c5 automatic will not go into 4th gear
(Cincinnati, Ohio, USA)
I took a 100 mile trip and car ran as normal. On my return I found that the car would not shift from 3rd gear to 4th. I had to drive home on expressway in 3rd gear. I check my gauges and oil temp, coolant temp and transmission temp were all normal. Before I take the car to a mechanic, is there a diagnostic check list to see if the solution might be a simple fix?