I replaced the T belt, water pump and seals on my 1991 LS400. Everything is 100% lined up and installed correctly. While apart I noticed the left cam sensor wiring was brittle, cracked and bare in spots so I cut, spliced and heat shrinked the damaged wires.When I started the engine it ran terrible. I disconnected the left cam sensor and it smoothed right out. pulled OBDll code 13. New sensor is on order.
I find it intriguing that it runs well on just the right sensor, are there two for redundancy sake? I assume the ECU compares the signals between the three for better performance and ignition accuracy?

 

Was the engine running properly before you do all those jobs?
If running good, you did something wrong. I'd simply confirm 3 signals using an oscilloscope for the quickest way to troubleshoot. Those signals should be phased like the chart below and also the amplitude should be around 5Vp-p. You get the cam signal every time you get 24 pulses from the crank position sensor.


Cam signals CH1 and CH2, Crank signal: CH3

 

Yes, engine was running perfectly before making the repairs (cam seal failure). I am confident that the fault lies with the wiring repair and the brittle nature of the left cam sensor harness. Due to the virus situation I am having to wait for the part to be shipped (ETA next Thursday) before I can reassemble and be sure of my diagnosis.
My question was more for curiosity sake as to why two sensors are used

 

Because there are two banks and all those gears and driven shafts have to be aligned correctly. Those 3 sensors are used to check those phases of shafts. In your case, the result of the check is not OK. It may be caused by the missing signal or the phase is incorrect. When one of cam signals is missing, the ECU uses a method to compensate the problem calculating internally. I worry that your ECU is not getting those signals properly.