Quick answer: The most common AC error codes are E1 (indoor temperature sensor), E2 (indoor coil sensor), E3 (outdoor temperature sensor), E4 (outdoor coil sensor), E5 (compressor overload), E6 (communication error), P1 (low refrigerant pressure), and P2 (high refrigerant pressure). Sensor errors (E1–E4) are often DIY-fixable with a $5–$20 thermistor replacement. Refrigerant errors (P1, P2) and compressor errors (E5) always require a certified HVAC technician.
The AC That Displayed E2 Every Morning — Until I Found the Loose Connector
The bedroom mini-split started displaying E2 every morning at around 6 a.m. — right when the overnight cooling cycle ended and the unit switched to fan mode. It would shut down, show E2 on the display, and require a manual reset from the remote before it would restart. I assumed the worst: a failed control board, an expensive repair. I pulled the front panel off the indoor unit, located the evaporator coil thermistor — a small bead sensor clipped to the evaporator coil fins — and found the connector had worked itself partially loose, probably from vibration over two seasons of use. I pressed the connector firmly back into its socket. The E2 error never returned.
That experience shaped how I approach common AC error codes: before assuming component failure, check the connections. Thermistor connectors, communication wire terminals, and capacitor connections all loosen over time from vibration and thermal cycling. A loose connection produces the same error code as a failed component — and costs nothing to fix.
How AC Error Codes Work and What They Tell You
Modern air conditioners — both split systems and window units — use a microcontroller on the indoor unit’s control board to monitor every measurable parameter in the system: room temperature (indoor thermistor), evaporator coil temperature (indoor coil thermistor), outdoor ambient temperature (outdoor thermistor), condenser coil temperature (outdoor coil thermistor), refrigerant pressure (high and low pressure switches or transducers), compressor current draw (overload protection), and communication signal integrity (between indoor and outdoor control boards).
When any monitored parameter falls outside its acceptable range, the control board logs an error code, displays it on the indoor unit’s LED display or remote control screen, and — depending on the severity — either continues operating in a degraded mode or shuts the system down to prevent damage. The error code is the control board’s way of telling you exactly which parameter triggered the fault. Understanding the code eliminates guesswork and focuses the diagnosis on the correct component.
The 5 Most Common AC Error Code Categories
1. Indoor Sensor Errors — E1 (Room Temp) and E2 (Coil Temp)
Indoor sensor errors are the most common AC error codes that homeowners can resolve without professional service. E1 indicates a fault in the indoor ambient temperature sensor — the thermistor that measures room temperature and tells the control board when the target temperature has been reached. E2 indicates a fault in the indoor evaporator coil thermistor — the sensor clipped to the evaporator coil fins that monitors coil temperature to prevent freezing and control defrost cycles.
Both sensors are NTC (negative temperature coefficient) thermistors with a nominal resistance of approximately 10kΩ–15kΩ at 25°C (77°F). Test with a multimeter set to resistance: disconnect the thermistor connector from the control board and measure across the two thermistor leads. An open circuit reading (OL or infinite resistance) indicates a broken thermistor. A reading far outside the expected range (below 1kΩ or above 50kΩ at room temperature) indicates a failed thermistor. Replacement thermistors cost $5–$20 and are available by brand and model number from HVAC parts suppliers.
2. Outdoor Sensor Errors — E3 (Outdoor Temp) and E4 (Outdoor Coil Temp)
Outdoor sensor errors indicate a fault in the outdoor unit’s temperature monitoring thermistors. E3 indicates a fault in the outdoor ambient temperature sensor — used by the control board to adjust compressor speed (on inverter systems) and determine operating limits. E4 indicates a fault in the outdoor coil thermistor — used to monitor condenser coil temperature and control defrost cycles on heat pumps.
Outdoor thermistors are exposed to weather and UV radiation, making them more prone to failure than indoor sensors. The diagnostic procedure is the same as for indoor sensors — disconnect and measure resistance with a multimeter. Outdoor thermistors cost $10–$30. Access requires removing the outdoor unit’s service panel with power off at the disconnect box. Check the thermistor wire for damage (rodent chewing, UV degradation of the insulation) before replacing the sensor — a damaged wire produces the same error as a failed thermistor.
3. Communication Errors — E6 (Indoor-Outdoor Signal Loss)
Communication errors (E6, U8, F6 — varies by brand) indicate that the indoor and outdoor unit control boards cannot exchange signals. Split-system air conditioners use a dedicated communication wire (typically a 3-wire or 4-wire cable running between the indoor and outdoor units alongside the refrigerant lines) to coordinate compressor speed, fan speed, and protection responses. When this communication is interrupted, the indoor unit displays a communication error and shuts down.
