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While pulling a 12.5k lb 5th wheel I had my overdrive light flash on 2 occasions and the service engione soon light came on as well. These 2 times were both while going uphill. When I got home I put used my ODB reader and got 3 error codes - P0221, P1211, P1783
This site has suggestions and then even more suggestions. Click on the Forums tab at the upper left and scroll over the list. There are several boards that can help you out: 7.3L engine, Super Duty, towing, etc. Try them out and I'm sure you'll get the help you need.