Check Engine

1994 Plymouth Acclaim; 103K mileage; v6

February 07: Check Engine light appeared; Code 51 registered. Car mileage dropped from 23 MPG to 19-20 MPG. New O2 sensor installed

Mid August 07: Same as above. Was told that first 02 sensor was defective; new O2 sensor installed

November 07: Back to square one. Same as above. Same code – 51. Fluids checked, diagnostic done; same code – 51.

There has GOT to be something else that is going on. It’s virtually impossible for 2 O2 sensors to be ‘defective.’

O2 sensors seldom go bad, or at least as bad as one would believe from this forum.
Any rough idle involved here? A vacuum leak can cause a rough idle, a lean signal (which means the ECM tries to go rich), and a drop in mileage due to the ECM going rich all of the time.
An O2 sensor is easily tested with a DVOM so there should be no guess work.
Any other obvious symptoms; bucking and jerking, etc.?

Is it time for the newer car? Maybe. If we are going to guess what to change next, maybe it,s the fuel injectors too. Catalytic converter may be going. Good luck finding a mechanic these days who feels like fixing an Acclaim.

code 51 doesn’t indicate a bad oxygen sensor. it indicates a lean running condition registered by the sensor and it’s maxed out trying to compensate. i would suspect something else is wrong with your car which is affecting your mixture…

Check for vacuum leaks and have the injectors cleaned by a shop that knows what they are doing, not a quick-lube place…

There are several ways that additional air can get into the intake manifold, and not be measured by the MAF or the MAP, and cause the engine to run lean and have lean misfire: cracked vacuum hoses, open egr valve, stuck open pcv valve, brake booster vacuum hose (or, leaking brake booster), cruise control vacuum hose, stuck open iac (idle air control valve), intake manifold gaskets.