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2005 Toyota Sienna Catalytic Converter Problem Cured by Marvel's Mystery Oil?

The check engine light was on in my 05 Sienna with 146k miles for about 4 days so I went to Autozone to have it scanned. Error Code PO420 Catalyst System Efficiency Below Threshold (Bank 1). The Autozone guy says I need a new catalytic converter but I can try a bottle of Marvel’s Mystery Oil and see if it cleans out the converter in a few days. I was appreciative that they offer the free scanning service so I bought the Mystery Oil for $5.50 and dumped it in my gas tank with absolutely no expectation that it would do anything to solve my catalytic converter issue. To my surprise, after about 4 days, the check engine light went off and has stayed off for the past 10 days. Is it possible that something could be diluted in a tank of gas, run through the combustion process and then do something to “clean” o cure a catalytic converter? Is it just a coincidence that check engine light randomly turned off and stayed off (so far) a few days after adding the Mystery Oil? Is it normal that this error code would come and go with no repairs being made?

Marvel bought you a few days, weeks or at most months. The cat (or O2 sensor) was probably just over the edge and the gas treatment was just enough to burn off some junk that was on it. Unfortunately there is no long term miracle fix for a bad cat. Once the platinum/palladium start to lose their catalytic properties there is no way for you or I to bring it back. Start saving and start checking for a good local independent mechanic to get it replaced. You may buy some more time the next time the code comes up again, but soon enough the light won’t go out.

Before you condemn the cat, check the O2 sensor, your mechanic can do that for you.

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I would speculate that the Marvel Mystery oil had nothing to do with the light going out, though it does have pretty good cleaning properties.

Most of the time, a code for Catalyst System Efficiency Below Threshold does NOT indicate catalyst failure. It indicates that the oxygen sensor is old and slow to respond. Voltage output is not the only criteria for oxygen sensors. They must also respond as quickly as the computer expects them to respond.

I am not a Toyota guy, but I expect that if you check your owner’s manual, you will find that at 12 years and 146k miles, your oxygen sensors are a couple of years overdue for replacement.