Once the lambda sensors have been tested to death or to confirm life, you might then be able to get onto the further question of fuel injection parameters.
If you have checked all components for normal operation, checked for air leaks, checked global parameters such as fuel pressure - fuel pump and fuel pressure regulator, and all those are fine and normal, then the fuel injection parameters finally might need to be looked at.
If the fuel trims are getting richer, it means that the ECU understands the data from the lambda sensor as being lean - obvious bit, but tells you that the ECU is doing the calculations, and is increasing the injector duration to compensate.
Injector duration, because this is Pulse With Modulation (PWM) is the way the ECU adds more fuel. If you see what the normal duration starts at, then increases to, you can tie this in with the % change of fuel trim.
With fueling, and controlling the injectors, you need to check the voltage at the ECU pins - i.e. before all the wiring, then at the injectors themselves to ensure that the voltage is actually not being reduced.
The voltage could be reduced by a bad bit of wiring - high resistance, or an LPG ECU, where there may be a relay or a thyristor/transistor, or even the voltage supply that are not normal. High resistance due to a failing component would reduce the voltage at the injector. The opening speed of the injector would be slower, due to the lower force applied, because of the lower voltage applied to the coil of the injector.
This would all need to be compensated by a longer duration signal from the ECU.
This is getting specialised, and you would need an oscilloscope to look at this.
If the fault appears over time with the engine running, it could indicate a component heating up as its failure mode.
which according to the wiring diagram shouldn't be affected by that wire!
