It could be a bad sender or bare wire somewhere. I can't remember the BOSS codes, one is sender not connected, sender out of range, short to ground etc. depending on your machine, the 2 and 6 should be written on a plate near the panel on older ones. Hopefully someone else will have the codes to help you out.
I copied this from one of Ken's old posts.
Boss Codes
AF = air filter, 3 = wire not connected
Codes are as follows
ECL engine coolant level
EC engine coolant temp
EP engine oil pressure
ES engine speed
AF air filter
H Hydrostatic fluid temp
b Battery (show on display in Lower Case)
FUEL fuel level
HF Hydraulic Filter
HP Hydraulic Fluid pressure
All these codes will be followed by a number, not all numbers apply to all codes
Example 1 is the engine shutdown code but there is no FUEL 1 or No AF1 because these are not serious failures.
Numbers are as follows
1 Shutdown engine in 30 seconds
2 Warning
3 wire not connected
4 wire shorted
5 High sensor voltage
6 Sensor no signal
7 Sensor out of range
If you see some sort of jiberish on the display you likely have a bad ground .
There are also code number 1.1 2.1 and 2.2
EC has a 1.1 shutdown and a 2.1 Warning code
ES hase a 2.1 warning code
b has a 2.1 warning bad battery and a 2.2 Warning battery voltaage
Therte are up to 4 relays
starter, glow plugs, park brake , not sure on 4th , I think it the fuel shut off soliniod on the Kubota engine models
Ken