Slowdown Light (very specific)

Are you getting 12 volts from the ignition at the solenoid plug? The ECU sends an earth/ground to the solenoid on the other pin at high rpms, but because the ECU output is transistorized, there may be a small voltage present with the key on (and at idle).

 

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12 volts should be on the green/black wire of the black bypass solenoid plug (i.e. not the blue one). The other wire is blue/yellow. This has the small voltage or earth on it.

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(EDIT)

I meant checking the voltage on each wire to ground, but anyway, your check is showing that the ECU is holding the voltage at 6 volts, which I recall is normal for low rpms. Did you monitor the voltage when driving? Depending on the car, the ECU doesn't rely solely on rpm values, but throttle position and possibly other factors. I haven't tested it myself on a static car to see if it the bypass valve behaves the same with the engine not under load. Anyway, for my peace of mind, could you recheck the voltage on the green/black wire only. The car doesn't have to be moving.

As you have fitted Technistrada dummy plugs to the main cat ECU thermocouple wiring, it's unlikely that the main cat systems are generating the warning lights.... unless you have a thermocouple ECU wiring or Motronic ECU issue. Do you have 12 volts power at the thermocouple ECU plugs? (on the green wire/pin 1)

 

I was under the impression that there were only two variations to the bypass system. Both had thermocouples on the exhaust bypass valve, but one car didn't have a thermocouple ECU for the bypass system. My confusion wasn't helped by this diagram in the WSM.

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That diagram also shows the thermocouple ECU for the left main cat as being on the right hand side of the car, which seems to be another possible variation.

Elsewhere in the WSM, there appears to be only two variations. Cars with and without bypass thermocouples, so I suppose your car may be normal for your country.

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Do you know if you have the correct Motronics ECU fitted for your car (assembly number and country)?

Oddly, the online parts manuals show no bypass thermocouple for any car.