CEL Fun - Fan Control Module Going Out?

[A4orce84]

Hey Everyone,

Over the past month I've noticed my coolant gauge struggling to get warm at the 12-oclock position. Sometimes it will fire up and get to the 12-oclock position in 5 minutes, and other times I will be driving for 10 minutes and it will still be sitting at the 3/4 mark:


The top coolant sensor (green) was replaced a few years ago, and as a sanity check, the bottom grey radiator coolant temp sensor was also replaced approximately 2 weeks ago. However, last night I was driving about 30 minutes on the highway and saw a similar situation to the above screenshot. The heater was on, and the coolant temp gauge stayed at the 3/4 mark.


I came home scanned my car last night and found the following codes:



Since the radiator Coolant Temp Sensor was replaced less than a month ago, does the above codes indicate that my fan control module is on its way out? TIA.



Thanks,
Asif

[old guy]

Sure sounds familiar. Clicky click®

Thermostat.

[A4orce84]

Thanks Fletcher, but shouldn't I have gotten a thermostat code if that was the case?


Thanks,
Asif

[old guy]

There is no code for the thermostat. There is a code for the thermostat heater and there are several codes for the mapped cooling system that can result from a faulty thermostat.

[A4orce84]

Is there any way to 'confirm' it is the thermostat? I guess the only reason I am hesitant, is the thermostat + j-plug was replaced 2-3 years ago on my car.....

Thanks for all your time and help so far!

[old guy]

Is there any way to 'confirm' it is the thermostat? I guess the only reason I am hesitant, is the thermostat + j-plug was replaced 2-3 years ago on my car.....

Thanks for all your time and help so far!

Yes there is. Read through the thread that I linked and you should be able to confirm that the thermostat is the issue.

[A4orce84]

Makes sense Fletcher! The symptoms do sound pretty spot on to what I am experiencing so far.

Question:
Is Behr a suitable OEM replacement for a thermostat? I ask because I am looking at purchasing the following:
https://jhmotorsports.com/shop/catal...18t-p-763.html

Along with a billet aluminum J-Plug from ECS. Thoughts?

Thanks,
Asif

[old guy]

Behr is an OEM supplier. Good choice. Aluminium J plug is a great idea!

[A4orce84]

Hey Guys,

Mini-update on my issue that I am still experiencing:

Replaced the Thermostat + J-Plug a few days ago...... and unfortunately, my coolant gauge is still taking a while to warm up and get to the 12-oclock position:


Average SHOULD be around the 5 minute mark to be at center.


Doing another scan of the car, again I am still getting the intermittent code for the FCM:


So, at this point the following has been replaced:
1. Lower Temperature Coolant Sensor (radiator)
2. Thermostat + J-Plug

I'm sure the next recommendation will be to replace the "Top Temperature Coolant Sensor" (green-plug)....but I did NOT receive a code for it, so I'm not sure what to check next.

Any assistance would be greatly appreciated.


Thanks,
Asif

[B6_A4_Kid]

Im having this same issue, very annoying. For me, sometimes it heats up normally and never goes past 1/2, other times like on the freeway, the gauge just drops to a little under a 1/4. I dont mind that, I just want my CEL gone! I only have the P1931 code currently. Not thread jacking, I am just hoping you figure out a resolve so I can do it too lol.

[old guy]

There are three possibilities that can cause the gauge in the display to take an excessively long time to heat up properly.

1) The block is heating up normally but the gauge isn’t registering correctly. This would be caused by a defective rear (Green) CTS. The Green CTS is a dual (four pin) sensor sending two separate signals. One signal goes to the ECM for fueling and temperature control and the other signal goes to the gauge in the display.
So it’s entirely possible that the gauge signal is faulty but the ECM signal is still good. There isn’t any feedback between the two so the ECM stays happy and no CEL or DTC.

2) You have a defective or sticking thermostat and consequently the block isn’t heating up normally.

3) Your thermostat is functioning properly but the ECM is forcing the thermostat to open early because it perceives a fault in the cooling system. If the ECM determines that something is out of whack the default position to protect the engine is to apply maximum voltage to the thermostat heater thus forcing the thermostat open at a much lower temperature than the mechanical set point.

