Exhaust ecu location

Is it not in the usual place on a panel behind the right hand wheel arch on a 1999 model?

Just follow the shielded wire from the thermocouple. It may be mounted on the front or rear of the panel. Perhaps slightly above the two vacuum solenoids.

 

Actually, this diagram shows the bypass thermocouple ECU next to the vacuum solenoids, not above them

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Anyway, follow the braided line to/from the bypass valve thermocouple in case someone has swapped over the braided line or electrical cable to the ECU. If you can see the wire colours on the electrical plug, the bypass thermocouple ECU should have a purple-black wire, not a plain purple wire. The other two wires are green and black (same on both)