This is on my '88 e28, but mechanically identical to what's in my e24 - M30B34.

I just tested the O2 sensor heating circuit by backprobing the connector on the harness side and got fluctuating voltage ranging from nominal to 6 volts (approx)- but no where near the constant 12v the Bentley manual suggests.

My next move is to replace the relay and retest.

I'm terrible at reading ETM diagrams - and don't yet know what half the symbols mean. I'll get there...but in the meantime could use any advice on where how to proceed if simple replacement of the relay doesn't solve.

TIA
-Drew