The PCM uses the engine oil temp sensor (EOT) to determine wait-to-start time. Jon is correct that glow plugs will remain energized after the light goes out. You may be able to find some data as far as resistance-versus-tempurature so you can ohm out your sensor to see if it is at fault, or potentially a broken or shorted wire to the sensor may be at fault. If anyone has better ideas, I'm sure they'll chime in.