Key fob won’t start the car

[Grantstoltz]

I’ve had my 12 A7 for almost for years. About a year into ownership, I started getting the “key not detected…” error on the dash and since then I’ve had to hold the key fob next to the DVD player to start the car.

It’s worth noting that the key fob unlocks and locks the car w/o problem, but the car doesn’t recognize when the keys are close and let me unlock the car when pulling on the handle - I have to press the unlock button on the key fob.

Is this likely a problem with the key (I’ve changed the battery about a million times) or is the a specific module I need to look at replacing?… if so, where does that puppy live?

Thanks in advance!


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[WideSuppie]

How many keys you have? I have 2 and changed only 1 key battery. Kept having reading issues. Changed BOTH and works.

[A6sport]

Is your 2012 A7 prestige trim?

[Alabama]

Is this likely a problem with the key (I’ve changed the battery about a million times) or is the a specific module I need to look at replacing?… if so, where does that puppy live?

Have you scanned the car for error messages?

[Grantstoltz]

Is your 2012 A7 prestige trim?

Nope - premium plus


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[Grantstoltz]

Have you scanned the car for error messages?

Yes, but nothing pops out specifically pointing at this issue. Is there a specific module I should be looking at in VCDS?


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[A6sport]

Nope - premium plus


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That’s the reason why you can’t open the doors without using the key fob MY 2012 premium plus did not have the keyless entry option.


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[Avantly]

Is this the advanced key (i.e. you can keep your key in your pocket)?

If so, holding the key up to the key spot energizes the NXP chip inside the fob so that it can handshake with BCM2. So, if it doesn't work while the key is not near the spot, you either have a problem with the RF circuit inside the key fob, or power to that chip... The same battery powers both the remote keyless entry (button press) as well as the NXP chip. If the button press works but prox start does not, then I'd take a look at the circuit board. You can pull the board out after you remove the battery carrier. It can be a little tricky, there is a detent in there but by grabbing the edge with stout toothed tweezers and wiggling it, it should come out just fine. The remote keyless entry (button press circuit) is on the opposite side of the battery, and the advanced key/prox circuit is on the battery side. \
I'd clean the battery contacts with a cotton swab and iso alcohol while you are in there, and if the battery contacts seem loose (test fit a battery with the board out) you can bend the base contacts up slightly (very carefully! not much tension) and bend the outer contacts inward slightly with your fingers.
Take a look at the coil in the prox circuit- It is a large square inductor looking thing, usually a white top, with winding visible on the sides. Gently grab it with your fingers and try to tilt it in all directions - Has it come desoldered from the board? Inspect the solder joints for all these components on the battery side - Is there any contamination visible? Do any of the solder joints look cracked?
The components you will see on this side: The (probably white) square coil - Prox coil; Small brownish 2-terminal device - Oscillator for the prox transmitter circuit; Small 8-pin chip - Prox transceiver IC; Small 3-terminal device - Prox RF front end transistor; Fat, rectangular chip with a bunch of pins on 2 sides - This is the NXP PCF7945AC chip with integrated antenna. You will also see a number of small (0402) components, and 3 of them are on the antenna trace at the end of the circuit board - You will see an interesting looping design of circuit traces on the opposite side of the board from the battery. Are all these tiny devices intact? There will be spaces for around 8 of them, but only 3 are populated, you can tell if one has broken off the board because it will leave indentations in the solder on the tiny pads.

Anyway, maybe visual inspection will reveal nothing but if holding the key up to the key spot still starts the car then this indicates that your NXP chip is still functional and maybe something is wrong with the prox transceiver circuit. That black rectangular chip with all the pins on 2 sides is actually capable of starting your car on its own - i.e. you could cut it off the board and hold it up to the key spot and it would still start the car. So if that's working, it seems that the problem must be outside of that chip.

[Grantstoltz]

Thanks for detailed write up! I followed your direction and didn’t find anything that stood out as problematic. Even compared the physical structure to another non-working key and there was no difference. Cleaning the battery contacts didn’t help either.

Is there any chance is a problem with the car and not the fob?


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[Alabama]

So, if it doesn't work while the key is not near the spot, you either have a problem with the RF circuit inside the key fob, or power to that chip.

Are there different antennas in the car to receive the "door unlock" signal and the "turn off immobilizer" signal from the key fob? Otherwise from your description, seems like time to buy a new key fob.

[Avantly]

Grantstoltz, IIRC the same sync data used for the immo transponder in the key is also used for the keyless entry buttons, so I think they key is OK. I therefore consider another possibility - OP is on to something regarding the possibility of the malfunction being on the car side - Reading the key via the prox antenna (and unlocking the doors by touching handle with key near) requires the car's active antenna system (I believe it is 5 antennas in A7) to be functioning correctly. The proximity key function and the keyless entry (button) function use different antennas. Before assuming that anything is wrong with the fob, I think you need a full scan to see if there is anything going on with the active key system in the car. Do you have, or know anyone with a VAG-COM tool? Or OBDEleven. You will not be able to scan the necessary modules with a standard OBD scanner.

Another question I forgot to ask is how many key fobs you have. If you have two, and they are both having the same problem, that is a very big hint that the problem is on the car side.

[omgContra]

When was the last time you changed the key fob battery? Could be as simple as changing them out, even though it still unlocks the car the battery might be low enough that it can't draw enough from the battery to get max range.

[Avantly]

When was the last time you changed the key fob battery? Could be as simple as changing them out, even though it still unlocks the car the battery might be low enough that it can't draw enough from the battery to get max range.

If the fob battery has enough energy to unlock the car with the buttons then IMO it will start the car from inside the cabin no problem. A low fob battery will also log a fault code in the BCM (it will tell you which fob has a low battery if you have multiple fobs).

[Grantstoltz]

Grantstoltz, IIRC the same sync data used for the immo transponder in the key is also used for the keyless entry buttons, so I think they key is OK. I therefore consider another possibility - OP is on to something regarding the possibility of the malfunction being on the car side - Reading the key via the prox antenna (and unlocking the doors by touching handle with key near) requires the car's active antenna system (I believe it is 5 antennas in A7) to be functioning correctly. The proximity key function and the keyless entry (button) function use different antennas. Before assuming that anything is wrong with the fob, I think you need a full scan to see if there is anything going on with the active key system in the car. Do you have, or know anyone with a VAG-COM tool? Or OBDEleven. You will not be able to scan the necessary modules with a standard OBD scanner.

Another question I forgot to ask is how many key fobs you have. If you have two, and they are both having the same problem, that is a very big hint that the problem is on the car side.

I do have vag-com but need some guidance into what module to look at that might represent the problem. Let me know and I can dump the output from that module.

Thanks again for the thoughts on this!


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[Grantstoltz]

Fixed it!

Ran vcds and found 03283 in the start auth module. Googled it and found that I should look behind the rear left door panel. Sure enough the cable was unplugged running to the BCM module. Good to go now! Thanks again everyone!


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[A6sport]

Fixed it!

Ran vcds and found 03283 in the start auth module. Googled it and found that I should look behind the rear left door panel. Sure enough the cable was unplugged running to the BCM module. Good to go now! Thanks again everyone!


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Good to hear, but tell me do you have the keyless entry option? IIRC, it was not available on the MY 2012 PP trim cars?

[Grantstoltz]

Sure doesn’t seem like it, no. Fixing push to start didn’t allow me to open the doors without pushing the unlock button.


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