Stop guessing: go online and read the machine's mind
Before you touch a single part, connect to the PLC and read what it actually sees. The fastest troubleshooters don't guess what the logic is doing — they watch it, live.
A faultline101 training module · the skill behind six hands-on scenarios
Going online: reading the machine's mind
The fastest PLC troubleshooting move is to connect, go online, and read what the controller actually sees — instead of guessing and swapping parts. The code doesn't lie.
The idea
The single most powerful thing you can do on a down machine is connect to the controller and go online. In monitor mode the true conditions light up and the false ones don't — the PLC shows you exactly what it sees at every input and drives at every output, right now.
A guesser argues about what the sensor 'should' be doing. A troubleshooter reads the input tag and knows what it IS doing. The PLC is not guessing, and once you're online, neither are you.
Going online also settles arguments fast: the HMI can be stale, the operator's memory can be wrong, the print can be out of date — but the live controller state is the ground truth. Read it first, and everything after gets easier.
Six faults you'll work in this module
You don't learn this by reading it — you learn it by doing it on six different faults, each drilling the same skill until it's instinct:
- The tech with the spare part already in hand. Two minutes online, zero parts.
- "The sensor is definitely fine". The tag told the truth the room was arguing about.
- The output that can't make up its mind. Scan order made the flicker; the fix is in the logic, not the card.
- Diagnosing blind from the paper. An hour of paper argument, answered in one connection.
- The screen says running, the machine says no. The HMI lied; the PLC didn't.
- Cold start on a machine you don't know. No prints, strange machine, still solved by going online.
The mistakes it kills
- Diagnosing from the printed program offline instead of connecting and reading live values
- Trusting the HMI graphic or the operator's story over the actual controller state
- Swapping parts before you've even read whether the PLC is commanding the output
Why it matters: Going online turns every argument about what the machine is doing into a fact you can read off the screen. It's the move that makes every other PLC skill possible.
Reading about it is one thing. Doing it under pressure is another.
faultline101 drops you into a running machine with the real evidence in front of you, and the readings respond to your choices — exactly like a real callout, on equipment you can't break. This is one of 35 modules and 210 scenarios in the trainer.
Works with no signal. Install it on your phone and the whole course — 35 modules, 210 faults — comes with you into the plant.
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