This can be because of an electrical signal fault from a sensor or it can be because the sensors are sending incongruent information that falls outside of the expected cooling maps. For example: The lower CTS in the radiator return hose reads a higher temperature than the rear CTS exiting the head. This will fall outside of the expected values since it would indicate that the coolant would be getting hotter instead of cooler as it flows through the radiator. The ECM knows something is wrong so it goes into protection mode.

Typically when the ECM is in protection mode it will do two things. It will turn the cooling fans on immediately upon starting the engine rather than after the normal warm up period. I have heard of some cases where this didn’t happen which could be explained by a fault in the Fan Control Module. The ECM will also apply full voltage to the thermostat heater.

[AudiA4_20T]

There are three possibilities that can cause the gauge in the display to take an excessively long time to heat up properly.

1) The block is heating up normally but the gauge isn’t registering correctly. This would be caused by a defective rear (Green) CTS. The Green CTS is a dual (four pin) sensor sending two separate signals. One signal goes to the ECM for fueling and temperature control and the other signal goes to the gauge in the display.
So it’s entirely possible that the gauge signal is faulty but the ECM signal is still good. There isn’t any feedback between the two so the ECM stays happy and no CEL or DTC.

2) You have a defective or sticking thermostat and consequently the block isn’t heating up normally.

3) Your thermostat is functioning properly but the ECM is forcing the thermostat to open early because it perceives a fault in the cooling system. If the ECM determines that something is out of whack the default position to protect the engine is to apply maximum voltage to the thermostat heater thus forcing the thermostat open at a much lower temperature than the mechanical set point.

This can be because of an electrical signal fault from a sensor or it can be because the sensors are sending incongruent information that falls outside of the expected cooling maps. For example: The lower CTS in the radiator return hose reads a higher temperature than the rear CTS exiting the head. This will fall outside of the expected values since it would indicate that the coolant would be getting hotter instead of cooler as it flows through the radiator. The ECM knows something is wrong so it goes into protection mode.

Typically when the ECM is in protection mode it will do two things. It will turn the cooling fans on immediately upon starting the engine rather than after the normal warm up period. I have heard of some cases where this didn’t happen which could be explained by a fault in the Fan Control Module. The ECM will also apply full voltage to the thermostat heater.

yep. You should get a VAG-COM and check to see if the reading matches the gauge, that'll rule out #1

[old guy]

I was just reading Clint's comment and realized that the last portion of my post is missing! Don't know what happened to it??

It's pretty easy to check to see if the thermostat is good and the ECM is causing the slow to heat problem. Simply unplug the thermostat heater before your next cold start. If the engine heats up normally (4→5 minutes to full temperature) that would be an indication that the mechanical portion of the thermostat is functioning properly and the ECM is the cause for the slow to heat up condition. The next step will be to figure out what is causing the ECM to apply power to the thermostat heater on start up.

Doing this obviously will give you a DTC but simply clear it after you determine if the ECM is causing the issue.

[A4orce84]

Thanks guys! I'm assuming since the thermostat was replaced that rules out option #2 as a culprit. I'll see if I can test the upper CTS and rule that out next.

Thanks,
Asif

[A4orce84]

Hey Guys,

Pulled out the the plug going into the thermostat heater, and as old guy predicted, the engine heated up normally (4→5 minutes to full temperature). I think this indicates my ECM is the one to blame for my coolant gauge not hovering at center / indicating that the engine is slow to heat-up and get to temperature.

Could this still be related to the intermittent code I receive regarding the Fan Control Module?

Thanks,
Asif

[old guy]

It does sound like your ECM is causing the thermostat to open too soon. If you reset all codes how fast does the P1931 come back?

On your next cold start monitor measuring blocks 130 and 131. Block 130 will show you the exiting temperature of the coolant at the back of the block and the exiting temperature of the coolant from the radiator. It will also show you the duty cycle of the thermostat heater.

What you should see is a steady climb in the coolant temperature exiting the block and no increase in the temperature exiting the radiator. The percent load on the thermostat heater should be minimal (around 3%). If you see both sensors heating up with no increase in the heater duty cycle (% load) it would indicate a faulty thermostat.

The block exiting temperature should climb to at least 80°c before you see any increase in the temperature of the lower radiator hose. If you do see an early increase in the lower radiator hose sensor observe the heater duty cycle. Normally it should not increase. If it does then we have confirmed that the ECM is causing the slow to heat issue.

