ThosL Posted December 29, 2015 Share Posted December 29, 2015 My sister had one of these codes, that her catalytic converter was below sensor level, though she recently passed emissions. If she runs premium gas would that help? Does any product help with these? The lady at Auto Zone after finding my cat. code came up on a 99 Outback with a 2.2 said probably the misfire or other less than optimum combustion triggered it. Is that right? Quote Link to comment Share on other sites More sharing options...
Fairtax4me Posted December 29, 2015 Share Posted December 29, 2015 If the spark plugs are old that can definitely cause this code. A good tune-up is often able to make it go away. Plenty of other things can cause the P0420 code as well, but a tune-up is the place to start if it hasn't had one recently. Quote Link to comment Share on other sites More sharing options...
DaveT Posted December 29, 2015 Share Posted December 29, 2015 Also look for a hole or leak in the exhaust before the second O2 sensor. Quote Link to comment Share on other sites More sharing options...
idosubaru Posted December 30, 2015 Share Posted December 30, 2015 1. post the code. probably an 0420 but best to be clear 2. nearly anything can cause an 0420 start with anything triggering the CEL (other than 0420) or causing poor gas mileage, exhaust leaks. simplest fix is an O2 extender - works often enough it's worth the $5 to never think about it again. Quote Link to comment Share on other sites More sharing options...
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