Instrumentation Engineer’s 3-Minute PT100 Check: Fix “No Output” on STT Without Replacing the Transmitter

TonyZhang
Feb 13
4 min read

1. “STT Has No Output?” – Often the Symptom, Not the Root Cause

On a recent project, an STT temperature transmitter had already been wired into the PLC analogue input. During pre-commissioning, that channel showed:

Temperature value fixed or not updating in the HMI
4–20 mA signal not within the expected range

The first reaction on site was predictable:

“The transmitter has no output. Is it faulty?”

Instead of immediately replacing the device, we started at the input. In many cases where the STT appears to have “no output / no change”, the underlying cause is a 3-wire PT100 wiring error. That drives the RTD measurement into an abnormal state, and the transmitter’s output follows into an error condition, which looks like “no output” from the outside.

One-line conclusion:

When an STT channel shows “no output / no change / abnormal output”, first verify the 3-wire PT100 wiring. Use a multimeter to identify the two wires from the same end of the element; those two go to terminals 21/22, the remaining wire goes to 23. Getting this step correct eliminates a large percentage of “no output” cases immediately.

2. Field Diagnostic in 10 Minutes: Loop First, Then Sensor

Step 1 – Verify the 4–20 mA Loop Actually Has Current

Before touching the PT100, confirm that the output current loop is alive.

Set the multimeter to mA range.
Insert the meter in series in the 4–20 mA loop (break the loop and insert the meter as a link).
Read the loop current.

Interpretation:

Measured current = 0 mA
Measured current non-zero but abnormal or not following temperature

Note: Step 1 is purely about the 4–20 mA loop. It is independent of the PT100 input wiring.

Step 2 – 3-Wire PT100 Connection: Identify the Same-End Pair, Then Land on Terminals

Most field errors do not come from exotic faults; they come from “guessing by colour” when landing the three PT100 conductors.

The correct connection logic for a 3-wire PT100 to STT is:

Two wires from one end of the RTD element (same end)
One wire from the other end

STT terminal assignment:

Terminals 21 and 22 → the two wires from the same end of the PT100
Terminal 23 → the single wire from the opposite end

The key is to identify the same-end pair by measurement, not by insulation colour.

Quick procedure to identify the same-end pair

Use a multimeter:

Set the meter to continuity or low ohms range.
Measure resistance between each pair of the three wires:

You will observe:

Two measurements will show higher resistance (full RTD element plus lead resistance).
One measurement (between the two same-end wires) will show significantly lower resistance, because both wires are connected to the same end of the PT100 element and differ only in lead resistance.

If continuity mode beeper is not reliable at ~100 Ω levels:

Use ohms range and select the pair that gives the lowest reading.
That pair is the same-end pair.

Then:

Land the same-end pair on terminals 21 and 22.
Land the remaining single wire on terminal 23.

One-line memory aid:

Same-end two wires → 21 & 22; the remaining single wire → 23.

Step 3 – Recognise the Three Most Common 3-Wire PT100 Wiring Errors

From site experience, most PT100-related “no output / no change” problems fall into one of three categories:

Same-end pair not landed on 21/22
Terminals mislanded (wrong terminal block)
Same-end identification based on colour or experience only

In all three cases, the transmitter is usually healthy; it is responding correctly to an invalid input connection. Correcting the wiring immediately restores normal output behaviour.

3. Design-Stage Measures We Now Treat as Mandatory

To avoid consuming commissioning windows on wiring-related issues, we recommend closing the loop at design and construction stage. Two low-cost measures have a high impact:

Drawing annotation
Construction / FAT checklist item

By treating these as formal steps rather than “good practice when time allows”, you prevent late-stage commissioning from paying the price for simple wiring mistakes.

4. How the Project Was Closed Out

On the referenced project, we did not replace the STT. Instead, we:

Used a multimeter to identify the two same-end PT100 wires.
Re-terminated the PT100 to follow the required pattern:

As soon as the RTD input returned to a valid state:

The STT’s internal measurement stabilised.
The 4–20 mA output immediately began tracking temperature changes as expected.

The key lesson was not “how to fix this once”, but:

Do not let a limited commissioning window absorb the cost of an avoidable wiring error. Build the PT100 same-end identification and landing check into standard practice.

If you are standardising STT or other temperature transmitters across multiple projects and want to codify loop checks + 3-wire PT100 wiring checks into your commissioning procedures, ODES can provide reference wiring guides and test templates.

📩 Email – tonyzhang@odes-electric.com

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