I received a MIL while driving the other day. I had it read and the code came up: P0421 - Warm-up catalyst system efficiency below threshold. According to the FSM, this indicates a faulty rear (downstream) O2 sensor. This is the first O2 failure I've had in the 15-year life of the car - not bad.
I purchased a new (4-wire) sensor for $98.22 including state theft. This O2 sensor screws into the front pipe which connects the front catalytic convertor (what Mazda calls the Warm-up 3-way catalytic convertor) to the rear (downstream) catalytic convertor. I put the car up on jack stands and went to work. I really hate working under the car this way.
Now, removing either O2 sensor from an aged exhaust system can sometimes be problematic (rust, corrosion, etc.) but in this case the old device came out relatively easily and in one piece. Why? Because about 5-years ago I replaced my rear (downstream) catalytic convertor and at the same time had to replace the front pipe which means I had to remove and reinstall the O2 sensor which probably loosened things up.
I installed the new device and now everything is back to normal. No more MIL.
Talking to some techs I was advised that if one O2 sensor malfunctions, the next one is not far behind so soon I will replace the front (upstream) device, as well, which screws into the exhaust manifold. The front sensor is much more expensive which is why I'm holding off for awhile. However, a failed front sensor is more disruptive than the rear since it has more of an effect on maintaining the proper air/fuel mixture.
All in all a fairly straightforward procedure since I didn't have any removal problems.
Happy Motoring!
I purchased a new (4-wire) sensor for $98.22 including state theft. This O2 sensor screws into the front pipe which connects the front catalytic convertor (what Mazda calls the Warm-up 3-way catalytic convertor) to the rear (downstream) catalytic convertor. I put the car up on jack stands and went to work. I really hate working under the car this way.
Now, removing either O2 sensor from an aged exhaust system can sometimes be problematic (rust, corrosion, etc.) but in this case the old device came out relatively easily and in one piece. Why? Because about 5-years ago I replaced my rear (downstream) catalytic convertor and at the same time had to replace the front pipe which means I had to remove and reinstall the O2 sensor which probably loosened things up.
I installed the new device and now everything is back to normal. No more MIL.
Talking to some techs I was advised that if one O2 sensor malfunctions, the next one is not far behind so soon I will replace the front (upstream) device, as well, which screws into the exhaust manifold. The front sensor is much more expensive which is why I'm holding off for awhile. However, a failed front sensor is more disruptive than the rear since it has more of an effect on maintaining the proper air/fuel mixture.
All in all a fairly straightforward procedure since I didn't have any removal problems.
Happy Motoring!
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