Max
Good job with your analytical approach.
Here is an old trick for ECU demand input check.
With car at normal operating temp and heater circuit stll hooked up, Unplug the 02 sig wire and hold the wire going to ECU in one hand..grab the Pos. bat post with your other hand..this gets 1/1.5 V signal to ecu . Monitor the 02 rection with DMM as you switch on/off of your hand at bat post.
Very High Tech, huh????
I actually use a DSO scope for this stuff so I can get cross count speed and V amplitude of the wave pattern... but this little trick works for a quicky test of ECU reaction to 02 sig input.
Another trick for 02 sensor reaction is to unplug sensor sig wire and monitor its output while you put you thumb over a decent size vac line..as you take your thumb on/off the vac line , you will see if the sesnor reacts to the lean condition you are causing...you are simply using the engine as a rich/lean condition generator...can do the same by partial blocking of intake air to cause rich condition...and on HFM systems , you can do the same busimply taking the vac line off the fuel pressure regulator. Crude, but effective and fast checks...........