code P0420

2008 HHR LS. Code P0420 keeps coming back. Started after I replaced cracked flex pipe with new Eastern Catalytic ECO-GM #50459 catalytic converter flex pipe. Replaced both o2 sensors and code P0420 is back. Dealer charged me $53 and said aftermarket parts are not to GM spec (I think they are full of BS). No other problems visible. Car runs great. How can I by pass the code? I heard you can use a capacitor and resistor to set .45 volts to ECM. What is the correct way to by pass this error. Only check engine light keeps coming on even after reset. HELP!!!
I'm 63 and on disability. No money....Sorry.