The engine computer cannot command fuel or spark if it is not properly powered.
Why this matters
No scan communication plus no check-engine lamp at key-on can be an important clue, but network faults can complicate this.
A module may receive one power feed while missing another, so checking only one fuse is insufficient.
First checks
• Check communication with the engine module.
• Verify constant battery feeds, switched ignition feeds, and grounds from the wiring diagram.
• Check ECU/engine-control relays and related fuses.
A practical test path
Begin with the easiest observation that can divide the problem into two paths. In a ecu power and ground 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.
Intermittent and control-system faults are easiest to solve when the vehicle is tested in the failed state. Capture voltage, module data, switch status, and indicator behavior before cycling the key repeatedly or disconnecting the battery.
Common mistakes to avoid • Condemning the ECU because the scan tool cannot connect without checking connector power and ground.
Stop and get help when… • Module replacement often requires programming and should be a last step after power, ground, communication, and output circuits are proven.
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 ecu power and ground, 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.