UniMac Commercial Dryer Error Code OP: Open Thermistor
OP on a UniMac 120 or 170 lb dryer means the control sees an open circuit in the thermistor that senses temperature. Turn the dryer off, let it cool and clean the lint screen. Leave panels closed and call us to test the sensor and its wiring.
Medium: the dryer will not heat, but the control stops safely and there is no direct hazard.
What does UniMac error code OP mean?
A thermistor is a small sensor whose electrical resistance changes with temperature. The control passes a small current through it and reads the result as heat in the dryer. If that circuit is open, there is no current, and the control cannot tell what the temperature is. OP stands for open, and it means a broken wire, an unplugged connector or a sensor that has died. Because the control cannot manage heat safely without a reading, it stops heating rather than run blind. The drum may still turn, but clothes will not dry. This is an older control line that many laundries still use, and we repair it. Compare it with SH, which is the shorted version, where the control reads an extreme temperature although the drum is not necessarily hot.
Affected models and control systems
This applies to UniMac 120 and 170 lb tumble dryers with the On Premise Micro control, such as the UA120L, in the TMB1268C troubleshooting family.
Code numbers and their meaning can change between control generations. We match the full model and control before acting on a code.
What you may notice
- OP on the display
- Drum turning but no heat
- Cycle ending early with damp clothes
Possible causes
Listed in UniMac's documentation
- Thermistor connector seating
- Wire run to the thermistor, for breaks or chafing
- The thermistor itself
- Control input for the thermistor
Other likely causes — what usually sits behind error code OP on this control. A technician confirms which one it is before any part is replaced.
- A connector pulled partly loose when the dryer was serviced or moved
- A wire stretched or chafed where it crosses a hinge or sharp panel edge
- A thermistor weakened by years of heat cycling that now reads open
Safe checks before calling for service
- Switch the dryer off at its power control and let the drum cool.
- Empty the lint screen and check around the door opening for packed lint.
- If you ever smell gas, walk out of the area and phone the gas utility from outdoors.
- Leave the panels closed.
- Tell us whether OP appeared suddenly or came and went, and call us.
How a technician diagnoses error code OP
- Read the code and confirm it persists after a power cycle.
- With power off and locked out, trace the thermistor wiring and inspect the connector for looseness or corrosion.
- Measure the thermistor resistance and compare it with the expected value at room temperature.
- Check the wire run for continuity from sensor to control.
- If the sensor and wiring pass, test the control input.
- Replace the thermistor or harness that failed, then run a heat cycle.
- The thermistor and its wiring are inside the cabinet near live control and burner parts.
- Reading resistance against temperature needs a meter and the right values.
- A dryer that heats without a good temperature reading can overheat, so it should not be forced to run.
How error code OP differs from similar codes
OP is the open-circuit reading and SH is the shorted one; both involve the same thermistor circuit but point to different wiring problems.
Technical reference
- Document
- Tumble Dryers 120/170 lb Troubleshooting TMB1268C, Part No. 70399901R2, Dec 2016 (pp.53, 65)
- Maker documentation
- UniMac owner support
- Edition
- Part No. 70399901R2, December 2016; models through serial no. 0907000048
- Models / control
- Tumble dryers 120/170 lb (TMB1268C family) micro control
- Section
- Hybrid Timer Control Troubleshooting - 31. Error Codes, printed p. 53 (PDF p. 55); On Premise Micro Control (RM) Troubleshooting - 36. Error Codes, printed p. 65 (PDF p. 67)
- Checked
- October 2026
We use the maker's document to read the code and do not reproduce it. Always confirm the complete model and control before applying these steps.
Related UniMac dryer error codes
Can I keep using the dryer with OP showing?
The drum may turn but it will not heat. The control stops heat on purpose when it has no temperature reading.
Does OP mean the thermistor is dead?
Not always. A loose connector or a broken wire gives the same reading, so a test comes before a part.
Is OP the same as SH?
No. OP is an open circuit and SH is a shorted one.
Need help with UniMac error code OP?
A technician confirms the cause on site and gives you the price in writing before any work starts.