Code 33 on Onan 7500 watt generator

We have an '04 Fleetwood Expedition with an Onan 7500 watt diesel generator. All of a sudden it won't work. Used it less than a month ago and it worked fine. Getting a code of 33 which means overheating but it runs for a minute or less so that can't be the problem. Belt is good, fuses OK, lots of air going through it, oil and anti-freeze levels are ok. Any other ideas of things to check before we have to take it to Cummins and spend hundreds of $$$? It does not pull out so access is limited.