to do that straightened things out -- killing power to the ECU by running down my battery. I had the ignition on II for over an hour on a cold day a few weeks ago while working on the car, and ended up needing a jump. After that, no more P0422.

Mileage has been steady before and after. The front O2 sensor is the one that kills your mpg when it goes.

Post by **Volvo4251** » 13 Mar 2012, 13:22

+1 to Matthew's comment.

The 98 V70 I had had a lot of O2 and catalytic convertor issues. There were at least two occaisons where the front O2 sensor went and the car literally stopped running. The engine couldn't get a correct air to fuel ratio and quickly died.

echase76 · Post by **echase76** » 14 Mar 2012, 20:17

I'm new to the volvo "game" but it is my understanding that after replacing an O2 you should pull your negative cable for 15 minutes to reset the ECU. That way it can relearn with the new O2 inputs and you kill any potential pending codes.

I'll be doing my rear O2 this weekend, so thanks for the write-up!

Post by **matthew1** » 14 Mar 2012, 22:22

Welcome, echase.

echase76 wrote:I'm new to the volvo "game" but it is my understanding that after replacing an O2 you should pull your negative cable for 15 minutes to reset the ECU. That way it can relearn with the new O2 inputs and you kill any potential pending codes.

Yea, I've recently learned to do this for more repairs than I did in the past.

1. have your radio code
2. pull the cable
3. step on the brake pedal to drain any residual charge that could keep the ECU's volatile memory alive

echase76 wrote:I'll be doing my rear O2 this weekend, so thanks for the write-up!

No problem.

red850 · Post by **red850** » 30 Mar 2012, 19:56

Thanks guys,
I recently replaced my O2 sensor but that p0422 code still came up. I think I'll try that battery thing.

goofy2 · Post by **goofy2** » 01 Aug 2014, 04:49

It has been intersting reading these posts. I have a 1996 T5 Wagon registered in Malaysia. There was a DTC error 153 thrown up. AS I had removed the catalytic converter I installed a spacer, but the mechanic installed it on the front O2 sensor as he couldn't find any tracesof a rear O2 sensor or its wiring. This strangely enough got rid of the 153 fault but a 231 fault came up. Could this be because the sensor being some distance from the exhaust is too cold to give a proper signal.