Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
Carrier 24ACC Air Conditioner Error Codes — What They Mean
The Carrier 24ACC is a Comfort series central air conditioner (condensing unit only). It uses a single-speed scroll compressor and a conventional contactor-based control system. Unlike Infinity-series units, the 24ACC communicates fault status through a diagnostic LED on the outdoor control board rather than through a communicating thermostat. The LED flash code identifies the active fault.
Carrier 24ACC Flash Code Reference
| Flash Code | Meaning |
|---|---|
| 1 flash | Normal — unit running |
| 2 flashes | High-pressure switch open |
| 3 flashes | Low-pressure switch open |
| 4 flashes | Compressor protection fault |
| 5 flashes | Control board fault |
| 6 flashes | Outdoor ambient sensor fault |
| 7 flashes | Discharge temperature sensor fault |
| 8 flashes | Communication fault (if connected to Infinity system) |
Common Causes by Code
- Code 2 — High pressure — Dirty condenser coil is the most common cause. The 24ACC coil must be clean on all sides — a garden hose flush from inside-out dislodges debris that compressed air only pushes deeper. Also check condenser fan motor: if the fan is spinning backward, high-pressure faults occur immediately.
- Code 3 — Low pressure — Low refrigerant charge (leak) or a failed low-pressure switch. On a 24ACC, low pressure can also occur in very cold ambient temperatures (below 45°F) — the unit has a low-ambient kit available to prevent nuisance trips.
- Code 4 — Compressor protection — The 24ACC compressor has an internal thermal overload. If the compressor has been running in high-pressure or high-ambient conditions, the overload trips and won’t reset for 20–30 minutes. The code also appears when supply voltage is low (below 208V on a 240V unit).
- Code 5 — Control board — Rare. Usually caused by a lightning strike, power surge, or moisture intrusion into the control box. Check for corroded terminals or burnt traces on the board.
- Code 6 — Ambient sensor — The outdoor ambient temperature sensor is on the control board. If the sensor reads out of range, the board will not operate in low-ambient or high-ambient protection modes. Check the sensor resistance vs. the temperature-resistance chart in the service manual.
Step-by-Step Fix
- Read the LED — Open the side access panel on the 24ACC outdoor unit. The control board LED is visible near the contactor. Record the flash code.
- For Code 2 (high pressure) — Turn off the unit. Inspect the condenser coil from outside — debris, cottonwood seeds, or pet hair often block the coil base. Flush the coil with water from inside-out. Confirm the fan blade is spinning in the correct direction (should draw air up through the top of the unit).
- For Code 3 (low pressure) — Connect refrigerant gauges (R-410A; EPA certification required). Check suction pressure — should be approximately 100–110 PSI at 70°F ambient. Below 80 PSI suggests low charge. Also check low-pressure switch continuity (should be closed above 40 PSI on R-410A).
- For Code 4 (compressor protection) — Let the unit sit off for 30 minutes. Check supply voltage at the disconnect — both legs should read within 10% of nameplate voltage. Check capacitor (start and run) before condemning the compressor.
- For Code 5 — Inspect the control board for burn marks, corroded terminals, or shorted components. Check fuse on the board (if present). Measure 24VAC from the transformer.
- Clear and test — After any repair, restore power, confirm the contactor pulls in cleanly, and monitor system pressures for the first 10 minutes of operation.
Parts Often Needed
| Part | Notes |
|---|---|
| Run capacitor | Dual run capacitor for compressor and fan; common failure |
| Contactor | Pitted contacts cause voltage drop and Code 4 |
| Low-pressure switch | Replace if Code 3 persists with correct charge |
| High-pressure switch | Replace if Code 2 persists with clean coil |
| Condenser fan motor | Check capacitor first before replacing |
| Control board | For Code 5 or persistent unexplained faults |
Technician Tips
Carrier uses different diagnostic systems across furnaces, heat pumps, chillers, mini-splits, VRF equipment, rooftop units, and communicating controls. Confirm the full model number and whether the fault came from the indoor board, outdoor board, thermostat, or equipment controller before matching it to a repair path.
Start with power, wiring, airflow or water-flow, sensor, condensate, and safety-circuit checks before ordering control boards. If the same code returns after one reset and the basic cause is not obvious, test the named circuit instead of repeatedly cycling the equipment.
Parts note: For 24ACC outdoor-unit faults, match the code to the listed condenser part: capacitor, contactor, fan motor, pressure switch, or control board. Do not order furnace ignition parts for this air-conditioner platform.
Need Replacement Parts?
Before ordering, match the HVAC part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
When to Call a Pro
Refrigerant handling requires EPA 608 certification. Any Code 3 investigation that goes beyond switch testing requires manifold gauges and refrigerant recovery equipment. If the compressor is failing to start (buzzing but not running), compressor replacement on a 24ACC is a major repair — get a cost estimate vs. full system replacement.