Outer Limits 4x4 Board

The other day i pulled up my driveway and noticed that 2 warning lights came on. The battery and the subtank. The battery one i figured out and had it checked, it is the alternator

However the subtank light worries me. I threw some diesel in the tank and pumped it into the main tank, it appears to be working fine.

Tomorrow i will be going to get the alternator fix, but im wondering why the subtank light would of come on. I dont have the users manual so i dont know what it means when it turns on.

Any ideas?

I had the same problem in a 2002 GU ST CoilCab. If you disconect all batteries it cleared the fault with the sub-tank

this happens when alt plays up. fix alt and all is sweet.