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2000 4Runner
406 Posts
CLEAN Your CAT - Fix stubborn P0420
Here's what I did: On 2005 Forester XT with P0420,
1. Removed downpipe/catalytic converters.
2. Separated into upper Cat and lower Cat.
3. In a 5-gallon bucket, mixed 4 gallons water, 10 oz. Citric acid and 7 oz Oxalic acid. (Purchased on Amazon - $20)
4. Soaked main Cat in acid overnght, rinsed completely with water.
5. Soaked upper Cat about 8 hours, then rinsed completely with water.
6. Reinstalled Cats, ran engine until hot, then cleared P0420 and it never came back. DONE.

P.S. For those of you that may be concerned about handling ACID, don't worry, just WEAR RUBBER GLOVES.
The oxalic acid is what's in "Bar Keepers Friend" cleaning powder that you buy at the grocery store, and
citric acid is commonly used in food to give it a "tangy" flavor.. Relax - it's not dangerous at all.
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So.. Obviously my previous post didn’t make any sense as blow back engine grease cannot get inside the rear Cat.

I know you said the code hasn’t shown up again.. Would like to know if this is still the case after almost a month..

Does anybody know more or less how long are the precious metals inside the Cats which perform gas exchange supposed to last? Maybe all it’s needed is actually replacing the Cat itself, which might not be worth it if your car is older like mine or if you need to pass emissions.. just rather get the $5 fix instead.. not really what I’d like to do since it’s just cheating the O2 sensor, but if it’s not going to affect performance, I might just go this route.

Wondering though if anyone has gotten the code back after replacing the Catalytic..

2001 Forester S
4 Posts
I'd like to put my 0.02 in on this situation as I recently encountered this problem. You can skip down to the last paragraph if you wanna bypass all the background stuff and read my eventual solution.

Background (because i need to contribute meaningful posts per forum rules)
I have an 2001 Forester S (USDM), that I bought with a blown head gasket last October. After after a few weekends, i was able to get the foz back together and take it to the smog check place. Having never had smog inspection issues with an OBDII car, I was instructed, after failing the test, to drive the car for 100 miles and return to get retested.

450 miles and two failed smog checks later, I was on my way home from work when the readiness monitors finally cleared and the P0420 appeared. "Fine," I said. Then I spent the next 4 days trying different catalytic converter cleaners and additives.

3 months, 1 timing belt, and 8 bent valves later, my opportunity to investigate the cause of the P0420 returned at last. First I replaced the O2 sensor, and drove the car for 2 days, while the light remained. I reset the codes and went through the drive cycles again (just in case the o2 sensor was the problem). 3 days later P0420 returned. Next, I tried filling the gas tank with 91 octane, along with the spark plug defouler for the rear O2 sensor. I drove the car for a day, while the P0420 made itself comfortable on gauge cluster. I cleared the trouble codes again and spent another 2 days going through the drive cycles upon which the P0420 came back. This whole time I was driving with expired tags and an out-of-state plate, because I needed the smog cert to finalize the title transfer, so I was really anxious and stressed every time I had to test drive the car, or go through the different drive cycles.
I was comparing the O2 sensor and A/F ratio sensor graphs during this whole time, and the graphs indicated that the catalytic converter was performing typically. I had a spare wideband that fit the Subaru harness, so I figured i'd swap that sensor and see how that went. 3 days and 3 drive cycles later, the P0420 - like an annoying roommate that sits in front of your computer, and goes through your AIM message windows without asking, popped back in. After reading some opinions on the O2 sensors, I opted to buy a genuine subaru wideband, just in case.
4 days, 1 subaru wideband sensor, and 3 drive cycles later, I have clearing subaru readiness monitors down to a T. And the wide-band that came with my AEM failsafe gauge is looks to be the same model as the subaru wideband, except the resistors are different. But, the P0420 code was still there. I wanted to exhaust all of my options before throwing down $500 on a cat for a $300 car.

The Solution
After reading that Cataclean was comprised of lacquer thinner, and that the exhaust stream temperatures are the parameters that need to be manipulated, I opted to go for the Scotty Kilmer Lacquer thinner route. Before heading off the buy the thinner, I started the morning with an Italian tune-up and drove around for a half hour, while my wife and daughter ate the breakfast I made for them (so I could go play with my car). At this point, I had about 6 gal. of 91 octane in the tank, so drove to the gas station and added about 4 gal of 100 octane, to have enough fuel to dillute the lacquer thinner. I put a gallon of thinner in my 10 gal mixture of 91 and 100 octane gas, and drove around for about 30 minutes - but the P0420 still present. Feeling defeated, invited my wife and daughter to join me in eating away my sorrows at an all-you-can-eat sushi restaurant. As I was going through the stages of grief, I figured that I would reset the DTCs (just-in case) and drive to a restaurant that was about an hour away from my house, just to have enough opportunities to clear the readiness monitors. On the way to the restaurant, my wife was operating the scan tool, while I went through the drive cycles; and after about 25 minutes, we were notified that the P0420 was pending. Struggling to hold back the f-bomb so as to avoid tainting my toddler's ears, I accepted the truth that the reader was telling me and the girls and I had our fill of tuna - for 6 people.
On the way home, my curiosity started nagging and I asked my wife to run the diagnostics. Lo, and behold, the readiness monitors were cleared, and there were no pending codes. Since my daughter was in a sushi coma and my wife was about ready to join her, I dropped them off at the house and drove to the smog check station and got passing certificate. It's been 3 weeks and still no P0420.

My guess is that the catalytic was in good condition until my foz's previous owner blew the head gasket and drive it around like that for some time. I imagine the combustion of the coolant/fuel mixture was fouling the surfaces of the converter, which is probably why the lacquer thinner helped with my case.
Update: After 2 months and 2500 miles that P0420 came back. At least it passed smog. I will try running just lacquer thinner again to see if I can repeat these results.
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