So, I bought a 2006 x-type a few weeks ago, and it's been pretty great. However, randomly and out of nowhere, yesterday the car would not start due to the immobilizer. I could unlock, lock, alarm/unalarm the car, but it would not start.

I have a second key, but it was at home and not on me, so I don't know if that would have done the job. We disconnected the battery, waited ten minutes, and all was well (-ish). I'd really like to troubleshoot this because the possibility of the car just not starting for funsies after I drive it somewhere is a bit concerning. The fact that it worked just fine before, and then didn't work, and then worked after a battery reset makes me really worried the problem is something other than the key.

Doing a bit of searching around, it seems like a lot of people have had this problem, and never quite managed to solve it, or they found short term solutions, but they would inevitably stop working.

It looks like there are a few likely trouble spots-

• Key transponder is wonky
• Ignition receiver is wonky
• Computer is wonky
• Electrical woes (??)

I saw guides to reprogramming the fob (doesn't seem relevant) and the key (does seem relevant) but I'm reluctant to monkey around with that until I have a better sense of what the problem is and if that solution makes sense. What can I do to find out what the source of the issue is?

 

Are you getting “Security Failure” message when you turn the key and it doesn’t start? Also check the ignition key hole gate. It might be stuck or broken.
I have an OEM original key (that came with the car) fob that for some reason, the transponder in the fob is no longer recognized by the ignition. Just a thought

 

Yep, it got the security failed message. It hasn't since, but it's troubling.

 

The FOB, meaning 4 button remote to lock and unlock doors, has nothing whatsoever to do with the security/immobilizer. You can remove it and leave it at home and will have no (additional) problems starting the car, so as you mention programming it is not relevant to this problem.

The key shell has a transponder chip in it. Some inductive field magic happens (which is why the transponder chip needs no battery) and the ignition receiver reads a series of bits from the transponder chip. The code on the chip must match one of the codes stored in the ECU. The dealer or a good locksmith can alter which codes are acceptable to the ECU. It sounds like you already know this but I'm just trying to be thorough. I agree with your analysis but I would say wonky transponder is highly unlikely unless you have somehow physically damaged your key. The transponder is sealed, it doesn't rust, it doesn't deteriorate due to UV rays or humidity, etc. Very robust. I would add in 1 other possible trouble spot: interference. Do you have a key to a different car on your key ring? It could be you randomly ended up with the other car key very close to the Jag key and the receiver read the code for the wrong transponder while normally the other key would hang down and be far enough away to not be scanned. Hoping it is something that simple.

 

As dh53 said, transponders don't fail. They CAN be DAMAGED, but if the glass tube is intact, they work unless subjected to some SERIOUS electromagnetic fields which kill the chip.

For an intermittent failure, I would check dh53's suggestion that another transponder was interfering -- it does NOT have to be a Jag key to do this. If this isn't possible, work backward from the loop that reads the code, key, looking for damage or corrosion.