Temperature gauge

[cully]

get some diagnostic equipment on there as the hdi use's a second temp probe to measure the temp for ECU reference

so the gauge sensor may be faulty but the Ecu sensor could be good compare the two to prove the gauge is telling the truth
both sensors are in the same package (blue plug right hand side of block in the thermostat housing)

but its probably the thermostat is stuck open like fuzi has said