When all cylinders lose spark, shared power feeds deserve attention.
Why this matters
A relay contact can supply voltage with no load but drop out when coils and injectors are active.
A missing ECU power feed may also cause no communication with the scan tool.
First checks
• Check ignition/ECU fuses.
• Verify coil power with key on and during cranking according to the wiring diagram.
• Check ignition relay outputs and module grounds.
A practical test path
Begin with the easiest observation that can divide the problem into two paths. In a ignition power supply 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 crank-no-start, use scan data and direct tests to determine which basic requirement is missing. Normal starter speed does not prove the engine computer has a valid position signal, correct fuel pressure, ignition output, authorization, or compression.
Common mistakes to avoid • Replacing all coils because none spark.
Stop and get help when… • Do not bypass fused feeds with oversized jump wires.
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 ignition power supply, 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.