V6 A Guide To Troubleshooting The Heating Cooling System


Having just gone through one heck of a time trying to figure out why my 2002 V6 Passat 4motion Wagon was not blowing warm air, I figured I'd try to put together a small "guide" to try and help people having the same problem. I did quite a bit of research and found some great information and some not so great information. With each search, I would find different information or suggestions, so I feel getting it all into one place may help someone else having the same or similar issues. Unfortunately it seems there are quite a few people having issues with no heat, or limited heat from the system.

Please note this is my experience with the V6 cooling system. I am a DIY person and enjoy working on cars, I've done timing belt jobs on several different VW / Audi engines and worked in the industry for a number of years. I am the second owner of my B5.5 Passat and know the service / maintenance history on the car. The car has had only G12 Antifreeze (pink) which I feel is an extremely important factor in helping diagnose what the problem may be.

The dealer will most commonly claim the cause of no / low heat is a bad heater core, or a bad thermostat. Both are extremely expensive repairs and very labor intensive. The parts in both cases are under $50, but the labor it takes to access either component is extensive. There are a variety of ways to troubleshoot the cooling system and help determine what the faulty component may be. It is also possible that the only problem is air in the system, a fairly easy DIY fix, even or someone with limited mechanical ability.

Here is a list of some common problems and cooling system components that fail:

Please note these are for the 30v V6 motor, while other engines may have similar problems, some of the components are in different locations and in some cases, easier to get at.

Cooling System Components:

Coolant Temp Sensor

The Coolant Temp Sensor has a high failure rate on both V6 and 1.8T applications, and can be the cause of some bizarre issues. The part is fairly inexpensive, so it is not a bad place to start. If the needle in your temp gauge never rises, this would be a good item to consider replacing. On the V6, the coolant temp sensor is located towards the passenger side, directly below the black throttle body pipe. Access is very limited, so i do suggest removing the throttle body pipe (less cuts on the hands, less frustration and only a few minutes of extra work). The newer versions have a green top section and are commonly referred to as a "green top" coolant temp sensor.


The thermostat and housing on the V6 motor are located behind the timing belt, directly underneath the passenger side camshaft pulley. Some say replacing the thermostat is possible without removing the timing belt, however I feel it is much easier to just remove the timing belt. If the car takes a long time to get up to operating temperature, or blows warm air at idle and cools off when driving, it could mean the thermostat is seized open. If the thermostat were seized closed, the car would typically overheat sitting at idle as coolant would not flow through the radiator.

Due to the location, the thermostat is quite a bit of labor. If you are having a timing belt job done, have the thermostat replaced. Quality timing belt kits should include a new thermostat and O-ring.

Water Pump

A bad water pump is typically fairly easy to diagnose. If the car is overheating at idle, a bad water pump or seized thermostat are likely causes. The water pump is also located behind the timing belt, and is just to the right of the thermostat housing. This is another part that should be replaced when doing a timing belt job and included with quality timing belt kits. The V6 uses a paper gasket to seal the water pump to the block and can be a common place for a coolant leak to develop. If doing the timing belt job yourself, make sure the gasket is set properly before putting everything back together. It is a horrible feeling putting the entire car back together and filling the system with coolant, only to find the gasket did not seat properly and you have a major leak. (ask me how I know…)

Heater Core

This seems to be the first conclusion for a bad part that everyone jumps to when dealing with heat issues on the B5 / B5.5 Passat. I feel the dealers set this trend (falsely) and have turned quite a few people away from VW's with the diagnosis. If the car has only had G12 coolant, then I feel the heater core is not likely the problem. If the car had other non G12 coolant mixed in, used water for a period of time, or was changed over to the typical "green stuff" coolant, then I could certainly see the heater core being the issue. If you smell a strong coolant smell in the car, it is likely the heater core is bad.

The heater core is located behind the dashboard almost directly in the center of the car near the firewall. The entire dash needs to come out in order to replace it. The parts are also typically under $50, however just as with the thermostat, the labor is extremely intensive to gain access so the cost to replace it is very high.

