P1631: Theft Deterrent Start Enable Signal Not Correct/ Main Relay Malfunction (Power Hold)

P1631 is a high-severity powertrain code that covers two related faults: the theft deterrent system is not supplying the correct start enable signal to the ECU, or the main relay is failing to hold power correctly after a start attempt. The engine management system requires a confirmed authorisation signal from the immobiliser or security module before it will allow the engine to run. If that signal is missing, corrupt, or arriving out of sequence, P1631 is logged and the vehicle may crank but refuse to start, or cut out shortly after firing.

The most likely causes include a fault in the security or immobiliser module, a communication breakdown between the body control module and the ECU, a failing or corroded ignition switch circuit, or a main relay that is not latching correctly under load. A flat or failing battery can also cause intermittent versions of this fault because the security handshake is sensitive to voltage during the start cycle. Begin by checking battery condition and terminal connections before moving on to the security system and relay circuits.

Because P1631 can leave the vehicle unable to start, it carries high urgency. Do not attempt repeated start cycles if the engine is cutting out, as this may complicate diagnosis. This fault generally requires a scan tool capable of reading immobiliser and body module data, so a workshop visit is the right first step unless you have access to manufacturer-level diagnostics.