So sorry to post a V40 question in the S40, but it seems that is where people look.

I have a 2001 v40 1.9T with 76k that has a recurring ECM fault code P0128. From the SAE book I have it says that "the PCM has determined that the input from the coolant temperature sensor indicates a temprature value that is below the thermostat regulating temperature." OK - So my (mis)logic tells me that could be one of two things. That the sensor is bad (or its wiring/plugs/etc) or the thermostat is sticking open. Since the fault comes up when it is cooler more then warmer (generally it will come un 3-4 days after being reset) I tend to believe it is the thermostat. Since they are only $8, as opposed to $70 for the sensor, I figure that is the place to start. The only odd thing is, the dashboard temperature guage is pretty much where it has been for the last 4 years.

So I'd like to know if anyone has a table of temperature sensor values that they would like to share with me (hint Tech, please!) so I don't have to buy something I don't need to and can start looking elsewhere for the problem.

Peter

 

Sorry I don't have a table for you to look at.There was 2 problems with what you are complaining about half of the time the thermostat is broke and the other half is the sensor.The thermostat has a gasket as part of the thermostat like an o-ring on the outside of the thermostat.You can remove the housing usually whithout causing any damage and can reuse.You will know right away if it is broke once you open it.

 

We shall see. The thermostat still had a decent spring, but it was a little sticky on the shaft. I replaced it, and pulled the sensor out. It seems difficult for me to believe the sensor could be bad unless the casing failed and allowed the bitmetallic thermistor to be contaminated with coolant or air. It gives a value, and that seems like it is either working or not. So it is back in, and yes the gasket was fine to reuse, thats good because they didn't give me one (there computer said to used sealant !&^%#!).

 

I guess there computers need to be updated.Those sensors are weird.In the 5 years I have been with Volvo I have seen many of those sensors go bad.They have pretty much been using the same style sensor since 1993.

 

I there, having the same problem. Replaced the thermostat twice since the dealers mechanic didn't like the non oem part from auto zone. Replaced the temp sensor and then the dealer replaced the software in the computer for 320$; the check engine came on the same day. Replaced the coolant because the previous owner put in 100 % antifreeze where it should be 50 50. Easy to check with a 3$ coolant density gauge. Can't get the car inspected with computer sending 128 codes even if the check engine light is off.

Tomorrow I'm going to check the connections to the ECM at the temp sensor terminals. From reading about the generic system obd2 I should get a low resistance <2 ohms to the car frame and get 5v on the other terminal with the ignition on. Keep you posted

 

I checked the wiring to the ECM and the coolant temperature sensor. I found tables on the web showing coolant temp sensor resistance vs temperature.

coolant tempo sensor

At room temperature one should get about 2250 ohms (measure after leaving the car overnight for a cool engine). I also found that the sensor should read about 2oo ohms when the engine is hot. I think these numbers can vary by +/_ 2o% by looking at published data.

wiring to the ECM

From web info on generic obd2 systems one can see that the resistance of one of the terminal on the ecu should be very low <10 ohms with respect to the engine block. In addition the other terminal to the ECM should be about 5v with the ignition on. Please note that the car doesn't need to be running for these tests and always be aware that fans (two in this case) could turn on even with the engine off.

Finding no plausible explanation for the continuing 0128 code I am looking into the IAT sensor next. From my reading on the web it is stated that the computer uses the Intake Air Temperature data as well as the data from the coolant temp sensor.

 

I'm sure where the iat sensor is located on my s40 1.9t. Can anyone help. I know it should be near the air cleaner. I can only find one connector but it has three contacts. Perhaps it's combined with another sensor?

 

For $320 the dealer should have also hooked up a DiCE tool and told you what the coolant temp sensor is reading as well as every other engine sensor including the IAT. Hell I can do that with my Chinese made DiCE tool.

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(Pure coincidence, I'm flying to Shenzen tomorrow night. One of these days I have to look up that factory.)

Comes with VIDA too. All you need is a 32 bit version of Windows XP Pro or 7 Pro or 7 Ultimate to run VIDA on your home computer. Does not work with the 64 bit version.

 

This code was always resolved by replacing the thermostat for me.

 

Good call. Same for me.

 

I replaced the Thermostat and the Sensor since the sensor was $10 at Amazon. This cleared the code for me.

Side note, the Thermostat from Amazon ($12) included the gasket also.

 

I've replaced my sensor and thermostat back at 75k when it threw the P0128 code and problem was solved until now...just starting throwing the P0128 code again...i'm at 145k now.
I get my stuff at Rock Auto. Cheapest sensor is $15 at Rock Auto and $47 at Autozone. Orielly doesn't have them

 

What is the difficulty level to replace the thermostat? I work on all of my American-made cars but never a Volvo. Thanks.

 

which engine? Take a look under your hood. on the 2000s vintage Volvos, you can follow the upper radiator hose to where it goes to the block, attaching to a domed aluminum cover. The thermostat sits under that cover, which is held in place by two torx bolts. You can replace the thermostat without draining the radiator on the those models (like my 2000 S40 ) but will lose about a cup or two of fluid. Also note that when you pop off the cover and push back the hose, you will find the engine coolant temp sensor, which screws into the housing below the thermostat. My two cents here: Go OEM quality and clean the parts when replacing the gasket/cover etc. Consider also replacing the temp sensor and the coolant reservoir cap while you have access.