If an engine won't start, what is the first thing you should check?

When diagnosing an engine that won't start, checking the battery and electrical connections is essential as these components are crucial for initiating the starting sequence. The battery provides the necessary power to turn the starter motor, which subsequently engages the engine. If the battery is weak or dead, or if there are poor connections or corrosion at the terminals, it can prevent the electrical system from functioning properly. This would lead to insufficient power being sent to the starter and other essential electrical components, effectively causing the engine not to start.

Furthermore, ensuring that all electrical connections are secure and free from damage or corrosion is critical. Faulty connections can interrupt the flow of electricity, which may also prevent the engine from starting. This step should precede checking other systems because addressing potential issues with the battery and electrical connections often resolves the problem without needing to investigate further systems, thereby simplifying the troubleshooting process.