Blog Details

Why New DDC Controllers Show “Temperature Out of Range” - Even with New Sensors

December 1, 2025

If you’re working with Johnson Controls CGM9090, you might have seen this strange issue: Temperature shows 999°F / –40°F / ??? / 270000°F

Even when:
1.  New controller
2. New sensor
3. Wiring is correct
4. Polarity is correct
So what’s really happening?

The Real Root Cause (Most Engineers Miss This) The UI port is NOT configured as a thermistor input. CGM9090 UI ports can operate in multiple modes:
1. 0–10V
2. 4–20mA
3. Binary input
4. Pulse input
5. Resistive (Thermistor Mode)

When the UI is left in default mode (Voltage Input) and you connect a 10k NTC sensor, the controller interprets that resistance as a random voltage → giving unrealistic temperatures.

That’s why you see out-of-range readings like:
1. 124°F
2. –40°F
3. 270,994°F
4. “???”
5. “Out of Range”
This is not a wiring fault — it’s simply a configuration mismatch.

How to Fix It (Takes 30 Seconds)
In CCT / FX Workbench / SCT:
1. Open the UI port used for temperature
2. Set Input Type → Resistive
3. Select the correct curve:
a. 10k Type II
b. 10k Type III (based on your sensor)
4.  Save → Download → Reboot (if required)
Instantly, your reading snaps to normal room temperature.

Pro Tip from Field Experience
1. If you see a huge, unrealistic temperature → that’s always a sign the UI is in the wrong mode.
2. If you see –40°F or 124°F → that indicates short/open.
3. If the reading fluctuates → check cable type or noise.

Takeaway
CGM controllers are powerful, but their UI ports are fully software-defined.
A small configuration mismatch = a big troubleshooting headache.
Once you understand this, troubleshooting becomes super easy.

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