Hello, first time post, long time lurker.

I have a 2003 XJR that keeps giving me a failsafe, limp mode and P1631 code. Usually happens after 40+ miles of driving, happens quicker and more frequently in warmer weather, almost immediately on hotter days.

Clearing codes with a handheld and waiting 30s-1 min allows you to drive again, sometimes for a few minutes, sometimes a few feet.

Worst instance it would crank, stumble
and stall. 90+ degree day, car in the sun. Had to remove the ecu and put in AC. Plugged back in and
made it 15-20 miles before it went out again.

Suspected a short in the ecu that flares up with heat but board looks perfect. Any ideas?

Forgot to add - that’s what I found when googling the code - not a lot of posts about it - I swapped relays twice to make sure it didn’t change. Definitely heat related - thinking g a failing ecu box fan may be the root cause...

Update - the small pc style fan that blows air out of the ecu box didn’t seem to be moving much air. I wired a larger pc fan in and it’s pushing a LOT more air out. Made it home yesterday with no failsafe or faults - fingers crossed!

Another update. Larger fan did not help. What else could it be??