'Key not found' has nothing to do with the fob battery. I have this issue on another vag car and it is really Pi**ing me off.
The 'Key Not found' error on the mfd is given when the immobiliser in the car is not detecting the coded rf chip in the fob. This is entirely passive and independent of the battery.
As you bring the fob close to the ignition and push in the key, the chip inside the fob is scanned by the car to find the corresponding code, 1/2 of which is stored (coded) in the car. When a compatible code is found, the immobilizer is turned off and the car will start. Whilst the key not found error is displayed, the car will not start which could be very annoying on the road.
One issue is with non-genuine fobs coded to the car by an independent locksmith. Some non-vw passive chips are not as as sensitive at being detected. If you have a known genuine VW fob, see if you get the same problems with that.
I think my problem is their firmware in the car. When I had one genuine key married to the car I had no problems, when I had VW code a second key, I started getting these issues on both keys. You can have several key fobs individually coded and stored in the car immobilizer control unit and I think my problem occurs when their controller scans for more than 1 key. I have not yet got a 100% solution other than suggestions to replace the immobilizer control unit and have it recoded to the keys. I smell a rat that a new control unit could have different firmware and a bug fix I will have to pay for.
Another issue could be if you are using any type of fancy aftermarket cover over the fob which could reduce the sensitivity of the rfid chip inside. But again, this is unrelated to the battery.
Every time you get a 'Key Not Found' message there will be a fault logged in the ECU and Immobilizer control unit. These will not put on the MIL light or put the car into limp home mode, but anybody scanning the car will see the DTC and will have to clear them.
I apologize for reviving this old post. We had the same issue with my car, and I found out that the battery voltage on my remote was way too low, so I just replaced them, and it worked fine. Search "Volkswagen Key Not Found on YOUCANIC"; you can read about it yourself. I hope this information will help others who are dealing with a similar issue.
CrossFox 2015, key not found message appeared this morning. Open and close doors works ok, but when start the engine it stops after showing that message. Replaced the 2032 battery on the key. But issue persists. Any help? Thanks
Two versions of the standard black key fob are used on the EOS - early and late. There's a different keyless system called KESSI on the newest. Mine is early and the immobiliser part inside is passive i.e it doesn't use the fob battery which is only used for door unlocking. Some have posted they replaced their fob battery and the immobiliser function then worked, these may be the later version design where they could have used an active battery driven code system?
The (important) immobiliser system uses the dash and ECU modules. The ECU contains the 'look up' codes linked to the fobs passive (or active) code generator. Immo. codes are stored for each fob you own. There could be a module fault or a fault with the ignition lock sensing coil with reduced sensitivity? The first thing a dealer or auto locksmith will do is use special security access software to erase all codes already stored and re-program to match your fob immo. codes. If the problem persists. they move on to more expensive parts. Assuming you haven't got some kind of cover over your fobs which could shield it, hold a fob right up against the lock barrel and see if the flashing red immo. led on the drivers door goes out? That confirms the fob immo. code is accepted.
Having explained that, I have another VAG that had the same problem intermittently during Winter after I had a second fob added some years ago. Dealers (who added the second fob) have seen the problem but only offered expensive module replacement solutions without guaranteeing a fix. The car was fine before the second fob was added and my suspicion is the problem is related to ECU firmware? The problem has gone away for a while but if it persists I'll consider an ECU immo. delete.
I apologize for bringing up this old post. I recently bought a 2015 VW Golf; the key fob LED works fine, which means the batteries are good. But the receiver doesn’t respond. I just manually unlock the vehicle and remove the receiver to check. On my way removing the receiver, I saw that the connector was removed. I re-connected it, then tested, and it worked fine! I learned this here: search for "Volkswagen Key Not Detected Fix on YOUCANIC” and you will see an article that goes over common causes and possible fixes; you can read about it yourself. I hope this information can assist someone else with a similar issue.