Carrier Mini Split H66 Error Code

Updated 2026-08-29 · review status: manual-verified · reviewed by MiniSplitDecoder Editorial Team Last verified: 2026-08-29

Check your model first

  • Carrier 45MHHAQ / 45MUAAQ High Wall (Comfort 22)

Match the model on your equipment label to the linked manufacturer documentation. The same code can mean different things across model families.

Quick answer

H66 (displayed by Carrier manuals as EH66) sits in the same group as EH62: an evaporator coil inlet temperature sensor (T2B) open/short fault that the indoor unit cannot trust Turn the system off, wait about 10 minutes, power cycle once at the breaker, and schedule service if the code returns. Do not open either cabinet..

Severity: medium · DIY difficulty: easy

Safety

Stop at the single power-cycle check. Do not open the indoor or outdoor cabinet, test sensors with a meter, or touch wiring - sensor and board diagnosis is technician-only.

What H66 means

H66 (displayed by Carrier manuals as EH66) sits in the same group as EH62: an evaporator coil inlet temperature sensor (T2B) open/short fault that the indoor unit cannot trust.

Code meaning varies by series

  • Carrier 45MHHAQ / 45MUAAQ High Wall (Comfort 22): EH66 - T2B evaporator coil inlet temperature sensor open/short (EH62 grouping)

Causes, ranked

  1. 1. the evaporator coil inlet temperature sensor (T2B group) is open or shorted common pro only

    Check: Requires resistance measurement against the Carrier service-manual sensor table.

    Fix: Technician: test the sensor, replace if open/short or out of spec.

  2. 2. Transient fault that clears after one reset common DIY

    Check: Turn the system off with the remote, wait about 10 minutes, then power cycle once at the breaker or disconnect if you can do that safely.

    Fix: If the code returns, stop resetting and schedule service.

  3. 3. Loose sensor connector or damaged sensor lead occasional pro only

    Check: Connector inspection is inside the unit chassis.

    Fix: Technician: reseat or repair the sensor connector and lead.

  4. 4. Control PCB input fault after the sensor checks out rare pro only

    Check: Board-level voltage checks only.

    Fix: Technician: diagnose the PCB sensor input; replace board if faulty.

Related codes

Cross-references for the same fault family, by brand:

Carrier H62 · Carrier H65 · Carrier EC56

Commonly searched as: "carrier h66" · "carrier error code h66" · "carrier air conditioner h66"

Sources

Related Mini Split guides

Mini-Split Communication Error: DIY Checks · Mini-Split Flashing Light Decoder: Blink Patterns · How to Reset a Mini-Split After a Power Outage · Mini-Split Sensor Errors: How to Test a Thermistor