Error E3 on Carrier 25HCC318A003 Heat Pump | Causes & Fix

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• Source: Official Carrier service manual
• Reviewed by: Geo Technical Editorial
• Published: March 15, 2026
• Updated: March 15, 2026

Safety notice: Start with user-safe checks only. Do not open sealed refrigerant systems, inspect gas-train components, or work on energized high-voltage parts unless you are qualified to service this appliance.

Error E3 on Carrier : Indoor coil thermistor fault. Follow the troubleshooting steps below to diagnose and fix this heat-pump issue. ## What Error Code E3 Means on a Carrier 25HCC318A003 Error E3 on the Carrier 25HCC318A003 indicates the indoor coil (heat exchanger) thermistor is faulty. The coil thermistor is used for freeze protection in cooling mode and coil over-temperature protection in heating mode. E3 means the sensor is open-circuit or short-circuit. The sensor is clipped to the indoor coil fins and is accessible after removing the indoor unit casing. When E3 appears, the Carrier heat pump pauses normal operation to protect internal components from potential damage. Not every E3 fault requires a professional engineer — many are resolved through careful inspection and routine maintenance. Work through each diagnostic step below in sequence, starting with the simplest check, to identify the root cause efficiently before considering part replacement or arranging a service call. ## Symptoms - E3 on display

• System not starting or stopping mid-cycle
• Possible coil icing or overheating ## Common Causes - Indoor coil thermistor fault ## Tools You May Need - digital multimeter
• Phillips screwdriver ## Step-by-Step Repair Guide | Step | Action | Result | |------|--------|--------| | 1 | Isolate AC. Remove the indoor unit casing. | Casing removed — continue to step 2 if the issue persists. | | 2 | Locate the coil thermistor (clipped to the indoor coil fins). | Thermistor located — continue to step 3 if the issue persists. | | 3 | Test resistance: at 25 °C expect ~10 kΩ. OL or 0 Ω = faulty. | Resistance measured — continue to step 4 if the issue persists. | | 4 | Replace the thermistor. | Thermistor replaced — continue to step 5 if the issue persists. | | 5 | Reassemble and restore power. Test cooling and heating. | E3 cleared — if the fault remains, contact a qualified technician. | ## Safety Precautions Isolate the AC before accessing the indoor coil. The coil fins are sharp — use gloves when working around the coil area. ## Estimated Repair Time and Difficulty Most heat pump error code E3 faults can be assessed and addressed in 30–90 minutes using the steps above. The diagnostic process requires only basic tools: a multimeter and standard screwdrivers cover the majority of checks described here. If a replacement part is required, allow 1–5 working days for delivery. A trained technician with access to the correct diagnostic equipment will typically resolve this type of fault within a single service visit. ## When to Call a Technician Contact a Carrier authorised service technician if: the fault persists after following all the steps above; if PCB-level or refrigerant-circuit work is required; or if you are not comfortable working with electrical or mechanical components. When calling, have the full model number, the error code displayed, and a brief description of any sounds or symptoms you noticed before the fault appeared — this helps the technician prepare the correct parts and tools. ## Frequently Asked Questions Q: Can E3 cause the coil to ice up? Yes — without the coil thermistor, the system cannot detect coil icing and freeze protection cannot activate. Q: Is the coil thermistor replacement a DIY task? Yes — it involves removing the indoor unit casing and replacing a clip-on thermistor. The thermistor must match the exact model. Q: What is the difference between E2 and E3? E2 is the room air temperature sensor; E3 is the indoor coil temperature sensor. Both are thermistors but mounted in different locations. ### Is it safe to keep using my Carrier with error E3? If error E3 does not indicate an immediate safety hazard such as a gas leak, electrical short, or water flooding, you can continue limited use while monitoring the appliance. However, running the heat-pump with an active error code increases the risk of further damage. Address the underlying issue as soon as possible. For high-severity errors, disconnect the appliance and contact a qualified technician. ### How much does it cost to fix error E3 on a Carrier ? Repair costs for error E3 on the Carrier heat-pump depend on the root cause. DIY fixes such as cleaning filters, resetting the unit, or reconnecting loose wires cost nothing beyond basic tools. If a component replacement is needed, parts and labor from a certified technician are the main expenses. Contact a Carrier-authorized service provider for an accurate estimate based on your specific situation. ### What does error code E3 mean on a Carrier heat-pump? Error code E3 on Carrier heat-pump models indicates Indoor coil thermistor fault. This error is detected by the appliance’s internal diagnostic system and displayed to alert the user that attention is needed. Refer to the troubleshooting steps above for guidance on resolving this issue. ### Is it safe to keep using my Carrier with error E3? If error E3 does not indicate an immediate safety hazard such as a gas leak, electrical short, or water flooding, you can continue limited use while monitoring the appliance. However, running the heat-pump with an active error code increases the risk of further damage. Address the underlying issue as soon as possible. For high-severity errors, disconnect the appliance and contact a qualified technician. ### How much does it cost to fix error E3 on a Carrier ? Repair costs for error E3 on the Carrier heat-pump depend on the root cause. DIY fixes such as cleaning filters, resetting the unit, or reconnecting loose wires cost nothing beyond basic tools. If a component replacement is needed, parts and labor from a certified technician are the main expenses. Contact a Carrier-authorized service provider for an accurate estimate based on your specific situation. ### What does error code E3 mean on a Carrier heat-pump? Error code E3 on Carrier heat-pump models indicates Indoor coil thermistor fault. This error is detected by the appliance’s internal diagnostic system and displayed to alert the user that attention is needed. Refer to the troubleshooting steps above for guidance on resolving this issue.