Take the heater ground out of the MS and run it straight to the engine block. Older LC-1s had a signal ground for running to the MS, but this one does not need to be grounded to the MS and would be better off grounding both its grounds to the engine block. However, running the logic ground to the MS isn't as bad as running the heater ground to it.

I'll be putting my NB O2 sensor back in for running the stock ECU so once the WB is out (not gonna cook it by running the engine without powering it) I'll try running the MS without the LC-1 and see what that does.

Grounds have always been a bit of a mystery to me. I understand how a ground loop works but my MS is grounded to the head (bolt near the TB) so if I ground the LC-1 to the same point how is it different to grounding it to the MS? I really should look into where all of my grounds go (2A,2B,2C,2D) One goes to the stud on the TB but the others im not sure.

If the MS has only one ground (the ground plane), all sensor grounds should return to it, without being connecte to the head/block/chassis on the way. IOW bring a wire from the MS circuit board ground plane out, and call this wire "sensor ground". Connect all sensor grounds to it and don't connect them to the head or block or chassis or anywhere else. This way the sensors have a direct, dedicated connection to the ground plane, so the MS sees a clean signal from them.

Bring a 2nd wire, a thicker one, from the MS circuit board ground plane out, (preferably near the pull-down MOSFET drivers) and connect this to the head, this will be your "power ground".

I do power and analog electronics design for a living (20 years now, I'm old), and I teach grounding and layout to the young 'uns in the company. ;)

If someone can point me to a schematic and layout of the MS, and the schematic of the sensor input circuitry, I can make a more detailed grounding scheme, and design a better input conditioning circuit for the CAS.

Hi Jason, Thanks for the interest. I did try running separate wires directly from the CAS round the outside of the car to the MS without any success. But I'm not going to start running permanent extra wires or clipping away at the original harness because that kind of defeats my PNP goal. My thinking in the matter is if its good enough for the OEM ECU then its good enough for me.

I think that we have narrowed the candidates down to either poor construction (e.g. me grounding the LC-1 through the MS) or me having a sick/dead/dieing CAS (has happened to others though very subjective)

I'm not sure about the MS. I think they consolidated all of the grounding into a plane bacause the 2.2 boards kept killing traces.

I have attatched the wiring diagrams for educational purposes. It has quite a good over-view of the grounding layout.