P0420 & P0430 Intermittent
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Mudding
Joined: Jan 2022
Posts: 163
Likes: 23
From: Michigan
P0420 & P0430 Intermittent
Codes triggered about a week ago. Reset with the GAP tool. Came back on this morning. Both set during moderate acceleration on the highway with outdoor temps around 65 f° and engine up to temp after 5-10 mins of highway driving.
checking the live values, my upstream values range from -320mA to 350mA and
my downstream range from 0.1v to 8v (unless I’m accelerating or coasting).
i reset the trouble code this morning while idling in the parking lot, within a minute my idle RPM went from ~650 to ~1200 and the viscous fan was at 1400rpm. Coolant temp stayed steady at 192 f°. No change after 5 mins and I shut it down. Had to get into work.
could this be a case of switching from winter gas to summer gas? I’ve been strictly filling up with Costco premium gas for the past 4 years.
checking the live values, my upstream values range from -320mA to 350mA and
my downstream range from 0.1v to 8v (unless I’m accelerating or coasting).
i reset the trouble code this morning while idling in the parking lot, within a minute my idle RPM went from ~650 to ~1200 and the viscous fan was at 1400rpm. Coolant temp stayed steady at 192 f°. No change after 5 mins and I shut it down. Had to get into work.
could this be a case of switching from winter gas to summer gas? I’ve been strictly filling up with Costco premium gas for the past 4 years.
Last edited by lenzda; May 5, 2026 at 12:07 PM.
Drifting
Joined: May 2026
Posts: 32
Likes: 0
420/430 is cat efficiency code that is extremely common to all brands.
From what I know from working on other brands it usually triggers when the downstream O2 readings go through wild swings, indicating that cats are not working at peak performance.
Usually I see those codes in colder months, in my understanding it's due to colder weather not allowing aging cats to fully warm up, especially if your heat shields are gone. Cats produce heat when they are working properly and they need heat to function properly, so it's a vicious cycle, low efficiency=low temp, low temp=decreased efficiency.
First "stealthy" repairs I saw was someone placing a capacitor across the O2 wires, hidden out of the way somewhere in the harness under the carpets so it can't be spotted because messing with emissions is a federal crime... but later the "antifouler" was openly advertised and sold as the device that will defeat 420/430 for $5... And the antifouler WORKS, it removes the sensor from the exhaust gas stream and being shielded form direct gas flow "blunts" the peaks and valleys of the signal, effectively tricking the monitoring system.
Now with everyone catching on they antifouler is no longer advertised as the cat fixer, but they are still out there.
From what I know from working on other brands it usually triggers when the downstream O2 readings go through wild swings, indicating that cats are not working at peak performance.
Usually I see those codes in colder months, in my understanding it's due to colder weather not allowing aging cats to fully warm up, especially if your heat shields are gone. Cats produce heat when they are working properly and they need heat to function properly, so it's a vicious cycle, low efficiency=low temp, low temp=decreased efficiency.
First "stealthy" repairs I saw was someone placing a capacitor across the O2 wires, hidden out of the way somewhere in the harness under the carpets so it can't be spotted because messing with emissions is a federal crime... but later the "antifouler" was openly advertised and sold as the device that will defeat 420/430 for $5... And the antifouler WORKS, it removes the sensor from the exhaust gas stream and being shielded form direct gas flow "blunts" the peaks and valleys of the signal, effectively tricking the monitoring system.
Now with everyone catching on they antifouler is no longer advertised as the cat fixer, but they are still out there.
Last edited by DimaPVD; May 17, 2026 at 07:08 PM.
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