his truck has AFR sensors on primary, they output a steady voltage of 3.3 volts in toyota. And ideally you need a graph to see it acting stupid. - The post cat ones are regular 4 wire o2 sensors.
If the sensor is acting like an oxygen sensor and swinging voltage - its dead and needs replacement (which I can guarantee because his mechanic doused them with cleaner)
That 3.3 voltage is actually a simulated voltage provided in the PID for diagnostics. you cannot tap it with a multimeter - as the actual sense is measured by amp pull to keep the sensor hot (just like MAF sensor)
It can also be something else - like literally a dribbling injector - as he has also changed the fuel pump for some reason. The fuel pressure regulator is built into the assembly and if its pinched or damaged, it will cause severe overfuelling and stress the pump too.