Bunn Slush Machine TEMPERATURE SENSOR OPEN: Low Product or Sensor
TEMPERATURE SENSOR OPEN means either the product is not covering the back of the drum, or the thermistor is unplugged or its wiring is broken. Check the product level first and add product above the minimum fill line. If the message stays on with a full barrel, turn the machine off and call us.
Medium: the machine may still run, but with the barrel temperature unreliable it can freeze too hard or too soft.
- Code
- TEMPERATURE SENSOR OPEN
- Maker
- BUNN
- Equipment
- Slush & frozen drinks
- Applies to
- ULTRA-1 / ULTRA-2 frozen drink dispenser
What does BUNN TEMPERATURE SENSOR OPEN mean?
The thermistor on the ULTRA-1 and ULTRA-2 reads the temperature of the barrel, and the control uses it to decide when to freeze and when to rest. The control also knows what a normal reading looks like. If the sensor circuit appears open, as if the wire were cut, the control shows TEMPERATURE SENSOR OPEN. The manual gives two meanings. The first is harmless: if the barrel is low, the product does not cover the back end of the drum, the drum gets colder than the control expects and the reading drops out of range. Adding product fixes it. The second is a real open circuit: the thermistor plug has come loose, the wire is broken, or the thermistor has failed. Because the control cannot trust the temperature, it cannot manage freezing well, and product quality can suffer. Checking the barrel level takes a minute and should always be done first.
Affected models and control systems
This page covers the older Bunn ULTRA-1 and ULTRA-2 frozen drink dispensers. The Ultra NX uses different messages.
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
- TEMPERATURE SENSOR OPEN shows on the display
- Barrel freezes too hard or does not freeze evenly
- Message clears when product is added to a low barrel
Possible causes
Listed in BUNN's documentation
- Product level below the minimum fill line, leaving the back of the drum uncovered
- An unplugged or loose thermistor connector
- A broken thermistor wire or failed thermistor
- A dead thermistor input on the control board
Other likely causes — what usually sits behind TEMPERATURE SENSOR OPEN on this control. A technician confirms which one it is before any part is replaced.
- A barrel that runs low during rush hours and stays below the fill line for long stretches
- A thermistor wire that has been pulled or pinched during a past cleaning or service
- A connector that has corroded from sticky syrup spills that reached the back of the machine
Safe checks before calling for service
- Look at the barrel and add product until it is above the minimum fill line.
- Watch whether the message clears after the barrel has product again.
- If it stays on with a full barrel, leave the back panel closed.
- If it still shows, turn the machine off and call us. Tell us which barrel it is and whether the level was low.
How a technician diagnoses TEMPERATURE SENSOR OPEN
- Read the display and confirm which barrel shows the message.
- Check product level against the fill line and note whether the reading changes.
- Check the thermistor plug and the wire for breaks or corrosion.
- Measure the thermistor resistance at the plug and compare it with the temperature.
- Replace the thermistor or repair the harness only after the control input has been proven good.
- An open circuit can hide behind a low barrel, so a repeat message needs a meter test of the thermistor and plug.
- Reaching the thermistor and its connector means removing panels on a machine with line-voltage wiring.
- Running with an unreliable temperature reading can freeze the barrel too hard or leave product soft.
How TEMPERATURE SENSOR OPEN differs from similar codes
The paired message TEMPERATURE SENSOR SHORTED means the opposite reading, a short, not an open circuit.
Technical reference
- Document
- ULTRA-1/ULTRA-2 Service & Repair Manual 41084.0000D, Troubleshooting p.19-21
- Maker documentation
- BUNN owner support
- Edition
- 41084.0000D 03/13 (copyright 2009 Bunn-O-Matic Corporation)
- Models / control
- ULTRA-1 / ULTRA-2 frozen drink dispenser
- Section
- Troubleshooting flowcharts: Torque Sensor Error p. 19, Temperature Sensor Error p. 20, Clean Filter Messages p. 21
- 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 BUNN slush & frozen drink error codes
Can I keep serving with TEMPERATURE SENSOR OPEN on?
If it cleared after adding product, yes. If it stays on, the barrel may not be managed correctly, so call us.
Will it clear by itself once I add product?
Often yes, within a short time. If it does not, the sensor circuit is the next suspect.
What is the difference between open and shorted?
Open means the control sees no connection, as if the wire were cut. Shorted means the reading is stuck at an impossible value.
Need help with BUNN TEMPERATURE SENSOR OPEN?
A technician confirms the cause on site and gives you the price in writing before any work starts.