Block 131 will show you the block exiting temperature as well as the specified block exiting temperature. It also shows the radiator exiting temperature and duty cycle of the thermostat heater. Once the block exiting temperature goes above the specified exiting temperature you should see a significant increase in the thermostat heater duty cycle. This will allow more flow through the radiator and bring down the block temperature to the specified temperature.

[A4orce84]

Thanks for the update Fletcher! You are a scholar and a gentleman sir! =)

Question:
Should I plug the sensor plug back into the thermostat heater before performing these log tests? Or continue to leave it unplugged (as it is currently)?

Thanks,
Asif

[old guy]

Clear any codes and plug the heater back in.

Here is a link to a thread that has some general information as to how the mapped cooling system functions. Clicky click® Might be worth reviewing the complete thread.

[A4orce84]

Hey Fletcher + Everyone

Wanted to give a mini-update to this coolant gauge issue. I have NOT been able to do any logging just yet, but I did end up taking my car to the independent mechanic in town.

I ended up getting the following diagnosis on the coolant gauge:


So he says "Coolant Migration" that affected some wiring at the top, near the green coolant temp sensor (or where it connects to). If I understand the mechanic correctly, basically when the thermostat sensor is plugged in, the green coolant sensor is getting an intermittent connection (due to Coolant Migration affecting some wiring) and the ECU thinks the Fan Control Module is at fault....or something along those liens. I wasn't sure about this diagnosis, since I haven't read anything like this on the board lately (searching came up with a few similar results), but wanted to post up and get some fresh opinions and feedback.

Thanks in advance everyone, your time and help is greatly appreciated.

Thanks,
Asif

[old guy]

Coolant migration is a common problem on certain model Jettas and early Passats. What happens is that the coolant sensor leaks and the coolant wicks through the wiring harness and causes corrosion at the terminal connections. Sometimes with catastrophic results costing $1,000+ repairs. there are several VW TSB's concerning this issue.

Apparently your mechanic has noted some coolant in the CTS harness connector and suspects it may be causing some of the four wires in the CTS to short. this could explain some of your erratic symptoms. As previously noted it appears that your ECM is applying power to the thermostat heater at start up since it is getting mixed signals from the sensors. It knows something is wrong so it wants to protect the engine from overheating.

It sounds like he may be on the right track toward a solution.

[A4orce84]

Makes sense, is this a common issue on our cars though (B5 / B6 A4s)? You mentioned Jettas and Passats...so I guess it isn't too far off that it could be related.

Thanks Fletcher! =)

[old guy]

Makes sense, is this a common issue on our cars though (B5 / B6 A4s)? You mentioned Jettas and Passats...so I guess it isn't too far off that it could be related.

Thanks Fletcher! =)

I have read a few instances with Audis experiencing coolant migration. but it has always been migration from the coolant level sensor rather than the CTS. Clicky click® In those cases the coolant migrates all the way to the ECM connections and causes real issues. If coolant migration is causing your issue it appears to be just at the CTS connection at this point.

[A4orce84]

Hey Everyone,

Wanted to bump this back up and close the loop here. Finally RESOLVED this issue and just wanted to give the break-down and final solution needed.

Background:
So, if everyone recalls my coolant gauge was acting wonky for several months. I ended up replacing the following parts due to a few recommendations:
1. Lower Temperature Coolant Sensor (radiator)
2. Thermostat + J-Plug
3. Top Thermostat Coolant Sensor (Green Plug)

After replacing all these parts, my coolant gauge would still act odd (sitting in C for 30 mins, etc.). Scanning the car I got an intermittent Fan Control Module (FCM) code, and decided to get the car inspected at a professional independent mechanic. After taking it to him, he said there might be some possible "Coolant Migration" that might explain the behavior. However, after taking a closer look (and replacing the green coolant temp sensor) I did not see any migration and the coolant gauge did not improve.

Resolution:
At this point, with the help of Mr. Matt Devo, we decided to tackle the FCM errors that the car was complaining about. I ended up getting a new FCM and my coolant gauge started behaving normally again.

Our theory is that the original FCM was causing a general fault, which put the ECU into "safe mode." This general fault opened the thermostat / put fan on full blast / lowered temp for the gauge and the Coolant Temperature Sensor was in fact reporting the correct temperature. Moral of the story, listen to the CELs your car complains about. =)


Thanks Fletcher (old guy) and everyone else who helped with the troubleshooting with this issue. Glad it's finally resolved!

Thanks,
Asif