A Gulfstream III had an engine fuel temperature indication that was pegged out high. Fuel temp sensors are resistance based. I was able to use the decade box to verify whether the bulb or indicator was faulty. I found that the indication was at it low limits around 75Ω and at the high limit around 125Ω. The bulb installed on the engine read “open”. The indication was correct after the bulb was replaced.
GIII ATA 28-08-00
Maintenace Manual 28-08-00