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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 2-10 months.

I'm pretty sure that the O2 sensor is failing, because all the symptoms disappear for a number of months 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 clearly, the O2 sensor probably isn't the root cause of the issue. Something is causing the sensor to fail soon after replacement. But we haven't found any other issues with the car.

What could cause the O2 sensor to fail? Any tips on trying to diagnose the issue?

2 Answers 2

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Coolant and motor oil will both contaminate an O2 sensor. Does the car burn oil, requiring frequent motor oil top-off on the dipstick? If so, do a compression test to check for worn cylinders. Do you have to add coolant to the reservoir more than twice in a year or notice the coolant becoming brownish? If so, use a coolant test kit to check for the presence of oil in the coolant, indicating a blown head gasket.

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  • I've never needed to top up oil or coolant on this car. Oil looks normal when changing. Coolant looks normal too. There's no unusual smoke or smell in the exhaust. Do you think a tiny (unnoticeable) gasket leak could kill the O2 sensor, or does it need to be so significant that it's noticeable?
    – ntoskrnl
    Apr 11, 2023 at 13:06
  • I doubt an unnoticeable leak would cause O2 sensor failure, especially if it has happened several times over years. Do you use any fuel additives, like octane boost?
    – Carguy
    Apr 12, 2023 at 4:09
  • What do the bad O2 sensors look like when they are removed? Are they shinny or blackened by carbon? An O2 sensor can be cleaned and bench tested in a vice and applying a propane torch to it for 20-30 seconds. After about three heat/cool cycles, it should generate about 9 volts on a volt meter. If a 'bad' one can be corrected easily, then the car likely needs a tune-up.
    – Carguy
    Apr 12, 2023 at 4:13
  • What is the octane rating of the gas you use? If you are using ethanol-free gas, it may be causing the problem. Also, if you are running E85, running the car in a very cold climate could foul the O2 sensor.
    – Carguy
    Apr 12, 2023 at 4:14
  • We use both E85 and regular 95E10 depending on prices. No fuel additives. The O2 sensors are sooty after removal, but I'm not sure if they become sooty before failure, or after failure when the engine starts running rich. I tested a couple of removed sensors with a torch, and one of them worked but the other did not. Running E85 in the winter could be the problem, but it's weird that the car was fine for years before this started happening. We also have an S80 with the same 2.0F engine and it's never had this issue.
    – ntoskrnl
    Apr 16, 2023 at 10:41
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The issue was caused by a dirty/faulty MAPT sensor (MAP sensor). This engine does not have a MAF sensor. The ECU doesn't indicate a fault with the MAPT sensor probably because the "ethanol content" fuel trim has enough range to compensate for the faulty sensor readings... or something to that effect.

We discussed the issue in these threads:

The sensor is fairly inexpensive and easy to change, although it is located in a tricky place at the bottom of the intake manifold.

MAPT sensor location

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