Check engine light on with code P0421, how do I fix it?

Hi,

The code is related to the warm-up catalyst operating below the threshold. The most common causes for this code are a faulty or worn catalytic converter, a faulty heated o2 rear sensor, an exhaust leak before the upstream sensor, or the PCM itself.

Here is how it works. The signal from the rear heated oxygen sensor should differ from the front heated oxygen sensor. That is because the catalytic converter purifies exhaust gas. When the catalytic converter has deteriorated, the signal from the front heated oxygen sensor becomes similar to the rear heated oxygen sensor. Once the PCM recognizes this, the code is set.

So, by any chance do you have a live data scan tool that we can compare the front and rear oxygen sensors? Also, are there any bad odors coming from the exhaust?

Let me know. Also, let me know if there are any exhaust leaks.

Take care,

Joe

Sounds like a plan. Also, if you have a live data scan tool, watch how the sensors react. The front and rear shouldn't be nearly the same. If they are, the converter isn't doing its job.

If you don't have a live data scan tool, here is a link that explains how to test an oxygen sensor using a voltmeter or multimeter.

https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor

Let me know.

Joe

Hi,

They could be related. However, I have a feeling we have a fuel issue related to the issue. Have any new codes turned up?

Joe

By chance, do you have the voltages for O2S1? I don't like how the voltage is somewhat flatlined in pic 3 above. As far as the fuel trims are concerned, they appear on target.

Let me know if you have that info. Also, the flatline may simply be from driving on a flat surface and not changing the throttle pressure.

Let me know.

Joe

I have a feeling it is sensor 1 that is bad. It shouldn't drop to 0v. It should stay between.1 and.9. Double check the connection. I say that because it seems to respond properly other than the drop to 0v.

Let me know.

Joe

Hi,

Sensor 1 is located in the exhaust manifold. I attached a pic below showing its location.

If you plan to replace it, here is a link you may find helpful:

https://www.2carpros.com/articles/how-to-replace-an-oxygen-sensor

Let me know if that helps.

Take care,

Joe

Have you tried clearing the code to see if it returns? It may eventually turn off after the system completes different drive cycles, but to speed things up, I would clear it and see if it returns.

If you have done this, then we need to start checking the wiring. Also, if you want, here is a link that explains how to test a sensor. If you have luck with parts as I've been having, you may have gotten a bad one. LOL

https://www.2carpros.com/articles/how-to-test-an-oxygen-sensor-02-sensor

Let me know.

Joe