downstream O2 sensors

1999 7.4L

I just replaced my fuel pump & now my downstream o2 sensors are 99% of the time flatlining between 85 & 115 mV.
Upstreams are fluctuating as they should between 80 & 800 mV.

Any ideas?

Did you replace the fuel filter when you changed the pump? 9 times out of 10 the reason why the pumps fail is because of a clogged filter. If it's clogged there will be low fuel pressure giving you the lean O2 readings. There could also be a vacuum leak somewhere, or an exhaust leak sucking in outside air causing the O2 sensors to go lean.