Troubleshooting guide

Why Is My Temperature Sensor Reading Incorrectly?

RTD versus thermocouple setup, wiring, compensation, placement and scaling explain wrong readings. Includes 2-wire RTD lead error.

Controls & Instrumentation · Last reviewed October 17, 2026

A temperature reading depends on the sensor, its wiring, the input that converts the signal, the scaling and where the sensor sits. A wrong reading has to be located in that chain. The first checks are the sensor type, the input configuration and the wiring.

Sensors are often in hot, pressurized or electrically live equipment. Removing a sensor from a process, working on a thermowell or wiring in a live panel belongs with qualified personnel following lockout and site procedures.

What you observe	Likely cause	What to check
Reading is a constant offset high	A 2-wire RTD, with lead resistance added to the sensor	Wiring: 2, 3 or 4-wire, and the input configuration to match
Reading is wildly wrong (very high or very low)	Input set for the wrong sensor type (RTD versus thermocouple, Pt100 versus Pt1000, type J versus K)	Input configuration against the sensor on the nameplate
Thermocouple reads wrong by a few degrees, varies with the panel temperature	Cold-junction compensation not working, or the wrong extension wire	CJC sensor, extension wire type and polarity
Reading shows the ambient, not the process	Poor placement: sensor not immersed enough, in a stagnant pocket or touching the wall	Immersion depth and location compared with the process
Reading is slow to follow the process	Thermowell, poor contact or a slow sensor	Response time, and the use of heat-conducting compound in the well
Reading is right at the transmitter, wrong in the PLC	Scaling or 4–20 mA range mismatch	Transmitter range against the PLC scaling
Reading is unstable	Noise on a long cable, a poor connection or a ground loop	Shielding, grounding and the terminals

Lead resistance in a 2-wire RTD

A Pt100 RTD changes by about 0.385 Ω per °C. In a 2-wire connection the instrument measures the sensor plus the resistance of both lead wires. If the leads add 3 Ω in total:

Error = 3 Ω ÷ 0.385 Ω/°C = 7.8 °C too high

A 3-wire connection measures the lead resistance and subtracts it, and a 4-wire connection removes it entirely. The 3-wire form works well when the three leads have equal resistance. A 2-wire sensor is acceptable only with very short leads, or when the lead resistance is measured and compensated.

Thermocouple points

A thermocouple generates a small voltage (about 0.04 mV/°C for a type K), which depends on the temperature difference between the hot junction and the cold junction at the terminals. The instrument must know the terminal temperature to compensate.
Use extension wire of the matching type and keep the polarity right. Ordinary copper wire adds new junctions and errors.
Setting the input to type J when the sensor is type K gives an error that depends on the temperature, and can be tens of degrees.

Write a useful problem statement

Sensor type, wiring (2, 3 or 4-wire, or thermocouple type), cable length and the input module.
The reading, the expected value and a reference measurement from a calibrated instrument.
The input configuration and scaling, and whether the error is constant or changes with temperature.
Sensor location and immersion, and recent changes.

Related tools and labs

Calculators, labs and simulators are part of the All-Access Pass.

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Questions

How do I check an RTD quickly?

With the sensor disconnected, measure its resistance and compare it with the table for its type. A Pt100 reads 100 Ω at 0 °C and 138.5 Ω at 100 °C.

Why is a 3-wire RTD common?

It compensates for lead resistance at a modest cost, provided the three wires are matched.

Does a thermowell change the reading?

It delays the response and, with poor immersion, can bias the reading toward the pipe or ambient temperature.

Educational material. Examples use simplified, hypothetical inputs and are not a design, a construction document or professional engineering advice. Confirm requirements with the codes and standards adopted where the work is built and with a licensed engineer.

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