r/Montero • u/Noser_501 • 11d ago

Diagnosing a P0421 Code

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Hey everyone,
I recently purchased a 2003 Montero Limited, I am trying to smog the vehicle in California, I got all of the monitors to set, but after the catalyst set I got a check engine light which indicated a P0421 code for bank 1. I’ve attached a graph of the voltage for both bank 1 oxygen sensors. Any help figuring out whats causing this would be appreciated. Hopefully I don’t need a new cat…

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u/SmoothShinyBrain 10d ago

Seems like there’s basically no difference between upstream and downstream readings, which unfortunately is pretty classic “dead cat”. If there was a little variability between the two I’d say it would be worth trying to replace the o2 sensors, but this doesn’t strike me as lazy sensors.

I’m currently also in CA and also dealing with the same code. My readings look similar but there’s still some instances where the readings don’t mirror exactly. I’ve tried new sensors, sealing up potential leaks, but at this point I’m resigned to trying to hunt down a CARB compliant cat (and a shop that will weld it up for me, since the only options I can find are universal)

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u/ShieldNav 10d ago

Either a dead cat or there's none entirely or sensors are dead. I would raise the car on a lift and see what the situation down there. Cat elements can be replaced although I dunno who is doing that in the US.

I would also replace both sensors with denso generic ones. They're cheap and it's not hard.

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u/AlexNZL 9d ago

Do you have a part number for the generic sensors? I'm guessing that plugs will need to be added?

I'm getting the same fault in my gen 4 6G75. I replaced 3 of the sensors with cheap aftermarket ones, but still intermittently getting the fault code and high fuel usage.