A locked engine can mimic an electrical starter failure.
Why this matters
A starter cannot distinguish between a failed starter motor and an engine that physically will not rotate; both can produce a heavy click and severe voltage drop.
Liquid in a cylinder can stop piston movement and cause hydrolock.
First checks
• If appropriate and safe, use the manufacturer’s procedure to rotate the engine by hand.
• Check for signs of hydrolock, severe overheating, oil loss, or accessory seizure.
• Consider a seized alternator or A/C compressor if the belt drive is locked.
A practical test path
Begin with the easiest observation that can divide the problem into two paths. In a can the engine turn mechanically? complaint, do not change several things at once. Record the initial condition, perform one test, and then decide what the result proves. If the test shows normal power, move downstream. If it shows a loss, stay with that part of the circuit until the source of the loss is found. Whenever possible, test the circuit while it is carrying its normal load rather than relying only on resistance or continuity checks with everything switched off.
For a cranking-system fault, the battery, cables, start request, interlocks, relay, solenoid, starter motor, engine mechanical freedom, and return ground form one chain. A missing result at any link prevents cranking. Measuring at strategic points is faster than removing parts.
Common mistakes to avoid • Repeatedly applying more booster power to a mechanically locked engine.
Stop and get help when… • Do not rotate engines by hand unless you know the correct crankshaft fastener and direction/procedure.
What to record
Write down whether the symptom was present during the test, the battery voltage or scan values you observed, and exactly what changed. For can the engine turn mechanically?, include temperature, key used, whether the engine cranked, and any warning or security indicators that were different from normal. Good notes prevent repeated testing and make professional diagnosis faster if you need it.