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I need help. I have a Mazda 3 2004 2.0L with a code P0421, according to Mazda obd2 definition the code is set by monitoring the rear sensor
heater oscillation with the front sensor, but my pcm is not turning on the heater and I though that the computer driver was bad, but I can turn on the driver heater with my scanner. My question is what is the criteria for turning the heater on or it should be on since I turn on the car?
The test monitor is TID $80 CID $D8 Read value: 1.00, Min: 1.50 and Max: 255.993. What I have done:
1. Change the O2 sensor
2. Checked the O2 heater 12V
3. Circuit integrity is OK and is a pull down circuit at the pcm
Thanks for your support.
Welcome to the forum!
For anyone reading along: "P0421 Warm up catalyst system efficiency below threshold."
That code is indicative of a failed catalytic converter.
The PCM compares the front o2 signal to the rear o2 signal so that it can determine if the catalytic converter is still working.
A large exhaust leak near the sensor, or the sensor being loose could mistakenly set this code, but it would be a very evident leak.
As for why the PCM isn't turning on the heater, I'm not sure yet.
How are you testing it?
Some vehicles don't turn on the heater for the rear oxygen sensor(s) until the vehicle is being driven. If the rear sensor signal on the scan tool is responding when driving the vehicle I wouldn't focus on the heater system but rather checking the signal while at cruise to see if it switching rich/lean.
Like Noah said, there are some variables like exhaust leaks, misfires or the rare faulty oxygen sensor setting a cat code, but in 15 years of troubleshooting I would say that 90% of the cat codes I have seen were due to a failed cat.
Noah and trouble shooter are correct unless you have a huge exhaust leak then it's very much likely the converter to follow trouble shooters suggestion to verify . If it is indeed the converter it may be worth a try to "clean" it I have had fairly good luck with this at a success rate of about 50% what I use is a can of seafoam and hose to hookup to a vacuum port on the intake (hose must have either a couple y connections or a few 45 slits in it to allow air yo mix with cleaner and to slow down the flow rate to about 10 mins for the whole bottle) after finishing this immediately take it for a drive and run it hard and don't be surprised when it smokes out your entire block for a few minutes
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