Check engine light

2000 Mercury Sabel. Currently 85K. Four times on long trips after filling the gas tank the check engine light comes on and won’t go off until the oxygen sensor is replaced. Seems to occur every 12-24 months. Only long trips and opnly when fueling on long trips - 150 miles or more. Seems to be when we squeeze the last bit of fuel in. My mechanic says it is a coincidence.

You have found the solution to your problem. You state “Seems to be when whe squeeze the last bit of fuel in”. Stop filling the tank when the pump clicks off. By pumping in more fuel, you are overtaxing the fuel recovery system, messing up the charcoal filter, and probably doing in the oxygen sensor. I disagree with your mechanic that this is coincidence. Either you didn’t tell your mechanic this tidbit of information, or he likes to sell oxygen sensors.

Please have the check engine code read and report back the exact number of the code, then we may be able to help. I suspect that the 02 sensor is correctly reporting that there is a problem, and it (the messenger of bad news) is being replaced when it is not the part really at fault.

Thanks for your comments. I figured overfilling had something to do with it. What code should my mechanic be looking for?

Overfilling the gas tank can cause a DTC code dealing with the EVAP (evaporative emissions control) system. There are several such codes.

What code should my mechanic be looking for?

He should look at all the codes.

You can buy a basic, vanilla OBD-II code reader for about $65 at AutoZone or other chain parts stores. Why not get one and throw it in the glove compartment? The OBD-II connector will be under the dash, and can’t be confused with much of anything else. When the check engine light comes on, you can use the reader to read the codes yourself and also to clear them (read the manual). That’s less trouble than a trip to the mechanic. The codes for EVAP problems P044x are different from those for a bad O2 sensor.

One or no bad O2 sensors in 85K miles would be a reasonable expectation.

As others will tell you, you really shouldn’t try to squeeze the last drop of fuel in. Conceptually, the problems from topping off should come from the EVAP canister getting flooded with gas, not the O2 sensors failing. But your wallet probably won’t notice much difference between the two.

You should also know that EVAP systems even without abuse are notorious for reporting problems that go away. The drill is when you get a P0440 or similar code, you tighten the gas cap, clear the code, and hope it doesn’t come back. If it does come back, you buy a new gas cap, clear the code, and hope it doesn’t come back. Only when you get an EVAP code that comes back persistently do you need to consult a mechanic.