Advanced and Intermittent Diagnostics
The hardest problems are the ones that won’t sit still — the intermittent that clears before you can test it, the fault with no code, the several faults interacting. These reward method and patience and punish parts-swapping, and “no fault found” almost always means the fault wasn’t present when you looked, not that there isn’t one.
The intermittent is usually a connection
An intermittent that comes and goes with vibration, temperature, or load is, most of the time, a connection or a ground — a corroded terminal, a loose pin, a chafed wire that opens when things move or heat up. That’s why the methodical electrical work (wiggle-testing connectors and harnesses, checking grounds, voltage-drop across connections) finds intermittents that parts-swapping never does. Chase the connection before the component.
Capture it and recreate the conditions
When the fault won’t reproduce on demand, make it show itself. Data logging captures live values over time so you see what happened at the moment of the fault, and freeze-frame records the conditions the code was set under. Then recreate those conditions — heat it, cool it, vibrate it, load it, flex the harness — to make an intermittent appear while you’re watching. A fault that only happens hot, or only over rough ground, tells you where to look.
Multiple faults and known-good comparison
When several things misbehave at once, look for a common cause — a shared ground, a power feed, a bus problem — rather than diagnosing each symptom separately. And comparing a suspect reading against a known-good value or an identical machine isolates what’s actually abnormal. Above all, don’t just clear the code and hand it back: an intermittent cleared without a cause found comes back, usually on the customer at the worst time.
Where it goes wrong
- Parts-swapping an intermittent instead of chasing the connection.
- Clearing a code without finding the cause and returning the machine.
- Not capturing data or recreating the conditions to reproduce the fault.
- Diagnosing multiple symptoms separately instead of finding a common cause.
Related
Electrical fundamentals and wiring diagrams · Reading fault codes · Diagnostic software and tools · Electronic controls and CAN bus.