Fault code: MID 128:04, PID 131A

I recently started getting a CEL and active code for exhaust back pressure sensor. CEL will only appear under light engine load/low idle or during engine brake use. CEL disappears and fault code goes passive, shortly after increasing throttle/boost pressure.
I’ve replaced the sensor and blew out both thermostat housing and manifold tube/pipe. Tested the voltage by back probing sensor while connected. 5volt supply is good. Grounding return tested at 0.2volt, which should be ok. But the signal wire fluctuates from 0.2-1.3volt, key-on, engine off.
The wiring at the sensor connector is very loose and wires are spongy.
I do have a minor exhaust leak coming from the flex pipe directly off the exhaust Y-pipe under the cab. Everything else is new.
I’m stumped as to whether this is actually a low back pressure issue from the minor exhaust leak, or sensor/wiring problem.
Any input would be appreciated.