I have a ’99 Volvo V70 GLT. I’ll try to be concise.

Check engine light came on, had the mechanic check it when I had it in for timing belt/water pump.
Said I needed a rear O2 sensor, a purge control valve and a new gas cap.
It seemed to running okay at this point, but a couple of weeks later it began to idle very rough and started puffing black smoke pretty badly.

I bought and installed a universal Bosch rear o2 sensor and installed it and disconnected the battery to reset the check engine light. And it seemed to run okay for a while until 50ish miles later the light is back on and black smoke again.

Just to check, I disconnected the battery again and reset the light. It again ran smooth, not perfect but much better. So universal sensor obviously didn’t do anything…it was just resetting something that “fixed” it temporarily.

I’m thinking that I may just need to get an OEM oxygen sensor instead of the universal one, but I wondered if it is just the sensor, why would it run okay for a little while after disconnecting the battery and resetting the light? Or is it something else more complicated..

 

Rear O2 sensor only cks how well the CAT is working; the front one is the one that controls your air/fuel mixture once in closed loop mode (after warm-up). Did your tech tell you what the code was?

Suggest you replace both O2 sensors with OEM; aftermkt universals do not work on these cars for whatever reasons..

Do a search; tons of threads start with " ... just replaced my O2 sensor with universal and it's still setting codes...WTF??"

 

Thanks, i've scoured the other threads as best i could and had narrowed it down to needing OEM or maybe a vacuum leak. I tried to check and didn't seem to find a vacuum leak to my knowledge.

I was really just curious about why it would work for a little bit after resetting with the battery disconnect. Just wanted to 'understand how it is supposed to work'...

Also, could anyone recommend a good place to buy the sensors online?

 

I found front and rear sensors made by NGK on FCP Groton...I know Bosch is the original manufacture, but are these equivalent?

The part reference numbers match the Bosch sensors..

 

Could try ipd, if you can find them on their crappy new site (Volvo Parts, Accessories and Performance Specialist Since 1963)
Did a search on "O2 sensor" and got one hit; a scan tool.

On fcp only the rear was NGK; didn't know they were OEM but i could be wrong; might call them..