WiremanWireman · 4–20 mA sensor reading wrong: troubleshooting for electricians · 2023 NEC ·
4–20 mA sensor reading wrong
Measure the loop current, then decide whether it’s the transmitter, the loop or the scaling.
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How it works
A 4–20 mA loop sends a measurement as a current: 4 mA is 0% of the range and 20 mA is 100%, so 12 mA is 50%. Current is the same everywhere in a series loop, so wire resistance doesn’t change the reading. Starting at 4 mA instead of 0 lets the system tell a real zero from a broken wire: 0 mA usually means the loop is open. Many PLC inputs read the current through a 250 Ω resistor, which turns 4–20 mA into 1–5 V.
Every question and answer in this guide. Tap “Step by step” to walk through it.
- What is the display or PLC showing?
- Zero, below range, or a faultTest: Check the loop supply at the transmitterExpect: The loop supply (commonly 24 V DC) at the source, and at the transmitter at least the minimum voltage in its manual.
- No voltage at the supplyNo loop power
- Supply good, 0 V at the transmitterOpen loop
- Full supply voltage at the transmitterThe transmitter isn’t drawing loop current
- Voltage present, but below the minimumNot enough voltage for the transmitter
- Pegged at maximumTest: Measure the loop current in seriesExpect: A steady current between 4 and 20 mA that matches the process (for example 12 mA at half range).
- Correct current, display wrongScaling or configuration error
- Current wrong for the processTransmitter calibration or sensing problem
- Above 20 mA or below 4 mAOut-of-range current: a fault or a short
- Current is noisy or jumpsElectrical noise or a grounding problem
- Reads, but wrong or jumpyTest: Measure the loop current in seriesExpect: A steady current between 4 and 20 mA that matches the process (for example 12 mA at half range).
- Correct current, display wrongScaling or configuration error
- Current wrong for the processTransmitter calibration or sensing problem
- Above 20 mA or below 4 mAOut-of-range current: a fault or a short
- Current is noisy or jumpsElectrical noise or a grounding problem
- Zero, below range, or a faultTest: Check the loop supply at the transmitterExpect: The loop supply (commonly 24 V DC) at the source, and at the transmitter at least the minimum voltage in its manual.