Sorry for delayed post, I had a shower and conked out on the sofa!
We had half a day looking at this yesterday. Here are the findings.
So - to start with a clean slate, we cleared the DTC's with the ignition on, but engine off.
Immediately (like, within a second) of starting the engine, the following fault codes appeared:
P0155-04 - O2 Sensor Heater Circuit Open (Bank 2 Sensor 1). Status:Present
P0155-08 - O2 sensor heater circuit malfunction (Bank 2 Sensor 1). Status:Present
For this reason, we can preclude any sort of fueling issue / imbalance, or erroneous input from any other sensors (such as CTS). Because the codes return immediately when the engine fires up, the fault is, exactly what the codes say - a problem with the heater circuit for Lambda sensor 1, on bank 2 (The pre-cat sensor).
Forgetting what I already know about the car, and applying a logical thought process, I think "what can be causing this", and the main suspects I arrive at
A) Lambda sensor failure itself (failure of the heater) - usually most likely suspect, although not with this car.
B) Wiring problem between the ECU and the lambda sensor (second most likely suspect)
C) Problem within the ECU itself (as we know, highly unlikely and improbably, but mentioned for completeness).
Now applying what we know about this specific car...
1) We can preclude an issue with the sensor. Apart from the fact it's been changed a couple of times, just to be
sure, we swapped the plugs - so, we plugged the actual B2S1 into the loom plug for B1S1, and the actual B1S1 into the loom for B2S1. More simply, we swapped the plugs for the two front lambda sensors. Despite doing this, and clearing the codes, immediately upon re-start, the same codes appeared - for bank 2 sensor 1 heater circuit.
So, based on this, we can 100% preclude this issue being with the sensor itself on B2S1.
So now we've ruled out the lambda sensor - let's look at the next most likely cause - a fault in the wiring, between the ECU plug, and the B2S1 connector pins.
Using the information very helpfully given by Dr G, we did the following tests, using a multimeter:
Bank 2 Sensor 1 (The one it's moaning about)
Pin 1 Grey (The live feed to the heater circuit) - a healthy looking 13.9 volts (engine running) - indicating there's no issue with voltage getting to the sensor?
The following were then tested using a multi meter on it's lowest ohm setting:
Pin 2 (Green wire) to pin 2 on X85 - 0.8
Pin 3 (Brown and Black) to pin 7 on X85 - 0.7
Pin 4 (Blue) to pin 9 on X85 - 0.7
On the face of things this looked okay, so we also tested it against one of the others (Bank 2 Sensor 2 - post cat) - and got exactly the same values.
I couldn't compare the bank 1 plugs, because the ECU Pins posted on here for B1 connectors didn't seem to match up (For example, X85/50 doesn't have any wire connected to it at the ECU end).
But as it stands, the values look fairly consistent, and don't indicate a wiring problem - although I still maintain it's the most likely cause.
We can rule out the ECU, as well as being indestructable - this one has also been swapped out, and the fault follows it.
The only thing I am unsure of, is the grey common wire feeding each sensor. At a first thought, it's clearly giving the sensor voltage which suggests that the circuit is okay. But, as DG says, it's fed from a common source - so, maybe is there a problem upstream of that source, between the common source and the ECU plug? I'm not sure what locations I would use to go about testing this, if so? (I'll ponder the wiring diagrams later, to see what I can see).
Because of (limited) access, I am in the mindset that the loom needs to be out of the car for a thorough inspection. And as we know, removal of that loom is not the most difficult job in the world.
I would like to identify and fix the fault, rather than "fit and hope" - but, we have found a loom from our famously known and trusted Heathrow based Omega breaker for good money, so given how strongly we can preclude the sensor itself, and the ECU - there is a serious temptation to just fit a known good loom - especially as the existing loom would have to come out to be thoroughly tested in any event?
I and I have no doubt Migalot would appreciate your comments and opinions, I feel we are SO close to resolving this problem, and it would be an unforgivable sin to scrap such a lovely and now fairly rare car (it really is in good shape!) for the sake of of what is a stupid electrical gremlin!
I don't mind going to Migalot and getting my hands dirty on / under the car, but I am the first to admit that whilst I get all the concepts, electrics are not my strongest point, and many heads are better than one. I really hope we can collectively get to the bottom of this and implement a permanent fix
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