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Error code reference

Heat Controller Air Conditioner Error Code 45

Condenser pipe thermistor sensor open/short circuit error.

Heat Controller codesHeat Controller air conditioner codes

Condenser pipe thermistor sensor open/short circuit error.

First step from the service manual

Measure resistance unplugged; normal is 5KΩ at 25°C (77°F). Measure voltage plugged; normal is 2.5Vdc at 25°C. If 0V or 5Vdc, sensor is abnormal.

The complete diagnostic procedure includes additional test steps, resistance specifications, wiring diagram references, and component test points.

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What 45 means on a Heat Controller air conditioner

Error code 45 on a Heat Controller air conditioner indicates an open or short circuit fault in the condenser pipe thermistor sensor. This sensor monitors the refrigerant temperature at the condenser coil, providing real-time data to the control board so it can regulate compressor operation, prevent high-pressure trips, and protect the system from overheating conditions. When the control board detects a signal outside the expected range, it flags this fault and typically disables compressor operation to prevent equipment damage.

The thermistor is a negative temperature coefficient (NTC) resistive sensor, meaning its resistance decreases as temperature rises. At 25 degrees Celsius (77 degrees Fahrenheit), the normal resistance value is 5 kilohms. The control board supplies a reference voltage to the sensor circuit, and under normal operating conditions at 25 degrees Celsius, the signal voltage at the sensor connector should read approximately 2.5 volts DC. A reading of 0 volts DC indicates a short circuit condition, where the sensor or wiring is pulling the signal line to ground. A reading of 5 volts DC indicates an open circuit, meaning the signal path is broken and the board is reading its full reference voltage with no load.

Causes of this fault range from physical damage to the thermistor itself to wiring harness failures between the sensor and the control board. Corrosion at connector pins, pinched or abraded wiring, and thermistor failure due to age or moisture exposure are all common contributors. Diagnosing the root cause requires unplugging the sensor and measuring its resistance directly, then cross-referencing that reading against the 5 kilohm specification at ambient temperature.

Source: manufacturer service documentation.

Common causes of 45

  1. 01

    Failed condenser pipe thermistor

    Part replacement

    The thermistor itself has failed internally, producing resistance readings far outside the 5 kilohm specification at 25 degrees Celsius. This is the most common cause and is confirmed by measuring resistance directly at the sensor leads after unplugging the connector.

  2. 02

    Damaged or corroded sensor connector

    Wiring / connection

    Corrosion, moisture intrusion, or a loose fit at the connector pins can cause the signal voltage to read 0V or 5V even when the thermistor itself is intact. Inspect the connector body and pins for oxidation, green discoloration, or bent contacts.

  3. 03

    Pinched or broken wiring harness

    Wiring / connection

    The wiring between the thermistor and the control board can be pinched during installation or abraded over time, creating an open or short circuit in the signal line. Visually trace the harness from the sensor to the board, looking for visible damage or tight bends against cabinet edges.

  4. 04

    Moisture or contamination inside the sensor housing

    Part replacement

    Water ingress into the thermistor housing can cause erratic or shorted resistance readings. This is more common in outdoor condenser units exposed to heavy rain or condensation accumulation.

  5. 05

    Faulty control board

    Part replacement

    If the thermistor resistance measures correctly at 5 kilohms and the wiring harness is undamaged, the control board's sensor input circuit may have failed. This is the least likely cause and should be diagnosed after ruling out the sensor and wiring.

See the test procedure for each cause

Frequently asked questions about 45

What does 45 mean on a Heat Controller air conditioner?
Error code 45 indicates an open or short circuit fault in the condenser pipe thermistor sensor. This sensor monitors refrigerant temperature at the condenser coil and reports back to the control board. When the signal voltage reads 0V (short) or 5V (open) instead of the normal 2.5V DC at 25 degrees Celsius, the unit logs this fault and may shut down the compressor to prevent damage.
Can I fix error code 45 myself?
Some causes of error code 45 are DIY-accessible. You can inspect the sensor connector for corrosion and check the wiring harness for visible damage without special tools. Testing the thermistor requires a multimeter: unplug the sensor and measure resistance across the leads, which should read approximately 5 kilohms at 25 degrees Celsius. If the sensor has failed or wiring is damaged, replacing the thermistor is a straightforward repair, though working inside the outdoor unit should be done with the power fully disconnected.
Is it safe to run my air conditioner with error code 45 active?
It is not recommended to continue operating the unit with error code 45 active. The condenser pipe thermistor provides temperature data that the control board uses to protect the compressor and refrigerant circuit from high-pressure or overheating conditions. Without a functioning sensor, the system cannot respond appropriately to abnormal operating temperatures, which could lead to compressor damage over time.
How much does it cost to repair error code 45?
If only the thermistor sensor needs replacement, parts and labor costs are generally modest, typically in the range of $75 to $200 depending on the service provider and part availability. If the control board is responsible for the fault, repair costs are higher, often ranging from $200 to $500 or more. Getting a diagnostic service call first is the best way to confirm which component has failed before committing to parts costs.
Will resetting the unit clear error code 45?
A power reset may temporarily clear the error code display, but the fault will return as soon as the control board re-reads the sensor signal and detects the out-of-range voltage. Because the underlying cause is a hardware fault in the sensor circuit, the code will not clear permanently until the defective thermistor, wiring, or connector is repaired or replaced. Repeated resets without addressing the root cause do not resolve the issue.

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