P0420 Suzuki Reno 2005
I have a Suzuki Reno 2005 and I failed to pass the emission test. My car is getting a code P0420 but my check engine light is not on, however the light works normally when I turn on the car or If a pass the switch. My question is: why the light is not on, and is it the sensors or catalyst converter? I already replace the O2 sensor and I friend told me to add a seafoam in the gas tank, and already did it. Somebody knows what I should do next? I don't want spend money in stupid way.
Thanks for your help