Odd inconsistent no-start
 

I've had this issue for a while now, but only recently has it become common enough to become a concern. Sometimes, when I go to start the car, I get a "click" from the starter relay in the false bulkhead, lights/acc turn off as usual when starting, but I get no crank. Often, turning the key back to off and trying to start again solves it and it fires right up. Occasionally it may take up to a dozen cycles of this before the engine actually cranks... This used to happen maybe once a week or less, but now seems to happen almost every day.

I've tried to do some diagnostics but hit a wall on what to check/research next.
There are no OBD codes stored (except P1111).
When starting and it fails, holding the key in start position for a few seconds doesnt make any change. Trying to "defeat" the little gate that won't let you turn to start more than once per key cycle, by releasing the key just before it locks, doesnt matter either (it still clicks and turns lights off, but doesnt start).
When the failure occurs, the security light isn't flashing, and there's no error or lockout codes on the console (which I would expect if it was a transponder read fault).
Waiting with the key in "on" for a few seconds before turning to start *seemed* to make a difference before, but not so much anymore.
I replaced the starting relay in the false bulkhead just in case, and that didn't make any difference.
Only got one key to try, so no way to test if it's a key fault for now.

Any ideas where to go next? Probing the relay/etc with a multimeter to read things while trying to start gets a bit tricky, needing a helper, and for the fault to actually occur that time.

It may sound odd but you may have a ground problem. Since you have replaced the relay I would recommend removing, cleaning and re-installng the ground strap that runs from the engine block to the chassis. It is located about 18 inches or behind the passenger side front tire. It is possible that the ground strap is corroded and not making a consistent ground. Free simple check.

I would make a jump lead for your start relay, that way you can test the starter motor and start solenoid to remove them from being the problem.

i have a similar problem where it won’t crank at all with the key but if you bypass the start solenoid it cranks. I have tried to no avail to find the problem which I suspect is the BCM module and has been at the jaguar experts for some weeks with no results.

If you leave the key in position 2 and jump the starter relay and it starts you will be able to narrow down the fault.

I'll double check the engine ground strap next time I'm under the car - thought I serviced it less than a year ago (cleaned, wire brushed, and greased contact points) and the car's pretty clean underneath rust wise. Always possible one of the bolts loosened I suppose, but it's never had a struggling/slow start, only no start, or starts perfectly.

On the relay bypass wire, I've actually got one in my glove box from diagnosing fuel pumps a while back. Hopefully I never end up needing two jumpers at once! I'll have to try the bypass method sometime if it becomes more frequent/reproducible of an issue. It makes sense that it could be BCM related since that is in the sequence of modules involved in the starting circuit. I'd expect though that it would throw a module fault code if that caused an issue.

It also occurred to me that I could run an LED or something wired to the relay coil so I could watch in realtime if a signal is being sent to turn the starter on, and it's just the starter itself not reacting. A bit involved, but if it's what it takes to find the solution.

To top it off, every engine start since I started this thread has worked on the first try. *shrugs*