code PO128

genna · Post by **genna** » 25 Oct 2007, 20:04

This code just appeared on my 2003 S40 and it says something about a problem with the thermostat temp. Anyone know exactly what this means?

Thanks

Post by **MadeInJapan** » 25 Oct 2007, 20:56

Faulty thermostat signal. Is your temp. gauge on your dash up more than usual? I would say a simple thermostat replacement will take care of this. Don't ignore it. You could over heat your car if it's not taken care of and that's really bad news for our aluminum engines.

mike_one2many · Post by **mike_one2many** » 11 Nov 2007, 14:08

My '03 V40 has the same issue.

I replaced the thermostat, of course. I didn't get one from the dealer, but got the top-end OEM replacement part.

For 3 months everything was fine, then the code came back. The computer actually says that the coolant temp is too low. The gauge is where it normally is (half-way up) and I live in Southern California, so it's not like the outside temperature changed much. Yes, people will say it got colder, but it is still around 60 F.

Anyone has any idea what else this might be? A coolant temp sensor?

Post by **MadeInJapan** » 11 Nov 2007, 14:15

Yeah, could be thermostat again but my guess is the actual sensor....strange that your gauge doesn't change. Reset the code and see if it comes back.

mike_one2many · Post by **mike_one2many** » 11 Nov 2007, 14:34

I would hate to think that the thermostat went bad in 3 months.

I reset the codes two weeks ago and they just came back today.

Do you know if the gauge and the codes are driven off different sensors?

Post by **MadeInJapan** » 11 Nov 2007, 14:43

Same sensor....could be the wiring to the sensor. Even good thermostats have been known to go bad early too....might have just gotten a bad one to begin with. I would reset the code again....see how long it takes for it to come back this time.

mike_one2many · Post by **mike_one2many** » 24 Dec 2007, 23:14

OK. I have reset the code 2 times and it has come back.

Time to replace the sensor. Any hints on where it's at and what's involved in replacing it?

Post by **MadeInJapan** » 25 Dec 2007, 06:43

Front of the engine in an aluminum encasing...has a radiator hose running from the casing to the radiator. Not sure on your model, but on other Volvo's it requires undoing 2 torx screws if you're going to replace the thermostat (highly recommended if you're changing the sensor out anyway). Be careful, they tend to be soft. Spray the screws with PB Blaster the night before and let them sit. You might want to buy a couple of extra screws from the dealer to be on the safe side. The sensor is located on the end of the aluminum housing. For the procedure itself- drain the coolant out down past the thermostat housing, undo the thermostat housing and exchange the thermostat. Unclip the sensor wire, and use an open ended box wrench to take the sensor out. Putting things back together is in reverse. For the thermostat, there should be a rubber o-ring that goes around its parameter. The sensor should also have a sealing ring/gasket with it. No other parts are required. Good luck!

Oh, and Merry Christmas.