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2002 MPV with extremely rough idle & P051 engine code

Greetings - the check engine light was on in my 2002 Mazda MPV (80K mlles) for a couple of weeks with no symptoms, but suddenly the car developed extremely rough idle below 1000 rpm. Runs okay otherwise. The local garage reported that the code was P051 (front O2 sensor), but they thought cleaning the throttle body & valves was the best solution … except $150 later, it didn’t (if anything, idle even more rough, light still on). They now think it is a valve problem not worth fixing. Checking the records, the MPV is due for a new air filter. Should I ask them to replace this and the oxygen sensor ($220) next?