Battery Cable problems of the 1997 Ford Expedition

Check engine light comes on stays on. At first, I was told this was being caused by a gas cap not holding pressure. I purchased a new gas cap which seem to take care of the problem and the computer was reset by taking the cable off the battery. This last about a month until I was told it was the evap can solinoid, which was replaced under the extended warranty purchased. The computer was reset by removing the cable from the battery. This lasted about a month and was told to check the gas cap and the computer reset by removing the battery cable. The check engine light will go out with fresh tank of high octane gas, but will reappear with lower grade of fuel after a while. I am told now that 1 of 4 O2 sensors are bad and need to be replaced. Furthermore, it is recommended that all 4 O2 sensors be replaced ($150/ea) at the same time. I am relucntant in believing the cause of the check engine light as I have had this problem way too many times to throw down $700 on something may or may not work. Ohio emissions test will automatically fail your vehicle if the check engine light is on. There needs to be record some place so that owners can fight back if they pass their emissions test, but can't get the certificate if the check engine light is on. I have wasted too much money and time trying to diagnose and fix this check engine light.