Onan 9kw starts and then quits

Last year my onan generator quit working. There is a blinking light code which told me it wasn’t pumping raw water. I changed the impeller, which was missing 9 fins, and it ran great for the rest of the year.
This year, it starts and runs for 30 seconds, and then shuts down. The manual said heat exchanger, so I pulled it and cleaned it out(found the rubber fins from impeller.)
I believe I’m getting an over heat code- I think as it’s just continued slow blinking lights
Any suggestions would be greatly appreciated

I am surprised after only 30 seconds of running there is a temperature issue. I would suspect something else.

Have you checked the fuel filters? no restrictions on the fuel lines?

What model of Onan 9k?

Many have a 30 second delay on the shutdown circuit. So, it could be, for example, the low oil pressure switch/circuit, or the 120v output check circuit, or, as you are guessing, the high temp switch/circuit.

I'll feel better commenting once I can look up the schematic, but normally the safety switches alarm by shorting to ground. So, if you very briefly disconnect the wire for each safety switch and keep it insulated from the block and ground, you can see if it runs longer. Of course if it is overheating or low-pressure, etc, there is an obvious danger.

Personally, when I do this type of thing, I like to put a mechanical gauge on oil pressure and measure temp with an IR temp gun. They are cheap at Harbor Freight, Amazon, etc, and it let's me verify that it is a false alarm vs real problem.