Unless your lucky enough to find the cause is oil contamination (easily cleaned) or an obvious wiring issue, then you need to get ut plugged into a diagnostic reader and see if there is/are fault codes logged. As said, the D2 TD5 is not OBD2/EOBD compliant as it predates it. Find somebody with Nanocom etc.
Launched in 2009, the Discovery 4 kicked off in Series 1 form with a choice of a 2.7-litre turbo diesel, a more advanced 3.0-litre turbo diesel with a twin, sequential turbocharger set-up, and a 5 Boot Floor. Probably the most common area for rust on any Land Rover Discovery. I have been told by the previous owner that the boot floor has been replaced, but i haven’t lifted the carpet yet to check out how well it has been done. Edit: Pic added – boot floor is ok, just needs painting to stop surface rust. Check Engine Light due to failed leak detection pump or oxygen sensor. Problem with your Land Rover? Our list of 74 known complaints reported by owners can help you fix your Land Rover.