Climatronic / Flapper

The Climatronic is what controls the blower motor setting, temperature setting and direction of airflow in the car. While I have never personally had one go bad (5 different cars), I don't doubt that some do. It is a fairly involved system with a variety of different motors to adjust the direction of the air flow. The most likely cause for poor heat / no heat and the Climatronic unit being the problem would be a bad flapper motor. You should be able to hear the flapper move when changing the temp from full hot to full cold. If you can not hear this happening, it could be possible the flapper is stuck or broken.

A list of problems to try and determine which component is at fault:

**Important Note:

The V6 cooling system is notorious for getting air trapped in the system. The air can be extremely difficult to get out and often times can ends up trapped in the heater core. One way to try and determine if air is stuck in the heater core… Sit in the car (running) with the heat temperature set to high and the fan speed on a medium / low setting. Slowly rev the engine up to 2000 rpm and let off. If air is in system, you should hear a gurgling sound coming from the heater core.

I feel that air stuck in the system is probably the number one cause for people having intermittent warm / cold heat issues and should be one of the first steps taken to try and solve the problem. If warm air is coming through the vents sometimes, check for air in the system.

The previous owner of my car took it to the dealer because of the heat issues, they said that most likely the heater core needed to be replaced. This was NOT the case and fortunately he decided against having the work done to replace it.**

V6 Common Heating Problems:

  • No warm air through vents at all, always cold air blowing
  • Hot air with engine under load, but cold when engine is idling
  • Warm air blowing, but not hot air

Now, lets go through each problem and try to come up with a few possible solutions.

Problem: No warm air through vents at all, always cold air blowing

Possible solutions:

- Check that the car is coming up to temperature on the gauge. If the needle of the gauge is never rising, the coolant temp sensor or thermostat could be bad. If the car comes up to temperature and stays there at idle and during driving conditions, then it is not likely a thermostat problem or coolant temp sensor problem.

- Check that both of the heater core hoses are warm / hot when the car is up to full operating temperature. The heater core hoses are located under the battery tray, just to the right of the battery. There should be a corrugated plastic shroud around both hoses which will need to be peeled back / removed to access the hoses. Note the bleed hole on the left hose, this is one hole to bleed air out of the system.

- If the heater core hoses are not getting warm, or only one of the hoses is getting warm, it is time to check the heater core for flow and sediment. Remember you are working with a pressurized heating system, if you pull the hoses while the car is hot, expect to get some hot coolant on you. I am not responsible if you burn yourself during any of these procedures.

To flush the heater core, remove both hoses from the heater core and attach one length of long hose to each side. Ideally these will be long enough to work outside of the engine bay (roughly 4 to 5 feet long). Place one hose in a bucket (preferably white to see if any rust / sediment comes out) and use a garden hose / nozzle to force water in the other hose. If any sediment comes out, continue to flush the heater core until no more sediment remains. Take caution to not put too much pressure though the heater core, if you are getting sediment, then it is possible the inside may be weakened.

For those that would like to take things a step further, several people (including myself) have used CLR to help clean out the heater core. Pour CLR into the hose connected to the heater core until it starts to flow out the drain hose into the bucket. Let the CLR sit and then flush the system out again. In my scenario, I flushed the heater core with water and did not get any sediment. Just to make sure, I put the CLR in and let sit for 40 minutes… I still did not get any sediment coming from the heater core. At this point, I ruled out the heater core as being my problem. Ensure that all of the CLR is flushed out of the heater core before reconnecting the hoses to the cooling system. If sediment is coming out of the heater core, it is likely the CLR will bring quite a bit more sediment out. It may require several flushes and CLR treatment to get the heater core cleaned out. Note that if you are getting large amounts of sediment, the heater core may need to be replaced in order to get the system functioning properly again, but the cleaning may also save you from having to replace the heater core. An hour of your time may turn into a $1500+ savings.

