Having a recurring "Key Not Detected" problem with a 2018 Mk3.5 Titanium with Start/Stop button. I would like to look for opinions as to what can be causing the problem.

Issue is that the car repeatedly fails to pick up the key inside the car and hence refuses to start with the push of the Start/Stop button. Funny thing is that if I push the key fob's unlock button to unlock the doors whilst I am sitting inside the car, then 90% of the time the car can start to recognize the key is actually inside and do the ignition. If that trick does not work, I dump the key into the centre console storage box to start the car.

Knowing that an insufficiently charged car battery is the usual suspect, I have already put the battery to charge overnight (the smart charger display said "full" at the time of disconnection). Still no luck.

Sometimes the Key Not Detected error message pop up whilst I am driving.

I have 2 keys and both have the same problem. As far as I understand, the transponder chip inside the key fob should be passive RFID and needs no battery to operate. Anyhow I replaced the key fobs' batteries, but the problem persists.

 I usually keep the key in my trousers pocket - quite sure the pocket cannot function like a Faraday pouch. 😃

Wonder if anyone has experience dealing with a similar problem? Is the problem likely caused by a defective receiver antenna hidden somewhere in the car and how can I diagnose which component has gone bad? Thanks.

55 minutes ago, Sim Lee said:

I have 2 keys and both have the same problem. As far as I understand, the transponder chip inside the key fob should be passive RFID and needs no battery to operate. Anyhow I replaced the key fobs' batteries, but the problem persists.

The transponder chip only works when you put the key in the emergency position.  Keyless starting requires a radio signal, which requires a fob battery.

However, if both keys are the same then it must be a problem with the car.  I think you're on the right track with a faulty receiver.  The front one is not far from the drivers knee, though I can't remember whether it's behind the dash or the centre console.  If the key is usually in your right pocket, you could try it in the left to see if it makes any difference.

8 minutes ago, TomsFocus said:

However, if both keys are the same then it must be a problem with the car.

Unless you have fitted poor quality batteries

25 minutes ago, TomsFocus said:

The transponder chip only works when you put the key in the emergency position.  Keyless starting requires a radio signal, which requires a fob battery.

However, if both keys are the same then it must be a problem with the car.  I think you're on the right track with a faulty receiver.  The front one is not far from the drivers knee, though I can't remember whether it's behind the dash or the centre console.  If the key is usually in your right pocket, you could try it in the left to see if it makes any difference.

I usually keep my key in my right pocket. Will try swapping to my left hand side to see if that helps.

I have previously used Forscan, but not for problem diagnostic. If I dig into the problem, would Forscan be useful in identifying which receiver is faulty?

Thanks.

How old is the car battery? If it's original it's done well to last 7 years and is due for replacement.

I only ask because when we owned a Mk3 with keyless start I had problems with it suddenly becoming unresponsive to the start button. Even charging the battery overnight didn't help. I decided to take a punt on a new battery, fully charged it before fitting and reset the BMS and that cured the problem.

2 minutes ago, Sim Lee said:

I usually keep my key in my right pocket. Will try swapping to my left hand side to see if that helps.

I have previously used Forscan, but not for problem diagnostic. If I dig into the problem, would Forscan be useful in identifying which receiver is faulty?

Thanks.

Forscan may or may not be helpful in this case.  As you already have it, it is definitely worth running a DTC scan to see if any key related codes come up.

21 minutes ago, DaveT70 said:

Unless you have fitted poor quality batteries

will swap to another set and check

thanks!!

the current car battery is less than a year old so I guess it should be relatively healthy. BMS was reset when it was fitted. I hope the car battery won't be dying that soon.

Quick question. Do you have a digital multimeter ?

1 hour ago, unofix said:

Quick question. Do you have a digital multimeter ?

yes, I took a voltage reading of the battery just now. it measured 12.5v.

It was the batteries in the keyfob I was more thinking about. They need to be 3.30V when new. Anything below 2.99V and they are finished.

To get an accurate check on the state of charge of the car battery you should ideally test it in the morning before you start the car. If the battery is only about one year old it's unlikely to have failed, but quite possibly needs a good 12 hour (or more) charge.

On 1/27/2025 at 6:14 PM, unofix said:

It was the batteries in the keyfob I was more thinking about. They need to be 3.30V when new. Anything below 2.99V and they are finished.

To get an accurate check on the state of charge of the car battery you should ideally test it in the morning before you start the car. If the battery is only about one year old it's unlikely to have failed, but quite possibly needs a good 12 hour (or more) charge.

Reporting back.

You are right. I put in a different batch of CR2032 into the key fobs this Monday. Drove for a number of trips in the past few days. The problem is gone.

The voltage of the old CR2032s that I took out measured 2.99V, despite having only been used for 2 weeks.

Happy days!

20 minutes ago, Sim Lee said:

The voltage of the old CR2032s that I took out measured 2.99V, despite having only been used for 2 weeks.

I too have been caught out with dodgy batteries recently, I have an oil level monitor for my central heating oil where the sender unit on the tank uses a CR2430 battery. As standard practice, I measured the outgoing battery, then the new one at 3.3v. It stopped working after 3 weeks, and another measure had seen it drop to 2.99v. It was an Energiser brand bought from Ebay with a battery seller with 100k + ve feedbacks.

I've now given up buying this type of battery online, and now purchase them from supermarkets who have a robust supply chain, albeit at a premium price...

I'll repeat what I have posted before, I too got tired of buying good brand Batteries that did not last so now I buy Supermarket own brand ones because no one will clone them. 

They have lasted fine for me so fair.