Modern HVAC/R equipment relies on electronic control boards, variable-speed drives,
and an array of sensors to optimize efficiency and protect components.
This lesson introduces the key electronic devices a technician will encounter,
explains how each works electrically, and outlines the diagnostic approach for
each type.
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1.7.1 — Electronic Control Boards
An electronic control board (ECB) replaces multiple electromechanical relays and
timers with a microprocessor that reads sensor inputs, executes a programmed
sequence of operation, and switches output loads. Understanding what the board
does — and what it does not do — is the foundation of electronic diagnostics.
Compares sensor values to programmed set points and limits
Switches output relays or solid-state switches to energise loads (contactor coils, fans, defrost heaters, solenoid valves)
Enforces timing functions (anti-short-cycle timers, defrost intervals, fan delay)
Monitors safety limits and locks the system out when a fault condition persists
Displays fault codes via LED flash sequences, seven-segment displays, or communicates codes via a communication bus to a thermostat or BMS
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Input Side
Low-voltage sensor signals and binary switch inputs (24 VAC or 5 VDC logic). Includes thermistor voltage dividers, pressure transducer 4–20 mA or 0–5 V signals, and dry-contact inputs from safety switches.
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Microprocessor (Logic)
Executes the sequence of operation firmware. Compares inputs to set points, applies timing rules, sets fault flags, and determines which output loads to energise. Not field-serviceable — board replacement is the remedy for logic failure.
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Output Side
Relay or triac outputs switch 24 VAC or line-voltage loads. Common outputs: compressor contactor coil (Y), condenser fan (O/B), indoor fan (G), reversing valve (O/B), auxiliary heat, defrost heater, crankcase heater.
Diagnosing a Control Board
Read and record any fault codes before clearing. The fault history often identifies the failure mode (e.g., “high pressure lockout,” “thermistor open”).
Verify supply voltage to the board: confirm the correct transformer secondary voltage is present at the board power terminals (typically 24 VAC ± 10%).
Check all inputs. With the diagram, verify each binary input (thermostat call, safety switch states) and each sensor signal is within the expected range. An open thermistor or a safety switch held open by a fault condition will prevent the board from energising outputs.
Check all outputs. If input conditions are correct but the load is not energised, confirm the board is outputting voltage to the load terminals. If board output is present but the load does not respond, the fault is in the load circuit, not the board.
Inspect the board visually (with power off and locked out) for burnt components, damaged traces, cracked solder joints, or evidence of moisture, pest intrusion, or arcing.
Replace the board only after confirming the root cause is in the board logic or its output drivers — not in an external sensor, wiring fault, or load failure that would damage a new board.
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Static discharge and board handling
Electronic control boards are sensitive to electrostatic discharge (ESD). Always handle replacement boards in their anti-static packaging until the moment of installation. Touch a grounded metal surface or wear an ESD wrist strap before handling exposed boards or connectors. A static discharge that is too small to feel can destroy a microprocessor or memory cell invisibly, causing intermittent failures that are very difficult to diagnose.
1.7.2 — Variable-Frequency Drives & ECM Motors
Variable-speed technologies allow fans and compressors to match their output to
actual load rather than cycling on and off at full capacity. Two technologies
dominate HVAC/R applications: Variable-Frequency Drives (VFDs) used with standard
AC induction motors, and Electronically Commutated Motors (ECMs) that have
the drive circuitry integrated into the motor itself.
Variable-Frequency Drive (VFD)
Converts incoming AC to DC (rectifier), then synthesises new AC at a controlled frequency (inverter stage)
Motor speed is proportional to output frequency: lower Hz → slower speed
Used with standard 3-phase induction motors on large condensing units, cooling towers, and AHU fans
Parameters are programmed via a keypad or PC interface: minimum/maximum Hz, accel/decel ramps, fault responses
Fault codes are displayed on the drive keypad; record before clearing
Electronically Commutated Motor (ECM)
Permanent-magnet DC motor with integral variable-speed inverter module in the motor end cap
Receives speed command as a 0–10 V, PWM, or communication signal from the control board
Common in residential furnace blowers, ECM condenser fans, and fan coil units
Highly efficient at part-load: 60–70% less energy than PSC motors at low speeds
Two-part assembly: motor (rotor + stator) and control module. Either part can be replaced independently on most models
Fault indications: motor LED flash codes or communication bus fault flags
Feature
VFD + Induction Motor
ECM (Integrated)
Motor type
Standard 3-phase AC induction
Permanent-magnet DC (brushless)
Drive location
Separate panel-mounted or wall-mounted drive
Integral to motor end cap
Speed command
Frequency (Hz) set by drive output
0–10 V, PWM, or serial signal from control board
Typical application
Large commercial AHUs, cooling tower fans, rooftop condensers
Residential/light-commercial blowers, small condenser fans
Field programming
Drive keypad / PC software
DIP switches or control board parameters
Diagnostic access
Drive display: fault codes, output frequency, current
LED flash codes on module; control board fault flags
Replacement unit
Drive replaced separately from motor
Module or full motor assembly replaced
VFD / ECM Diagnostic Approach
Record all displayed fault codes before clearing; photograph the keypad or LED sequence.
