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Volvo trouble codes/ oxygen sensor types

Hey All,

I’ve got a 1994 Volvo 960 with about 130,000 miles on it. It’s a really nice wagon for its age.

A couple of weeks ago, I was having some heating problems with the car and replaced the thermostat. My heating problem was fixed.

When I finished with that project, I noticed the car was running a little rough, whereas it’s always idled very nicely in the past.

About the time the car started idling rough, the “Check Engine” came on and has remained on since.

With a Chilton manual, I was able to check the trouble codes on the car and came up with “Long-term fuel trim, idle.” A little poking around translated that to what I think is the car needing a new heated oxygen sensor.

I’m guessing that if that’s the only trouble code I’m getting and that’s the indication, that there are good odds replacing the oxygen sensor will solve the problem. I’ve just never fixed anything off the indication from a trouble code before.

While we’re at it, any thoughts on the differences and/ or strengths and weaknesses of a Nippon Denso oxygen sensor over a Bosch sensor?


I posted this on Saturday, but didn’t get any replies, so I thought I’d see what happens if it makes its way back to the top of the pile.


I’m suspicious that you have a problem after replacing the thermostat. Is it posible that you knocked a vacuum hose loose, for example, and caused the problem? Did you spill antifreeze on the oxygen sensor?

Well, I don’t know for sure whether I spilled antifreeze on the sensor. It’s certainly possible it happened. Some antifreeze did drip down the side of the reservoir while filling, but can’t say if it made it to the sensor. I guess I could go look at all of the vacuum hoses.

Coincidences do happen, but like markmast I’d first be suspicious of something getting inadvertently messed with while doing the thermostat.

The thing about the O2 sensors is that it is their job to report values back to the computer so that it can adjust the fuel trim - basically the fuel/air mix. You might be getting this code because your O2 sensors are working perfectly well - and instead you need to look for the cause of a lean or rich running condition that the computer can’t adjust for.

A vacuum line knocked loose (resulting in too much air - leanness) is a good first guess.