Have 02 civic ex 1.7 (D17a2) with P0420 . Upstream and downstream sensors are new. Sensor heaters ohm out at 8.6Ohms, downstream sensor passes power test but fails ground test. Upstream sensor fluctuates aprox every second, downstream fluctuates aprox 2-3 seconds and reads below 0.5volts which is not correct.

Ran new ground to chasis plug for down stream and now reads 12.38 (battery volts) but still reads under 0.5volts and fluctuates too much.

 

what brand sensors did you buy these cars only like DENSO or NTK. also those sensors are very sensitive i wouldn't buy them online only from a local parts store.