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11 fast troubleshooting checks

Eleven faults that show up on almost every startup, what usually causes them and the first thing to check.

Before you start

Look at the equipment before you look at the screen. Most of these come down to power, wiring, signal type or a setting, and the fastest path is to measure at the device.

1. Temp reads extremely low or high (e.g. −40 or 300+ °F)

   Likely causes

   Open or shorted sensor; wrong thermistor type configured; wiring on the wrong input

   Check first

   Measure resistance at the controller with the sensor disconnected and compare to the sensor's chart

2. Temp reads a few degrees off everywhere

   Likely causes

   Wrong sensor curve; lead resistance on a long run (matters for 1k RTDs, rarely for 10k thermistors); sensor in a bad location

   Check first

   Confirm the sensor type setting; compare at the sensor with a reference

3. AO commands but the device doesn't move

   Likely causes

   No actuator power; wrong signal type (0–10 V vs 2–10 V vs 4–20 mA); wiring reversed

   Check first

   Measure the output signal at the actuator, then the actuator supply

4. Valve or damper moves backward

   Likely causes

   Normally open device configured as normally closed; actuator direction switch

   Check first

   Check the device spec, actuator direction setting, and output reverse-acting option

5. BO commands but equipment won't start

   Likely causes

   HOA in Hand or Off; VFD not in Auto; safety circuit open; wired to the wrong relay contact

   Check first

   Check HOA and drive mode, then the safety chain, then NO vs NC contact

6. Status says running when it's off (or the reverse)

   Likely causes

   Current switch setpoint wrong; belt broken; status wired to the wrong input

   Check first

   Look at the equipment, then adjust or replace the current switch

7. Analog value scaled wrong

   Likely causes

   Range or units configured wrong; 0–10 V vs 2–10 V mismatch; wrong transmitter range

   Check first

   Compare the transmitter nameplate range to the BAS scaling

8. MS/TP devices drop offline or are intermittent

   Likely causes

   Duplicate MAC; missing or extra termination; baud mismatch; star wiring; shields grounded at both ends; Max Master too low

   Check first

   Check MAC addresses and termination first, then baud and wiring layout

9. Device won't discover on BACnet/IP

   Likely causes

   Wrong subnet or VLAN; duplicate device instance; BBMD or firewall blocking UDP 47808

   Check first

   Confirm IP settings and device instance, then the network path

10. Network point value is stale

    Likely causes

    COV not subscribed; poll rate too slow; device offline; wrong object mapped

    Check first

    Compare to the equipment display and check the poll or COV setting

11. Controller resets or loses program

    Likely causes

    Undersized or shared transformer; voltage sag on equipment start; failed backup battery

    Check first

    Measure supply voltage during equipment starts

Prefer paper? Download the one-page cheat sheet (PDF).