Another day another puzzle!

[spanishfly]

First trip out in 14 years, Dehler WBX originally 2.0 lite Oettinger though replaced with DJ driving down a motorway, temperature gauge falling gradually over 2 or 3 hours...?
Heating working fine and not loosing water so assuming not airlock.
Gauge now reads minimum, checked a spare but maybe wrongly wired binnacle also reads cold and doesn't move.
Q. Do sensors fail slowly or is it more likely to be a wiring issue?
Thanks Hive mind....

Problem solved, wire came adrift under plug cover...!