The last four gas fill-ups, I've used 89 octane and STP High Mileage additive, and today I finally got the P0420 code (that I typically get twice a year) when I tried to accelerate too fast for too long up a hill going 60 MPH. MO started shaking, engine was sputtering and I had loss of power. I backed off, coasted down the hill at 55 MPH, then pulled over. Stopped, restarted and was on my way and MO was running fine. About 15 miles later the SES light came out and I took my scanner and read it. Reset the light while driving, and it never came on during the last hour of my journey. First time I've gotten that code this year. Looks like it's back to 93 octane and another can of lacquer thinner...
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