4 to 20 mA Loop Troubleshooting: Reading the Number Before You Replace the Transmitter
Read the current first
Before touching anything, measure the loop current. The number tells you most of what you need:
| Reading | What it means |
|---|---|
| 0 mA | Open circuit. A cut wire, a loose terminal, a blown fuse, or a dead loop supply. |
| Under 3.6 mA | Below live zero. On most transmitters this is a deliberate downscale fault indication. |
| 3.8 to 4.0 mA | Process at or below the bottom of range. Usually genuine. |
| 4 to 20 mA | Normal operation. Your fault is elsewhere. |
| 20 to 20.5 mA | Process at top of range. |
| Over 21 mA | Upscale fault indication, or a wiring short across the loop. |
| Around 24 mA and pinned | The transmitter has failed to its upscale alarm, or something is feeding the loop that should not be. |
This is the whole point of a live zero. If the standard were 0 to 20 mA, a reading of 0 would be ambiguous: it might mean an empty tank or a severed cable, and you would have no way to tell. Starting at 4 mA makes a dead loop unmistakable.
Where to put the meter
The loop is a series circuit, so current is identical everywhere in it. That means you can break it at any convenient point and read the same number. Practically:
- At the PLC terminals. Easiest access, and it tells you what the card is actually seeing.
- At the transmitter. Tells you what the transmitter is actually sending.
- At any junction box in between.
If the reading at the transmitter is good and the reading at the card is bad, the fault is in the cable or a terminal between them, and you have just halved the problem. Halving the problem is the whole method. Do not start at one end and work along; start in the middle.
Many transmitters have test terminals that let you measure without breaking the loop. Use them if they are there.
The order to check things
1. Is the loop powered?
A two-wire transmitter is powered by the loop itself. No supply, no current, and the transmitter is not faulty. Check the 24 V supply and its fuse before anything else. This is the single most common cause and the one most often skipped.
2. Is there a break?
0 mA with a healthy supply means an open circuit. Work from the middle. Check terminals under the screws, not just the visible wire; a strand that has broken inside the insulation reads open and looks perfect.
3. Is the polarity right?
A 4 to 20 mA loop is DC and polarity matters. Reversed at the card usually gives you nothing at all. It is a five-second check.
4. Is the burden resistance too high?
Every device in the loop drops voltage. Add up the transmitter's minimum operating voltage, the resistance of the card, any indicator, any isolator, and the cable resistance. If the total exceeds what the supply can deliver at 20 mA, the loop works at low readings and fails at high ones. A loop that reads fine at 30% and pins at 80% is very often a burden problem, not a transmitter problem.
5. Is the card configured for the right signal?
A card set for 0 to 10 V will read nonsense from a current loop, and a channel configured 0 to 20 mA will misreport a 4 to 20 mA device by the offset. Check the hardware configuration before you doubt the field.
6. Is the scaling right in the program?
If the current is correct and the displayed value is wrong, the fault is in software, not in the field. See NORM_X and SCALE_X analog scaling.
Two-wire, three-wire, four-wire
- Two-wire (loop powered): the same pair carries power and signal. Most common for pressure and level transmitters.
- Three-wire: separate supply positive, shared common, signal out.
- Four-wire: transmitter is separately powered and outputs current independently.
Wiring a two-wire device as though it were four-wire, or the reverse, is a frequent commissioning error. Read the transmitter label, not the previous drawing.
Noise and shielding
Intermittent readings that jump when a motor starts are almost always an earthing or screening problem, not a transmitter problem. Two rules:
- Earth the screen at one end only, normally the control panel end. Earthed at both ends, the screen carries circulating current and becomes an aerial.
- Keep signal cable away from motor cable. If they must cross, cross at right angles. Running a 4 to 20 mA cable alongside a VFD output cable in the same tray is a reliable way to manufacture a fault that no amount of transmitter replacement will fix.
What "the transmitter is faulty" usually means
In training and on site, the running order of actual causes is roughly:
- Power supply or fuse
- Terminations, especially under screws in junction boxes
- Cable damage, often where it enters a gland
- Wrong card configuration
- Burden or supply voltage margin
- Software scaling
- The transmitter itself
The transmitter is last on that list, and it is the first thing most people replace. Measuring the current at two points takes two minutes and tells you which half of the loop to look in.
Practise with a meter in your hand
Fault finding is not a reading skill. On the Automation Engineer Program you calibrate loops, break them deliberately, and find the faults with a multimeter and a loop calibrator, which is what an interviewer means when they ask how you would approach a dead signal.
Related: I/O loop checking during commissioning, The fault-finding interview question.
