I've got a Mercedes C200 (W204) from 2011 (facelift model) with a working air con (air blows out) but not getting cold or hot.

I've taken it to a local mechanic that in less than 1h diagnosed it as a leaky air con and immediately said it needs a new condensator which will set me back about 2000€.

Now I'm not a mechanic by any means but am pretty handy and from the research I've done it seems to me like the way to determine where a leak is or if one exists at all is by using a dye and a UV light and/or a sniffer. I find it hard to believe that in the amount of time he had the car for that he did all that, but also I've gone around with a UV light after I got the car back from the mechanic and couldn't find anything myself, which leads me to believe he didn't use a dye at all, although he may have used a sniffer.

So I want to try to attempt a diagnosis/repair myself. I'm looking for some validation on the steps to perform based on my understanding of what needs to be done.

  1. Assuming there there is a leak, am I correct in the assumption that one should not be too concerned with air in the system so at this stage and therefore shouldn't get a vacuum pump? Also I don't know how much refrigerant is still in the system if any at all (and have no way of properly disposing of any that might be in there).

  2. Am I correct in assuming as well that at this stage it makes no sense to invest in a 3 hose system with gauges + vacuum pump and instead just get one of those cheap R134a bottles with a gauge of its own?

  3. I am finding it hard to find R134a bottles, instead mostly finding "R134a replacement" bottles like this, are they interchangeable and safe to use and - assuming there's still some refrigerant in the system - can it be combined with what's left in the system?

  4. From what I gathered now the process is to first connect the hose to the low pressure port, turn on the car and the AC system and measure both the pressure in the low pressure side as well as the air temperature coming into the car. If the pressure is too low then add some refrigerant (with dye) to the green mark, ensure that the compressor kicks in and that the temperature drops inside the car, disconnect the kit from the low pressure port, leave the car running for at least 20m to allow it to circulate through the system. A day later check the pressure and ensure it hasn't changed.

I've had this car since 2014 and as far as I know no maintenance has ever been done on the air con. So assuming that the pressure stays the same, I've now just introduced air into the system (arguably small amounts but still). Is this something I should be worried about? Is there a way of topping up the refrigerant without introducing air into the system that doesn't involve completely emptying out the system first and using a vacuum pump?

Of course if the pressure drops and/or I see a leak with a UV light then it needs to be repaired and that's a whole different story but at least this initial part is something I can do alone I think.

Any help would be appreciated.

  • Welcome to Motor Vehicle Maintenance & Repair! Jun 2, 2022 at 13:32

1 Answer 1


An air conditioning system will not affect the heating system. If you are going to diagnose this yourself, it is best that you find out why you have no heat. The heating system is easier and safer to diagnose and the problem could very well be the solution to the air conditioning as well. It could be a blend door actuator that regulates the temperature through the HVAC system. Make sure your engine coolant is full and there is no air in the system. I'm just assuming there is no overheating going on. Once those things are confirmed good, bring the engine to operating temperature, turn your heater on and both heater core hoses should feel hot. If they are and you still get no heat in the car you may very well have a blend door problem. Once you have heat and your air conditioning still doesn't work:

Does the A.C. pump turn on? If not you might then have pressure problems. On order to check the A.C. pressures properly and safely you will need manifold gages and research how to read them to determine what the next steps of diagnosing is required. Before I had manifold gages I always told myself that if I work on A.C.without the proper tools and information, there is a good chance that I would cause a lot of damage and could possibly get seriously injured. But I hope you may find your problem before you get to the A.C.

  • In regards to the heating... there's hot air coming from the left vent (driver side) but no hot air coming from the middle vent. I'm not sure about right vent (passenger side) as I haven't had many sit there during the cold months when this started being an issue. There is no overheating going on and engine coolant is full. I haven't checked whether the compressor is actually kicking in or not when set to cold, will check it later today.
    – JohnUbuntu
    Jun 2, 2022 at 15:54
  • Since heat is only coming out on the driver side vent you have a blend door actuator that is not working properly for the middle and passenger side vents. You still could have an AC problem as outlined in @Jupiter's response, but you should get this fixed first and see if that also resolves the AC issue.
    – MJH
    Jun 2, 2022 at 17:31
  • Got this online. It's supposed to be for calibrating the climate control for Mercedes. It would be the first step for me. "Press and hold the RECIRC a DEFOG button simultaneously until you get blinking lights. This lasts about 30 seconds and will reset the system which may help." If you can listen to the actuators and blend doors while they calibrate. It may give you a clue.
    – Jupiter
    Jun 2, 2022 at 17:42

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