mypros+
Error code reference

Aprilaire Hvac Error Code E5

High Temperature Sensor Thermistor Failure

Aprilaire codesAprilaire hvac codes

High Temperature Sensor Thermistor Failure — the signal from the High Temperature Sensor is open or shorted, most likely due to a loose connection at the Internal Control board or a failed sensor.

First step from the service manual

Turn the ON/OFF toggle switch OFF, remove side panel, insulation and electrical box cover, remove connector for the High Temperature Sensor and measure resistance across the terminals.

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

MyPros+ Max

Get the full diagnostic procedure for E5.

  • Step-by-step tests with expected resistance, voltage, and continuity values
  • Wiring diagram references and connector pinouts from the OEM service manual
  • Verified part numbers when replacement is required
Get the full procedure

$29/month · Cancel anytime · Or $9.95 3-day pass

What E5 means on a Aprilaire hvac

The E5 error code on an Aprilaire HVAC unit indicates a High Temperature Sensor thermistor failure. The control board has detected that the signal from the High Temperature Sensor is either open (no electrical continuity) or shorted (abnormally low resistance), both of which represent readings outside the sensor's normal operating range. Because the control board relies on this thermistor to monitor high-limit temperatures and protect the system from overheating, it shuts down or flags the fault when the signal falls outside expected parameters.

The High Temperature Sensor is a thermistor, meaning its resistance changes predictably with temperature. Under normal conditions, the sensor produces a resistance value that corresponds to the actual temperature it is measuring. When the circuit reads as open, resistance is effectively infinite, usually pointing to a broken wire, a disconnected connector, or a failed sensor element. A shorted condition produces near-zero resistance and can result from a damaged sensor, pinched wiring, or moisture intrusion in the connector or wiring harness.

The most common physical cause is a loose or unseated connector at the Internal Control board rather than a fully failed sensor. Vibration during normal operation can work connectors loose over time. The first diagnostic step involves turning the ON/OFF toggle switch to OFF, removing the side panel, insulation, and electrical box cover to access the sensor connector, then measuring resistance across the terminals with a multimeter. If the resistance reading is open or shorted and does not correspond to expected thermistor values at ambient temperature, the sensor itself is likely faulty and will need replacement.

Source: manufacturer service documentation.

Common causes of E5

  1. 01

    Loose connector at the Internal Control board

    Wiring / connection

    Vibration from normal HVAC operation can gradually loosen the connector that links the High Temperature Sensor to the control board. Reseating the connector firmly is the first step to check before replacing any parts.

  2. 02

    Failed High Temperature Sensor thermistor

    Part replacement

    The thermistor element inside the sensor can fail, producing an open or shorted resistance reading regardless of actual temperature. A multimeter measurement across the sensor terminals will confirm a failed sensor if the reading is open-circuit or near zero ohms.

  3. 03

    Damaged or broken sensor wiring

    Part replacement

    A wire in the sensor circuit may be broken internally or have a damaged insulation causing a short to ground. Visually inspect the wiring harness from the sensor to the control board for pinching, cuts, or burn marks.

  4. 04

    Moisture or corrosion in the sensor connector

    Wiring / connection

    Water intrusion or corrosion buildup in the connector can disrupt the electrical signal, causing the control board to read an open or shorted condition. Inspect the connector terminals for discoloration, oxidation, or signs of moisture.

  5. 05

    Faulty Internal Control board

    Part replacement

    If the sensor and all wiring check out correctly with a multimeter, the control board itself may have a defect preventing it from reading the sensor signal accurately. This is the least likely cause and should only be considered after all other components are confirmed good.

See the test procedure for each cause

Frequently asked questions about E5

What does E5 mean on a Aprilaire hvac?
E5 on an Aprilaire HVAC unit means the control board has detected a failure in the High Temperature Sensor thermistor circuit. The signal from the sensor is either open (no continuity) or shorted, making it impossible for the system to get a valid temperature reading. The unit will not operate normally until the fault is resolved. The most likely causes are a loose connector at the control board or a failed sensor.
Can I fix the E5 error myself, or do I need a technician?
A knowledgeable DIYer comfortable working with electrical components can attempt the initial diagnostic steps. This involves turning the unit OFF, removing the side panel, insulation, and electrical box cover, then using a multimeter to measure resistance across the High Temperature Sensor connector terminals. If the issue is simply a loose connector, reseating it is straightforward. Replacing the sensor or the control board is a more involved repair and may be better handled by a qualified technician.
Is it safe to continue running my HVAC with an E5 error?
It is not advisable to continue running the unit with an active E5 error. The High Temperature Sensor is a protective component that monitors for overheating conditions. Without a functioning sensor, the system lacks the ability to detect and respond to dangerously high temperatures, which could result in equipment damage or a safety hazard. Address the fault before resuming normal operation.
Will resetting the unit clear the E5 error code?
Simply resetting the unit by cycling power will not permanently clear the E5 error if the underlying fault still exists. The error will return as soon as the control board attempts to read the High Temperature Sensor signal and finds it open or shorted. The fault must be corrected, either by securing the connector or replacing the sensor, before the error will clear and stay cleared.
How much does it cost to repair an E5 error on an Aprilaire HVAC unit?
If the issue is only a loose connector, the repair cost is minimal, potentially just a service call fee ranging from around $75 to $150 depending on your area. If the High Temperature Sensor needs replacement, parts and labor together typically range from $100 to $300. If the Internal Control board is at fault, costs can rise to the $200 to $500 range or more, depending on the specific unit and labor rates in your region.

Related Aprilaire hvac codes

All Aprilaire hvac codes

78,000+ service manuals · 160,000+ error codes · 55+ brands

Get Started

$29/month · Cancel anytime · Or $9.95 3-day pass