SiteIt's 1am. The wrapper is stood with "waiting for turntable home" on the HMI. The turntable is home. Your fitter has checked the sensor, cleaned it and set the gap. Still nothing.
Somewhere between that sensor and the program, the signal has gone missing. Or the PLC is seeing it fine and something else is holding the machine. You need to know which, and you need to know fast.
Most I/O faults are found by following the signal in a straight line, one point at a time. It sounds slow. It's quicker than guessing.
Get the drawings and the I/O list
Before you open the panel, find the electrical drawings and the I/O list. You want three things for the signal you're chasing: the input address, the terminal number, and the fuse or breaker that feeds it.
No drawings? Then the panel labels and the wire numbers are your map. If they're missing too, that's a job for days. Note it down. The next person to chase this signal will thank you.
Inputs: work from the field to the card
Start where the signal starts. Is the sensor or switch actually changing state? Its LED tells you, if it has one. For a plain switch or a contact, you'll need a meter.
Then go to the input card. Check the input LED for that address. Then measure at the input terminal, against the card's 0V or common.
- 24V at the terminal and the input LED on: the PLC can see it. Your fault is in the logic. Go to the next section.
- 24V at the terminal but no input LED: suspect the card, or a missing common on that group.
- No 24V at the terminal, but the sensor is switching: the fault is in between. Look at the cable, the junction box, the marshalling terminals and any fused terminals on the way.
- Nothing anywhere: check the supply to the sensor. A blown fuse can take out a whole group.
That last split is where half-split fault finding earns its keep. Measure at the junction box halfway along. You've cut the problem in half with one reading.
Watch for sinking and sourcing. Fit a PNP sensor to a card that wants NPN and the sensor LED looks perfect. The input never comes on. Our proximity sensor guide has the full list of what "like for like" has to match.
When the input is there but nothing happens
If the input LED is on and the machine still won't move, the PLC is seeing the signal. Something else in the program is holding it. That might be another interlock, a timer, a step it's stuck on, or a fault bit that needs clearing.
The HMI may tell you. If not, you need to go online and watch the logic. Going online is a separate skill with its own risks, so we've written it up in going online to a PLC to fault-find.
Outputs: work from the card to the field
Now the other way. The PLC has turned an output on, and the thing it drives hasn't moved.
Check the output LED. If it's on, measure at the output terminal. Then follow it out. Most outputs don't drive the load direct. They go through a fuse, an interposing relay, or a valve terminal on a bus.
- Output LED on, no voltage at the terminal: suspect the card or its supply.
- Voltage at the terminal, interposing relay not pulling in: check the relay coil and its 0V.
- Relay pulled in, nothing at the load: check the relay contact and the fuse after it.
- Voltage at the solenoid, valve not shifting: now it's a mechanical or air problem. See solenoid valve faults.
Live testing is live work
Most of this has to be done with the panel live. That makes it live work. HSE's guidance on the Electricity at Work Regulations says that "testing of live exposed conductors using a test instrument is live work" (HSR25, paragraph 50). Live work is only allowed when it's unreasonable for the kit to be dead, reasonable to work live, and suitable precautions are taken (regulation 14).
Fault-finding a 24V signal often passes that test. But the 24V terminals sit inches from 230V and 400V. Shroud what you can. Use GS38 leads. Set the meter before the probes go on. GS38 says a meter should be "set to the correct function and/or range before the connections are made" (GS38). A meter left on amps across a supply is a flash waiting to happen.
Don't force an output to see what moves
It's tempting. You go online, force the output on, and watch. On a machine with people near it, that's how someone gets hurt. A forced output ignores the interlocks the program would normally check.
HSE's guidance on control systems puts it simply: "Isolation and the immobilisation of equipment by mechanical means are the best methods of ensuring safety during maintenance operations" (HSG238). If you must drive an output by hand to prove it, clear the machine, isolate what you can, tell everyone, and have a second person watching. Then take the force off before you walk away.
The skill gap at 1am
Most mechanical fitters can clean a sensor. Fewer can read an input card, meter a common and tell a sourcing card from a sinking one. Fewer still can go online. On a lot of sites, that skill lives on days, or in one person.
When it isn't on shift, the wrapper waits. Every hour stood is lost output, idle labour and late trailers. Be honest about which of your engineers can do this work, and where the gaps are. Our piece on fitters, electricians and controls engineers can help you map it.
What to do on Monday
- Put a printed I/O list and the drawings in every panel, or a QR code to them.
- Label every input and output terminal with its address.
- Check which engineers on each shift can trace a signal at the card. Write it down.
- Make "no forcing outputs without a second person" a site rule.
Paragraph numbers refer to HSR25, third edition (2015).