Persistent issue with A/C

[calm_your_ham]

Hi all,

I’m hoping to get some insight on an issue I’m having and if anyone else has had the same/similar.

So I bought my A4 B8 in May 2025 and the car was sold with the A/C not working. The button would work as if the A/C had no issues, staying on when pressed, etc. I had the system checked by Halfords who told me that the system had about 450 grams of gas in it and held pressure so they only topped it up 100 grams, but didn’t empty the system out and so on, just recommended replacing the condenser unit which I didn’t do after they quoted me £400.

Fast forward to 25°C heats in June in Scotland and I thought I should get around to fixing it. I looked up the codes I got and looked through some forums and it all pointed to the infamous pressure sensor issue. I ordered the new sensor from Autodoc, replaced it, A/C still didn’t work but this time the button would only stay lit up when I held it, turning off as soon as I let go. One thing to note is when replacing the sensor, I didn’t get the little squirt of gas that everyone mentions.

Eventually I dropped the car off with a mechanic who did a proper recharge, pulling out 70 grams, doing a leak and pressure test and filling it up with 550 grams. The system supposedly held pressure for 20 minutes and the A/C finally worked as it should.

A week later, I was out with friends, A/C working fine, then overnight it was no longer working in the morning, but the button stays lit up like it did at first. So 2 days ago I left the car at the same mechanic who tested the system again and have now said that there is a leak in the system, but they did a bubble test and cannot find the leak anywhere and want to do a further investigation.

Just wondering if anyone has any ideas or had the same issue? I don’t know if they tested the sensor for leaks (I’ve seen a couple forums where that was the issue) but where could I be looking next? The car is throwing up the same codes about the lower limit being exceeded and “No Signal/Communication” with sensor. Could it be the sensor has failed again for some reason and leaked all the gas?

TIA everyone

[Magnet]

Welcome Xavier,

Before moving on, what brand of sensor did you buy from Autodoc?

Regards,

Gareth.

p.s. Any particular reason for buying from Autodoc with  their delivery charges? 

[calm_your_ham]

1 minute ago, Magnet said:

Welcome Xavier,

Before moving on, what brand of sensor did you buy from Autodoc?

Regards,

Gareth.

p.s. Any particular reason for buying from Autodoc with  their delivery charges? 

Hi Gareth,

It was a MAHLE brand sensor 🙂

The exact website I bought the part from was OnlineCarParts and I didn’t know they were an AutoDoc “reseller” from what I know, only found out when I got the shipping details saying it was coming from AutoDoc. Didn’t really concern me that shipping was £8

[Magnet]

Many thanks,

You should be OK with genuine Mahle.

You will find there are a number of sellers linked to Autodoc. 
‘Didn’t concern me that the shipping cost was £8’

You are not Welsh then! 
Just let us know when high shipping costs become something you can’t live with and we can suggest other places to try - if it helps. 
The no leaks found/ now leaking confuses, and I think you are wise to suggest rechecking the sensor fitting. 
Please let us know how you get on Xavier. 

[calm_your_ham]

Just now, Magnet said:

Many thanks,

You should be OK with genuine Mahle.

You will find there are a number of sellers linked to Autodoc. 
‘Didn’t concern me that the shipping cost was £8’

You are not Welsh then! 
Just let us know when high shipping costs become something you can’t live with and we can suggest other places to try - if it helps. 
The no leaks found/ now leaking confuses, and I think you are wise to suggest rechecking the sensor fitting. 
Please let us know how you get on Xavier. 

Only thing that did concern me was the new sensor having no sort of serial number or writing on it, it was just blank. It came in a Mahle box but obviously that’s easily done. I just didn’t think much of it until now seeing as the sensor change had altered something and was then working.

[cliffcoggin]

Bear in mind there could be a variable gas leak at the compressor depending on whether it is running or not. That would not be unexpected on a 15 year old car.

I was offered some sort of additive for the gas in my car that was supposed to reduce leakage, but as a cynic about such things I declined the offer. I have no idea what it is called so you would need to research it yourself if interested in trying it.

[spartacus 68]

Watch this video from SPR autos in Manchester. They are Mercedes specialists, but I think this illustrates the issue with air con generally, regas being seen as a fix by some and Halfords which just don’t cut it, you name it.

It’s a complicated bit of kit. A bubble test is just that, spray on and observe. Plus if you go for nitrogen test (they charge £40), then ideally it could be on for a couple of hours to see if pressure drops.

Condenser is a good shout. It’s taking brunt of rain and wind and general debris. Any leak on the reverse you won’t see. And if garage just check for 20 mins, then it won’t reveal true cause.

Pressure sensor, genuine is also a good shout. On Audi B8 it’s fairly accessible. SPR made tear mistake and had to strip it down again.

Would be useful to pull codes on the car. Search out specialist with real world reviews. Unfortunately these things are never cheap. 

https://mercedescarservicing.co.uk/youtube/