Verify incoming line voltage and phase balance at the drive input terminals (within ±10% of rated, phase imbalance < 2%).
Verify the speed command signal is present and within range (e.g., 0–10 VDC from the control board to the VFD or ECM module).
Check drive output frequency/voltage to the motor (VFD) or confirm the ECM module is receiving and acting on its command signal.
Inspect drive heat sink, cooling fan, and cabinet for blocked airflow, excessive dust build-up, or evidence of overheating.
For ECMs: verify the control module connector is fully seated and free of corrosion; low-voltage connector issues are a common cause of erratic speed behaviour.
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VFD DC bus voltage — wait before working
After disconnecting power to a VFD, the internal DC bus capacitors can retain dangerous voltage (400–700 VDC on a 208–240 V drive) for several minutes. Always wait the discharge time stated on the drive label (typically 5–10 minutes) and verify DC bus voltage has dropped below 50 VDC with a meter before opening the drive enclosure or handling wiring inside it.
1.7.3 — Thermistors (NTC & PTC)
A thermistor is a resistor whose resistance changes significantly with temperature.
HVAC/R control boards use thermistors to measure refrigerant, air, coil, and
ambient temperatures without the expense of a full thermocouple or RTD system.
Two types are used, with opposite behaviours.
↓Ω
NTC — Negative Temperature Coefficient
Resistance decreases as temperature increases. The most common
thermistor type in HVAC/R. Used for: suction/discharge temperature sensing,
coil temperature (defrost termination), indoor/outdoor ambient sensing, and
return-air temperature. Typical resistance at 25°C: 5 kΩ to 10 kΩ;
increases sharply as temperature drops.
↑Ω
PTC — Positive Temperature Coefficient
Resistance increases as temperature increases. Used as
self-regulating current limiters (crankcase heaters, motor start assist),
and as motor winding overtemperature protectors. A PTC placed in a motor
winding will sharply increase resistance when overheating, reducing current
flow before the winding insulation fails.
How a Thermistor Input Circuit Works
The control board supplies a small reference voltage (typically 5 VDC) through
a fixed resistor to the thermistor. As temperature (and therefore thermistor
resistance) changes, the voltage at the junction between the fixed resistor and
the thermistor changes. The board’s analog-to-digital converter reads this
voltage and converts it to a temperature reading using a look-up table stored
in firmware.
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Worked Example — Diagnosing an NTC Thermistor
Complaint: control board displays “coil sensor fault” and disables defrost.
The coil NTC thermistor is a 10 kΩ type (resistance at 25°C = 10 000 Ω).
Read the fault code to confirm it is a thermistor open or short fault (not a temperature limit fault).
Disconnect the thermistor connector at the control board. Measure resistance across the thermistor leads with a DMM.
Interpret the reading:
O.L. (open): thermistor is open-circuit. The sensor has failed open. Replace the thermistor.
Near 0 Ω: thermistor is shorted. Replace the thermistor.
Reasonable value (e.g., 8–15 kΩ at room temperature): thermistor itself is likely good. Suspect the wiring (broken wire, corroded connector) or the board input circuit.
If thermistor reads reasonable resistance, check the wiring from the sensor to the board for continuity and shorts to ground.
If wiring is good, consult the service data for the expected voltage at the board input terminal; if 5 V reference is missing, the board input circuit may be damaged.
Fault Reading
Likely Cause
Board Behaviour
Action
O.L. (open)
Thermistor failed open; broken lead
Board displays open sensor fault; may default to failsafe mode or lockout
Replace thermistor; repair lead break
Near 0 Ω
Thermistor shorted; leads shorted together
Board reads implausibly high temperature; may lockout on high-temp fault
Replace thermistor; inspect for pinched wiring
Plausible but drifted
Thermistor aging; moisture in connector
Temperature offset errors; control hunting; reduced efficiency
Compare reading to calibrated thermometer at same location; replace if offset > 2°C
Beyond thermistors, modern HVAC/R systems use several other sensor types to
feed the control board with data needed for precise, efficient operation.
