Small Engine Troubleshooting Problems? Ask a Mechanic Now.
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The first thing that I would look for is a wire that is shorted or grounded which would cause this. If you find nothing there then I would look at the sending units for each of those gauges.
I am in agreement with you that it is doubtful that both sensors would go bad at the same time but it could happen.
At this time I don't feel that I will be able to help you any further, I will opt out of your question which will allow for another expert to assist you if they feel that there is anything they can add.