Problem: Hot air with engine under load, but cold when engine is idling

Possible solutions:

Note: This was the exact problem I was having. After flushing the heater core, replacing the coolant temp sensor (was getting fault codes), replacing the thermostat, water pump and timing belt components (was due anyhow). I started up the car to find it came up to temperature much quicker, but still was blowing barely warm air at idle and extremely hot air with the engine under load. I think my problem all along was air trapped in the system.

Many VW dealerships now use a vacuum fill system to pull all the air out of the cooling system while refilling. If you do not want to deal with, or get sick of dealing with the bleeding procedure, specifically requesting the dealer vacuum fill the cooling system may be an option.

- Check heater core hoses on firewall, make sure both hoses are getting hot. If they are not, it may be worth trying to flush the heater core for peace of mind.

- Try to bleed air from the cooling system. The heater core hoses are the highest point in the cooling system, you may want to change that. Remove the (3) screws from the coolant reservoir and suspend the reservoir above the heater core hoses. A short bungee cord attached to the hood latch can help hold it up so it becomes the highest point in the system. I put a plastic zip tie through one of the holes on the coolant reservoir to hook the bungee cord into.

Slide the clamp back on the left (passenger) heater core hose. Note the weep hole in the top of the hose, the hole will be used to help purge air from the system. Pull the hose back an inch or so on the fitting but do not remove. With the car running and up to operating temperature, try to squeeze the hose so air is purged through the hole. If the hose is back far enough on the fitting so the hole is totally exposed, it may let air out of the system, but it can also let it back in. If the engine is revved during this, expect to see coolant shooting out of the weep hole when the throttle is released. The main thing I found to watch for were small bubbles coming when I squeezed the hose (think using soapy water to try and find a leak in a bicycle tire). Having a second person rev the engine up to 3k RPM, then let off… while I waited a second for the system to build pressure and squeezed the hose to purge air from the system. It took awhile (nearly an hour), but I was finally able to get the gurgle sound to go away from the heater core and the car to blow hot (not just warm) air both at idle and under load. Each little bit of air out of the system would give warmer and warmer air when the engine was at idle.

There is also a bleed screw at the front of the engine, just under the ignition coil. I did purge a small amount of air from this screw, but it seemed to cause more air issues so I stopped using it.

Problem: Warm air blowing, but not hot air

Possible solutions:

- Check for air in the cooling system, if no air in the system, then check for blockage in the heater core by flushing. Clean out sediment (if any) from heater core (see process above) and refill / re-bleed the cooling system. If there is still no change in heat, it may be time to consider a new heater core, but make sure there is no air in the system. If you flush out the heater core but still have not solved the issue and want to ensure there is no air in the system, have a dealer or indy shop vacuum fill the cooling system or purchase a kit to DIY.

Final Notes:

I do sincerely hope that this may help someone down the line. I spent countless hours and days problem solving and researching trying to determine what was causing the "no heat" issue. I did have some maintenance that I knew needed done anyhow (timing belt, water pump, thermostat, coolant temp sensor, etc…) so I do not consider the money spent on that as wasted, but more of a means to an end. I will not lie, when the car came up to operating temperature after replacing all the parts and still blew cold air at idle, I wanted to cry.

My best advice to anyone going through any of these issues would be do not assume the heater core is at fault, or the thermostat. Start trouble shooting one thing at a time and eliminate candidates that are known to be fine. A thermostat replacement or heater core replacement is major cash if not doing the work yourself. Even if you do the work yourself, it is quite a bit to tackle and I am fortunate the Passat is not my only car. I was able to work on things a little each evening for a week before reaching the solution.

Knowledge is power. Power is in numbers. I collected quite a bit of data from a variety of different sources and tried to put it all together in one place. I do hope that people will comment and continue to add other thoughts, ideas and solutions to this thread. With any luck, we can save others from spending large amounts of money on needless repairs.

Good luck!

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