A supply that changes when a device is connected needs testing under the relevant load. A normal disconnected reading does not prove the circuit can support operation.
Preserve both conditions
Record the sensor identity, connection state and exact readings obtained by the technician. Include other devices affected at the same time. Shared circuits must be understood before an ECU is blamed for a collapsing reference.
Do not repeatedly connect unknown substitutes or apply external voltage to the signal wiring. The workshop should follow the manufacturer’s isolation and measurement procedure. A repair recommendation should identify what causes the change and verify the connected operating state afterwards, rather than treating the disconnected voltage as the only relevant result.
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