Control Board Troubleshooting- The Ins ‘n’ Outs

inputElectronics work just like you do; they determine what’s happening by looking at inputs like temperature and pressure, and decide what to do based on what they “see”.

The only difference between your brain and electronics is you use your eyes, ears, and hands to determine what’s happening, electronics uses sensors like thermistors and transducers.

Electronic control systems essentially have three components:

  • Inputs
  • Control Board
  • Outputs

To diagnose electronic controls you need to know how to check inputs and outputs.

There are two types of inputs and outputs:


Also called Digital or Discrete. is the same as open or closed contacts on a relay or switch. Think of it like the on/off switch on a radio.


Is a variable signal. Think of it like the volume knob on a radio. There are a few types of analog signals, but HVAC typically uses 0-9 volts dc or 4-20mA, but there are others, so pay attention to the wiring diagram of the equipment.

Examples of Binary inputs:

  • High / Low pressure switch
  • High temperature safety
  • Thermostat
  • Damper end switch
  • Dirty filter switch
  • Fan proving switch
  • Time clock

Examples of Analog inputs:

  • Suction or discharge pressure transducer
  • Supply air temperature thermistor
  • Humidity sensor
  • Supply static pressure sensor
  • Building pressure sensor
  • Room temperature thermistor


Checking a binary input is simple; Set your multimeter to the same voltage as the circuit being tested, and place a meter lead on each terminal of the input. Zero volts is closed, a voltage reading is open.

Checking analog inputs varies according to the type of signal:

To check a mA (milliamp) signal, set your multimeter to the mA scale, disconnect one of the sensor’s wires, and connect your meter in series with the sensor by placing one lead on the wire that was removed, and the other on the terminal it went to. The mA value will read out on the meter.

To check a 0-9 volt dc signal, set your multimeter to the volts DC scale, place one meter lead on each of the control wires; the meter will give a direct readout of the signal.


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Patrick is Zen HVAC’s diagnostic and training guy.

Patrick started in the trade the day he left technical school and never looked back.

He’s served in various technical and training roles in the HVAC industry but specializes in system troubleshooting and diagnostics, retro commissioning, and technical training.

His moto: If I can understand it, anybody can.

Patrick uses the Zen common sense approach to teach

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Patrick’s Dislikes- Taking work too seriously. Anything unintuitive. Emoticons :(

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