[StManson]

I have my 2006 Scion XB with 580,000mi and it started throwing P0420 & C1201. It used to be intermittent, but it's been on for over a week now. Gas mileage did drop to 26MPG from the normal 30MPG, but I'm not sure if that's because it's the "summer blend."

Any tips on where I should start? Never seen the C1201 before. Cat was replaced almost 2 years ago now.
Thanks!

 

[jawhn]

I do not know the answer to your question, but holy sheet 580K miles - that's awesome, bump.

 

[superbee 4406]

the gas blend is most likely your culprit for gas milage
the current administration has quietly raised the ethanol level from 10% to 15%
which kills your gas mileage
you really don't know what they are pumping into your car today.

 

[timmcal]

Could be a bad O2 sensor or wiring causing it to “see” a bad converter. Could also be a bad leak in the exhaust manifold Causing the ratios to show an out-of-range making it think the converter is bad.

 

[Ryan1]

Google both Codes then you can narrow it down after you look at the results. Most likely an O2 sensor. My issue was the up stream O2 Sensor.
Also would fill up the tank and pour a bottle of B12 Chemtool in it as well. Might be a fix.

 

[Thunder]

Only burn 87 octane. NOT 88 octane. More isn't better. 88 is 15 percent ethanol. 87 is only 10 percent. About 4 mpg worse using 88 is roughly expected. Not worth the 5¢ "savings"