P0421 & Spark Plug fouler
 

2010 CX-7 2.3L, code P0421.

I used spark plug fouler in 2nd O2 sensor. Now getting P0137 & P0139.

Maybe i damaged the sensor when reinstalling with fouler? or Fouler is too long?

Remove the anti fouler. If your new Mils go away and you only have p0420, you can fix it with a diode on the signal lead of the sensor.

do you have a diagram and more details about this ?

Nope. But did it on my ms6 for over a year. Rear o2 has 4 wires, two are for the heater, one is reference votage, and one is the signal to the ecm.
Cut the signal wire and splice in a diode with the stripe away from the sensor, it drops the voltage just enough for the p0420 mil to turn off.

a standard black diode with a silver stripe or a zener model ?

Standard black diode.