The main O2 sensor keeps failing in our 2008 Volvo V70 2.0F (flexifuel/E85). We've replaced the sensor several times in the past few years. We've tried every available brand of sensor, including OEM parts from Volvo, but they've all failed within 1-10 months.

I'm pretty sure that the O2 sensor is failing, because all the symptoms disappear after it's replaced. I won't go into too much detail, but the symptoms include running poorly, running rich, and OBD codes related to the main oxygen sensor.

But the O2 sensor isn't the root cause of the problem. Something is causing the sensor to fail.

But there doesn't seem to be anything wrong with the car. It works perfectly after the sensor has been replaced. There are no OBD codes (except the oxygen sensor codes when it fails). The car doesn't seem to be burning oil or coolant (I've never had to top up either, the fluids look normal, and there's no weird smoke/smell in the exhaust).

The sensor is contaminated with soot when I remove it after failure, but I'm not sure if it becomes contaminated before failure or after failure (when the engine starts running rich). I did a propane torch bench test with one of the failed sensors, and something flammable (fuel?) vaporized from the sensor. After the bench test, the sensor started working again, but it failed after a month or so as usual.

We thought it might be caused by running on E85 fuel in cold weather, but last winter we used mostly regular gasoline and the sensor still failed. We also have an S80 with the same 2.0F engine and it's never had this issue.

I'm completely puzzled by this problem. There seems to be nothing wrong with the car but it still keeps failing. Any ideas, is there anything I could try? I have the S80 so I can compare/swap parts if necessary.

 

The only theory I can think of is that it's running rich, which fouls the O2 sensor over time. Maybe it's only a slight issue and it's only running rich when in open loop control mode (before the O2 sensor heats up and kicks in and dials down the mixture).

It's not burning any oil, so it must be getting too much fuel.

I'm thinking it could be caused by
- Bad MAF sensor (but this should cause an OBD code)
- Bad fuel injectors (I wonder how I could test this)
- High fuel pressure (is there some kind of fuel pressure regulation? I wonder how this could be tested)
- Bad thermostat causing it to not reach operating temperature. There's no engine temperature gauge on this car. But I think this should also cause an OBD code.

Anything else that I could possibly test?

 

have you done the P code to Volvo diag code conversion table lookup to see what it actually means ?

 

you dont have an oil temp guage?

O2 sensors are electrically preheated by the ECU
heres a pdf printed from VIDA describing the bosch me functions, goes into some detail on the O2 sensors

 

I have not looked up the Volvo DTC codes. I found a lookup table for P2 platform cars, but this is a P3 so I wonder if the codes are the same? Do you happen to know where I could find a table for P3 cars?

P3 cars don't have an engine temperature gauge in the dashboard (or anywhere else as far as I know).

Thanks for the pdf, it's very helpful. This car has an B4204S4 engine, which is a bit different in some aspects. Do you happen to have access to the pdf for that engine?

 

maybe with the VIN, but just using the year and type info you have posted already i'm only getting b5 and b6 engines

 

Could be the same problem as mine. Check this out. I registered to this forum because I found your post.

https://volvoforums.com/forum/2001-2...e-dies-113680/

 

I replaced the MAP sensor (or MAPT sensor) as discussed in the thread linked above. Car works fine so far and we'll see in the coming months if this fixed the issue for good.

 

Here are also the VIDA printouts for the B4204S4 engine for future reference (courtesy of lrock).