I just had a fuel system cleaning done and valvoline syn3 was used.20 miles down the road CEL came on and was giving a 0420 code.Im wondering if the carbon being cleaned out is burning off and giving the code.Is the a possibility.It came on after the intake and throttle boddy cleaning and i had it turned off it went a couple days and came back on but im thinking because i had a full tankof gas and they put bottle of cleaner in the tank and im at a half tank now.is it still cleaning and burning the carbon and throwing the code.

 

Could be.I'd burn as much of that tank as I could w/out getting too close the bottom.I would then fill up again,reset the CEL and see if it comes back on.

 

How many miles? Its quite possiably the o2 sensor is bad.

 

Could also be a crack in the manifold or broken header bolt allowing fresh air to get inside the exhaust stream