I have a 1998 S70 with 180,000 miles on it. The cooling fans stays on after the car shuts off--even when cold. The check engine light just came on. Have not had it checked yet, but could a sensor be bad?

Sounds like more than one problem could be present.

Does the fan start right away and keep running, whenever you start the engine (even if the engine is cold)?
Does the fan stay on forever or go off after some minutes, after you cut the engine off?
Also did the fan start to run too long or too often BEFORE you first saw the check engine light?
Does your dashboard temp gauge read ok?
Is the engine overheating (whether or not the gauge reads ok)?
Does the radiator feel much colder than the engine, once the engine is warmed up?

All these things could narrow down what might be wrong.

Some cars have a separate thermal switch which is supposed to turn on the radiator fan whenever the radiator gets hot, regardless of anything else that is happening. The fan could run on if this is stuck, or if another control circuit such as a/c is forcing the fan on despite the switch being ok.

I don't think that a radiator temp sensor would trigger the check engine light, but as the other posts say, a bad ECT sensor would do that.

Actually what can happen is that when your t-stat gets stuck in the open position the temp sensor will notice that the engine isn't warming up fast enough (so many degrees per minute) When that occurs, the engine ECU will set a code and you could be fooled into thinking that it's your sensor. I learned that the $60 way. Could've spent $10 on the stat but spent $60 on the sensor and t-stat. I'd start with the stat first.
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'98 XC '95 855 GLT '83 245 DL (245 for sale! e-mail me if your looking to buy); The Purple Neon has been sold (sniff) All over 120k

Thanks. I will give it a try.

coolant temp sensor is out. Replace t-stat at the same time and you'll be good as new.
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'88 240, '92 745, '98 v70 -John, Tampa Bay

jda2000 pretty much called it. The temp sensors go bad in these cars all of the time and you have to pull the thermostat housing out of the way to get to it so you might as well just toss in a new thermostat while you have the old one out. I replace them all of the time for posting codes as well as leaking coolant right through the center of them. If they post a code for that sensor it will put the fan into default mode and it runs at half speed all of the time as well as running on for a few minutes after the car is shut off.


Mark

I had the car checked by a local garage and the code indicated a coolant sensor fault. I ordered a new one and also a thermostat. Should fix the problem.

Thanks.

If replacing yourself, be carefull with the 2 screws on the thermostat housing. The rear screw (torx T30 I believe, or it might be T40) is very difficult due to the fuel rail being in the way. You really need to buy a slender (around 6 inch long) torx bit and avoid using a connection piece on a ratchet. I did the later and ended up rounding out the inside of the screw and then it was a bugger to get out. If you do end up messing up the screw(s), www.fcpgroton.com (you'll have to call them) sell the screws for only a buck a pair. Good luck...otherwise it's a very easy repair, but to make things easier, syphon out the expansion tank coolant before you start so you have minimum spillage. If you do this, you really don't even need to take any hoses off or drain the radiator at all.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

Thanks again. I did notice the rear torx screw and am concerned about rounding out the screw head.

Spray some PB blaster on it and let it set over-night before trying to take it out. Mine came out fine without it, but if it's never been changed and you have high miles, the screw head can be wrenched off...heard of this happening to someone over on Volvospeed. Ughhhhhh......
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.

I did spray some PB Blaster on them and they come right off. I have the sensor and thermostat now. Looks like a simple job to replace. Thanks again.

Good deal...yes, it is rather simple...just stuff on the fuel rail (metal fuel pipes) are in the way. Make sure you have a long torx bit that will clear this, otherwise, you'll bugger up the torx screw doing the job like I did the first time.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.