P0420 Code

That’s the post cat sensor, firewall side. I’m dealing with P0430 which is the same sensor on the radiator side…it’s for the “Catalyst System Efficiency
Below Threshold”
Mine didn’t start until I ran 2 cans of SeaFoam top engine cleaner through my motor. Fixed my misfire codes, but now I’ve got the new code…

I had P0420 in my 2011 I4 Terrain and it turned out the be a bad catalytic converter, not a cheap fix. I would check the 02 sensors first since those are cheap and easy then go from there.

I am hoping just a 02 Sensor. Check engine light went away today.

Sngtrkr20 said:

I am hoping just a 02 Sensor. Check engine light went away today.

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Yep…mine comes on, I clear it, and then I’ll check again, and nothing’s even pending, but then it’ll come on again later…

PO420 is a death sentence to older higher mile vehicles in Cal and other strict states. As the cat is the most expensive replacement part of a vehicle. A faulty cat needs to be replaced to pass smog emissions. The smog referees are very critical that the correct one is used. In Cal, it must be CARB certified which is a very expensive procedure for the manufacturer.
If you determine your cat needs to be replaced, be sure to find and correct the reason why it failed before installing a new one.
The only good news; if disposing of an OEM, check online for the highest price. I was able to get over $200 for my old one. The buyer paid shipping to NJ. That helped offset the expensive of the new genuine part.

BandDirector said:

Yep…mine comes on, I clear it, and then I’ll check again, and nothing’s even pending, but then it’ll come on again later…

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Turns out it is a bank 2 O2 Sensor

Sngtrkr20 said:

Turns out it is a bank 2 O2 Sensor

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Good to know…I may just replace mine next too…

BandDirector said:

Good to know…I may just replace mine next too…

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I am going to replace both of them. Bosch. like $24 each

Sngtrkr20 said:

I am going to replace both of them. Bosch. like $24 each

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Cueing John (Repairman) . . .

garagerog said:

Cueing John (Repairman) . . .

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I see it, , use only Genuine GM sensors, factory GM parts are branded that way now, and an extra $25 ea. for them is worth it over the Bosch.
On any brand vehicle the OEM factory sensors should be the only ones used to minimize issues. Aftermarket is a last resort if OEM are not available.
Bad front or primary sensor could have affected the cat but do not be disappointed if it is not the cure for the P420.
Slow driving, urban and no long pulls up hills, etc. do not get the cats fully up to temperature to help them burn off buildups. That's where the (no offense PC disclaimer) the Italian tune up does come into play getting cats hot on a regular basis so they can burn off buildups.

Bosch does make good dishwashers tho. . . .

garagerog said:

Bosch does make good dishwashers tho. . . .

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I'll disagree on that, I used to service those POS's.

Bosch makes them for GM

Replaced both the downstream with Bosch. All good to go.

On my 2010 traverse, replaced all 4 with bosch at 100,000
They were listed as the oem part.

Got rid of vehicle with 197,000.
So the bosch did good for me in this case.

rbarrios said:

On my 2010 traverse, replaced all 4 with bosch at 100,000
They were listed as the oem part.

Got rid of vehicle with 197,000.
So the bosch did good for me in this case.

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Bosch is OEM. Pull a Delco out of the box and it will have the long Bosch part # on it like 0258010261 and the Bosch script