Could be something as simple as a poor connection on the rear sensor and you unplugging and plugging it back together cleared it up.
And if memory serves me, 0420 would be the rear sensor reading poor performance from the cat. So it shouldn't be the front one.
97 Sahara, 74 Renegade, 72 CJ5, 55 Willy's (the Beast) Boss 302, 6:34 gearing, Bose 901 speakers.
God I miss that Jeep. Forgot the kinda Jeep, 80 SX4.