Cooling System Problems?

Check engine light on, thermostat/coolant temperature sensor failure

Tutorial: Thermostat Assembly Replacement

Vehicle: 2004 Audi A-4 Quattro 1.8L Turbo


This vehicle has no drivability concerns at all. the check engine light is on with the codes displayed above. CTS (coolant temperature sensor) signal values are within range in relevance to the ambient temperature. The reason for the diagnostic trouble code to even set is from prolonged warm-up cycle. Most likely the thermostat is hanging open slightly or out of range causing it to open early. The CTS have failure also and do the location of the part it is better replaced as an assembly and also sold as an assembly.

Thermostat Assembly Replacement:

First you need to drain the coolant. The petcock is located at the lower radiator hose fitting (left lower part of radiator). If your having trouble removing petcock drain plug, don’t force it or even break it. Just remove the coolant fan switch sensor above it and the coolant will drain just fine. Remove the upper radiator hose. Remove the coolant reservoir, there’s one screw holding in place and a sensor you need to disconnect below the reservoir bottle. You can keep coolant hoses attaches just set the reservoir to the side out of your way. Remove serpentine belt using either special serpentine belt removal tool or a large flat head pry bar, but be careful as to not break the tensioner. Wedging it between the pulley and the tensioner bracket. After you release the tension off the belt do not completely remove the belt just slide it off the alternator and leave it in its place for quick re-installation. You now need to disconnect the battery but before doing so make sure you’re not going to end up with a radio lock code. If so use a memory saver but be careful because your circuits will still be live. It’s a good practice to always use a memory saver when disable an auto electrical system as to prevent things such as drivability concerns. In some situations it will require complete power disassemble but then it just need to explained to the customer properly. Remove the four 5mm Allen bolts holding the throttle body to the upper plenum, disconnect drive motors connector and remove. Remove the four 7mm Allen bolt holding the alternator, disconnect reference connector and power cable then remove the alternator. When removing the throttle body take care to the gasket behind it as it’s metal and re-usable as long as you don’t crease it or anything.

Now you have a lot better access to the thermostat assembly, heck you can actually see it now!!! Disconnect coolant temp sensor connector and remove the 10mm bolt holding the coolant by-pass metal pipe. To completely move the coolant pipe out of your way you will need to also remove the 13mm bolt located down the pipe towards the firewall, below the intake plenum. Do not remove the pipe , just move it out of your way. Slide off the lock clip for lower radiator hose at thermostat assembly. Slide hose fitting off the thermostat housing assembly. There is still a smaller hose above the housing. I find it easy to remove the two 5mm Allen bolts holding the housing assembly to the block. This way you can pull the housing out and get better access to the hose clamp. Remove the hose clamp tension and then remove housing and replace it. For installation just follow these procedure in reverse. When filling a cooling system I usually use a vacuum fill system. See picture attached. The vacuum fill system makes it easy and quick. You will not have to worry about bleeding the cooling system. This vehicle has a coolant bleed port at the upper radiator hose metal cross over pipe. See picture attached. Always look for a coolant bleed port or even check the factory service manual for one before using the vacuum fill system as this is the best way to fill the cooling system if available. Clear your codes and run the vehicle till fans cycle twice. Monitor your CTS signal and make sure vehicle is reaching normal operating temperature. Congratulations you should know have the procedure down and this is actually a pretty common failure that pays good time. My book time shows 2.7hrs and diagnostic time if needed. I usually will not charge diagnostic time for an easy problem such as this. I don’t consider hooking up a scanner and pulling codes, diagnostics.

This post was written by: Martin Hand


If you find this information helpful please consider a donation. These articles, questions and comments are very time consuming so even a small donation gives me motivation to keep educating automotive owners. Donations will allow us to continue open questioning/comments, automotive education and repair tutorials in the future as the business grows. All proceeds go to the expansion and maintenance Thank You

Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Muslimah says:

    Thank you very much for all your videos and help. I have ornagiil radiators on my 1997 528iA with 223000 km and 1998 M3 with 128000 km. Generally, the 6 cyl BMW cooling is good (except for the plastic pump impeller, which in case of my 1997 was metal as I pulled it out and checked), What this video didn’t cover is the electric AC fan. I had the resistor for the first speed failed so it was always off when AC was working and which may have resulted in the overheating.

Leave a Reply

Your email address will not be published. Required fields are marked *