Each sensor type has a distinct signal type and diagnostic method.
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Pressure Transducer
Converts refrigerant or fluid pressure to an electrical signal. Most common
output: 0–5 VDC or 4–20 mA, proportional to pressure over
a calibrated range. Used to monitor suction pressure, discharge pressure, and
oil pressure. The control board converts the signal to a pressure value using
a calibration curve, then calculates saturation temperature for superheat and
subcooling control.
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Airflow / Differential Pressure Switch
A differential pressure switch proves airflow across a coil or heat exchanger.
When fan speed is sufficient, the pressure difference across the coil deflects
a diaphragm and closes (or opens) a set of contacts, signalling the control
board that adequate airflow is present. Used as a safety input on heat pumps,
gas furnaces (pressure switch), and dedicated outdoor air systems (DOAS).
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Humidity Sensor (Humidistat)
Measures relative humidity in the air stream or space. Capacitive-type sensors
(most common) change capacitance as moisture is absorbed; the circuit converts
this to a 0–10 VDC or 4–20 mA signal, or a digital value on a communication
bus. Used to enable dehumidification modes, prevent over-cooling (latent heat
control), and trigger ventilation sequences in energy-recovery systems.
Pressure Transducer Signal Types and Measurement
Signal Type
Typical Range
How to Measure
Fault Indication
0–5 VDC
0 V = 0 psig; 5 V = full-scale pressure (e.g., 500 psig)
Voltmeter DC across signal and ground wires; confirm 5 V supply reference is present
0 V with supply present = sensor or wiring shorted; 5 V at all pressures = open signal wire or sensor failed high
4–20 mA
4 mA = 0 psig; 20 mA = full-scale pressure
DMM in series (mA function) or measure mV across a known shunt resistor; clamp micro-amp meter
Under 4 mA = open wire or failed sensor; over 20 mA = short circuit; exactly 0 mA = loss of supply power to sensor
General Sensor Diagnostic Steps
Read and record fault codes. Most boards flag specific sensor faults (e.g., “suction pressure sensor out of range”) and often show the raw sensor value in a diagnostic menu.
Verify supply voltage to the sensor. A pressure transducer or humidity sensor needs its supply voltage (typically 5 VDC or 12 VDC from the board) before it can output a valid signal. No supply = no valid output.
Compare the sensor signal to an independent measurement. Use a gauge manifold to read actual refrigerant pressure; compare to the pressure the board calculates from the transducer signal. A large discrepancy indicates sensor drift or failure.
Inspect connectors and wiring for corrosion, moisture, or mechanical damage. Crimped or corroded signal wires are a leading cause of erratic sensor readings, especially on outdoor equipment.
Confirm signal is within the expected range. A 0–5 V transducer reading 4.95 V at low pressure, or 0.05 V at high pressure, indicates it is at its rail — likely stuck or miswired.
Replace the sensor only after confirming the supply and wiring are correct; a faulty board input or miswired connector will damage a new sensor if the root cause is not corrected first.
Sensor Faults — Effect on System
Open or out-of-range thermistor: board may lockout, use default value, or disable the function that sensor controls (e.g., defrost)
Pressure transducer failure high: board may see falsely high pressure and lockout on “high pressure” even when actual pressure is normal
Pressure transducer failure low: board may see falsely low pressure and allow compressor to run into actual low-pressure conditions unprotected
Humidity sensor drift: dehumidification activates too early or too late; space humidity may be outside comfort range without alarm
Differential pressure switch stuck open: board may not allow the system to start (airflow proven-off = no call-to-operate)
Preventing Sensor Failures
Coat connectors on outdoor sensors with dielectric grease during installation to prevent moisture ingress
Secure sensor cables with proper clips; avoid contact with sharp metal edges or vibrating components
Verify sensor compatibility (voltage/current type, range) when replacing; installing a 4–20 mA sensor on a 0–5 V input will not work and may damage the board
Include sensor reading checks in annual PM procedures; compare readings to independent measurements to detect drift before failure
For humid or wet environments, select IP-rated sensor housings and use conduit for wiring runs
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Correlation is a powerful diagnostic tool
Modern control boards often display multiple sensor values simultaneously in a diagnostic or “status” screen. If the suction pressure transducer reads 70 psig but the board-calculated saturation temperature for R-410A does not match a thermistor temperature measured at the same location, one of the two sensors has drifted. Comparing correlated quantities (pressure vs temperature, supply vs return air temperatures, airflow differential pressure vs measured fan current) quickly reveals which sensor is suspect — without disconnecting anything.