i thought i had posted this last year but i guess i didnt..
in case anyone else is battling with the CEL (i think my code was p0420) and cant pass smog check
I have about 150k miles on my 2001 qx4
replacing the cats in california is crazy expensive
I bought a can of cat cleaner on amazon and simply followed instructions after filling up 1 tank of high octane ( i normally roll with the mid grade)
I let the CEL do its full reset and was able to pass smog. a week or two later and the light came back on but im good for 2 years.

Have your O2 sensors ever been been replaced? Although not a routine maintenance item, O2 sensors do not last much past 100K miles. 60K to 90K miles normal replacement times. A scanner would indicate if the voltage reading in the downstream, right bank sensor, is fairly steady like it should be or if it shows significant voltage variation, which could cause the P0420 code to get flagged. If it is the passenger side cat, that's just sad. Extra sad in California

hi. good question. i did replace the downstream O2's back at 85k miles because they were throwing the codes and because they are relatively easy and accessible. i never replaced the upstream sensors because I dont think they are the cause of 0420 and 0430 and instead I believe those codes indicate the CATs are bad.

do you agree with that assessment?

regarding replacing the cats... they are crazy a** expensive to replace at a shop and looking how little space there is to work i doubt a regular human could get in there to remove all those rusty 20 year old bolts.

i know (from a friend of course, not me )... that those little cat extender tubes do work but you are not supposed to use them and in California..the smog check guy actually goes under the car and looks for them!

mvmcali wrote: ↑

Wed Oct 04, 2023 5:05 pm

regarding replacing the cats... they are crazy a** expensive to replace at a shop and looking how little space there is to work i doubt a regular human could get in there to remove all those rusty 20 year old bolts.

i know (from a friend of course, not me )... that those little cat extender tubes do work but you are not supposed to use them and in California..the smog check guy actually goes under the car and looks for them!

You could always ditch CARB (and communism) and move to a free state.

VStar650CL wrote: ↑

Wed Oct 04, 2023 6:10 pm

You could always ditch CARB (and communism) and move to a free state.

... and get a better performance quality gasoline compared to California's unique and pathetic blends of life sucking additives.

mvmcali wrote: ↑

Wed Oct 04, 2023 5:05 pm

do you agree with that assessment?

yeah ... those are catalytic system fault codes. The downstream O2 sensors act as a witness to what has occurred upstream, taking into account all the metered air inputs, temps, what the upstream O2 sensors are doing to regulate fuel supply and then measures the efficiency of the catalytic converter based on all that is happening upstream. If the measurement is not within the expected range, those codes get flagged. The question is always whether or not the witness is lying and if not then what has gone wrong upstream. Bad cats are certainly a possibility but then its a question of why are they bad. Lazy and slow to react upstream O2 sensors can cause unburned fuel to burn up in the cats, damaging them or clogging them. You mentioned a cat cleaner product temporarily got your CEL to turn off which I don't think would be likely if the cats were damaged. If your upstream O2 sensors are the original 22yr, 150K mile old sensors, they are long overdue for a change. I can only hope it's not too late to try additional cat cleaner and see if you can get that code to clear.

mdmellott wrote: ↑

Wed Oct 04, 2023 7:30 pm

VStar650CL wrote: ↑

Wed Oct 04, 2023 6:10 pm

You could always ditch CARB (and communism) and move to a free state.

... and get a better performance quality gasoline compared to California's unique and pathetic blends of life sucking additives.

As a happy resident of one of two states where you can build a street legal go kart, I vote for aviation fuel.

Same issue with my Path which is why I got a bargain on it (Smog was due on it too at the time). Sellers were upfront about the check engine light and code. Both O2 sensors had been replaced (Denso I believe) (Bank 2). However they weren't driving the vehicle much anymore so I took a chance thinking I could blow out the cats with a long drive or also try the Lacquer Thinner trick (haven't done it and don't know if it's bad for vehicle or not) (Bought the last gallon just as store was phasing out / stocking shelves with CA compliant formula).
I had smog station reset the OBD codes, drove it through it's drive cycle (using a mish-mash of different instructions I found), Immediately parked it near Smog Check. Next morning drove it the block there and it passed. About 5 miles later the dash light came on. Since then I've put some miles on the Path and then one day the light went out. Has stayed out. Don't know if that really means anything (have to buy a scanner, any recommendations welcome on brand/model) but I don't have to worry about it for 2 years until the next smog.