I have a 2000 S40 with 72,000 miles on it. Recently, the last six months I have been pulling DTC 128 Coolant Thermostat (Coolant Temperature Below Thermostat Regulating Temp). I clear the fault and then it usually takes a couple of days, about a week, for it to return. I was assuming that the thermostat was faulty and that I needed to replace it but was told that it might also be the cooling temperature sending unit.

Any thoughts or ideas?

Reporting back... the thermostat change seemed to do the trick for me... I have driven over 500 miles since I changed it and the code has not returned. Also, I brought it in for my state's new OBD-II based emissions test and it passed... so the computer I assume ran through all of its tests and I don't seem to have a problem any longer.

I had originally though it might be the temp. sensor b/c the temp. needle was in the 3:00 position as it always was...

Good luck...

I had the same code one night last winter when it was -10 degree F. The thermostadt was changed with no further problems. The temp gauge did not show the problem for me either.
Thanks,
Craig

I also have a 2000 S40 with 69,000 miles on it... I have been pulling the DTC 128 - Coolant Thermostat for the last month or so... last weekend I replaced the thermostat and now I'm waiting to see if it the code returns before visiting the Volvo parts department again... if it returns I intend to replace the Engine Coolant Temp (ECT) Sensor and hopefully that will do it... when I removed the old thermostat, I noticed that the gasket around the thermostat was split... I'll let you know if the code returns...

u naad a t stat and ect sensor, very common