I too am suffering from the infamous P0128 code on my 2000 V40. I have had the thermostat replaced. Had the temperature sensor changed (twice). Also, I have had the 105K service done and frequent tuneups (~30 K miles on plugs).

My mechanic tells me the V40 is supposed to have two sensors (in his manuals). He can find only one. He thinks since the code is coming back with the new sensor, it may be the computer.

So:
are there two sensors
is this a computer problem

I do have a roll of black electrical tape handy to cover up the light

 

there are two. The most common one is near the thermostat housing.

The other one is on the block, but I'm not sure where. Sorry.

 

The coolant temp sensor is located near the thermostat housing. Try that and if it still comes back then I believe some people have solved this problem by using a genuine volvo thermostat. If it still comes back, then you can go ahead and use the black tape lol.

It is common on these cars for this code to come up for no reason. Hope this helps.

 

Fought this battle for 18 mths, changed thermostat and temperature ranges several times, changed IAT sensor twice, (all part changes with Volvo parts). After each change light would stay off 2 - 3 months and than back on. I was ready to junk this Volvo v40 that my wife loved.
Last stray was when a/c started to work when it wanted to and the temp guage went from normal to pegged hot and then back to normal when engine was not overheating. Took it to the Volvo dealer and $640.06 later no CEL and everything working correctly 5 months later thru some of the hotest weather of the year last fall.
They replaced 3 relays, IAT sensor, Thermostat kit, flushed cooling system, and installed new anti-Freeze. Tech said a faulty engine coolant sensor can shut ac system when reading incorrectly and continued operation with the CEL on will affect other systems.
I am not a tech, so I dont know the validity of this statement.
Anyone needing the p/n of the parts replaced can e-mail me at dan42@charter.net.
I am not suggesting this will solve your problems, just stating what solved the problem for me.

 

Well, I just got the P0128 temperature lower than thermostat regulating temperature...again.

Please post the part numbers and names. I'll send an e-mail as well.

Since I've changed the sensor and thermostat (x2), and the fluid. I need to look for that other sensor (if there trully is one) and the other bits.

 

Disregard the other temperature sensor issue. P/N W0133-1889714 is a temperature sensor, but it is for the Intake Air Temperature (IAT) or Fuel Air Explosive (FAE) temperature. I've seen it listed as both, but believe volvo calls it an iat.

What is the correct temperature thermostat? I've seen 160F, 180F, and 195F all listed as OEM replacement thermostats. Since I just changed mine and the guage looks about right, I'm starting to think that maybe I got the wrong temperature thermostat.

 

I have had same issue (P0128) for a few YEARS now-replaced thermostat with genuine Volvo part. Flashed computer at dealship, and CEl P0128 still came back within a couple of thousand miles. Dealer says computer needs a software upgrade, but there is a risk that this might kill the computer and replacement would be upwardss of $700. Anyone have thoughts on engine computer software upgrade? I'm told that this can only be done at Volvo dealership. Is this true? Just recently have had issues with engine cranking but not turning over in the morning when it has sat overnight, but I think this is non-related.
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2003 V40 117K miles