Most oxygen sensors last about 60000 miles before needing replacement.
Replacing the sensors is a good thing ...... Not one but all four is best.
The sensors are part of the circuit that controls the fuel mixture. This could definitely foul a plug or cause a misfire due to fuel mixture. Too rich a mixture will cause a buildup of carbon in the cats and to lean a mixture will cause them to run hot and possibly burn the honeycomb inside. Not to mention fouled plugs.
Needless to say fuel mixture is critical. Does this Vehicle have plug wires? It may be time for a new set.
On some newer vehicles there is a coil pack (module) for each cylinder. In some cases one coil pack for two cylinders. I don’t know your vehicle. Have the hi-voltage ignition leads / coil packs checked. Some times you can see the ignition leads arcing at night. A sure sign that the insulators are dry rotted.
I recently replaced the oxygen sensor and complete exhaust system on my 1990 300SE. Including the cat. What a differance!
I did this after a tune up with plugs wires cap and rotor and still failing the emmissions test.....
When I put the car on the Dyno I found the power curve droping off over 3300 RPM. I cut the cat open and the honey comb was melted ....and broken-up in pieces. The car has 135000 miles on it. The new exhaust made all the differance.
Did they check the emmissions before and after? Was it Dyno tested for performance?
-A