Will a bad ecm cause engine not to start?

Yes, a faulty Engine Control Module (ECM) can absolutely prevent an engine from starting. The ECM is the "brain" of the engine, controlling fuel delivery, ignition timing, and numerous other critical functions. A malfunctioning ECM can fail to send the necessary signals to allow the engine to crank and/or fire. The symptoms can vary widely depending on the nature of the ECM failure.