The most common causes are: a loose terminal connection at the indoor or outdoor unit’s control board (the communication wire terminals loosen from vibration), a damaged communication wire (pinched during installation, chewed by rodents, or corroded at outdoor terminals), a power supply failure at the outdoor unit (the outdoor unit cannot communicate if it has no power), and a failed control board (indoor or outdoor). Start with a power reset and check all terminal connections before assuming board failure.
4. Refrigerant Pressure Errors — P1 (Low Pressure) and P2 (High Pressure)
Refrigerant pressure errors indicate that the system’s refrigerant pressure is outside the safe operating range. P1 (low pressure, also LP or E9 on some brands) means the refrigerant charge has dropped below the minimum operating pressure — almost always caused by a refrigerant leak. P2 (high pressure, also HP or E8) means refrigerant pressure has exceeded the maximum safe limit — caused by a blocked condenser coil, a failed condenser fan, or refrigerant overcharge.
⚠️ Critical legal and safety note: All refrigerant pressure errors require a certified HVAC technician. In the United States, handling refrigerants without EPA Section 608 certification is a federal violation carrying fines up to $44,539 per day. For P2 (high pressure) errors specifically, check the condenser coil for debris blockage before calling a technician — a blocked coil is a DIY-accessible fix that may resolve the high pressure fault without a service call. For P1 (low pressure), do not attempt to add refrigerant — the leak must be located and repaired first.
5. Compressor Protection Errors — E5 (Overload, Locked Rotor, or Discharge Temperature)
Compressor protection errors (E5, H6, P0, C5 — varies by brand) indicate that the compressor has shut down on a protection fault. The compressor protection system monitors current draw (overload), motor speed (locked rotor), and discharge temperature (the temperature of the refrigerant leaving the compressor). A protection fault means one of these parameters exceeded its safe limit.
⚠️ Critical note: Do not attempt to restart the compressor repeatedly after a protection fault. Repeated restart attempts after a protection fault cause permanent compressor damage. Clean the condenser coil, check supply voltage, and perform a single power reset. If the protection fault recurs immediately, the compressor requires professional diagnosis. Compressor replacement on units older than 8–10 years often approaches the cost of a new system — always request a full replacement cost comparison before authorizing compressor replacement.
How to Diagnose and Repair Common AC Error Codes: Step-by-Step
Step 1: Record the Error Code and Perform a Power Reset
Before doing anything else, record the exact error code displayed on the indoor unit or remote control. Take a photo if possible — some units clear the error code display after a reset. Then perform a power reset: turn off the air conditioner at the thermostat or remote, turn off the circuit breaker for both the indoor and outdoor units, wait 5 minutes, restore power, and wait 10 minutes before calling for cooling. If the error code does not return after the reset, the fault was temporary — monitor the unit for recurrence. If the error code returns within minutes of the reset, the underlying fault is active and requires diagnosis.
Step 2: Diagnose Sensor Errors (E1, E2, E3, E4) with a Multimeter
For sensor error codes, turn off power at the circuit breaker before opening any panels. Locate the thermistor — for E1 and E2, remove the indoor unit’s front panel and filter; the thermistors are small bead sensors on short wire leads, clipped to the evaporator coil (E2) or positioned in the airstream (E1). Disconnect the thermistor connector from the control board. Set a multimeter to resistance (Ω) and measure across the two thermistor leads. At room temperature (approximately 25°C / 77°F), a good thermistor reads 10kΩ–15kΩ. An open circuit (OL) or a reading below 1kΩ indicates a failed thermistor. Before replacing, check the connector and wire for damage — press the connector firmly back into its socket and retest.
Step 3: Check Communication Wire Connections (E6)
For E6 communication errors, turn off power at the circuit breaker. At the indoor unit, remove the electrical cover panel (the panel that exposes the control board terminals). Locate the communication wire terminals — typically labeled S, N, or COM on the terminal block. Check that each wire is firmly seated in its terminal and that the terminal screws are tight. Repeat at the outdoor unit’s terminal block. Also check the communication wire along its entire run for visible damage — pinching, rodent damage, or corrosion at outdoor terminals. Restore power and observe whether E6 returns. If it does, the control board requires professional diagnosis.
Step 4: Clean the Condenser Coil (P2 High Pressure)
For P2 high pressure errors, turn off power at the outdoor unit’s disconnect box. Inspect the condenser coil fins on all sides of the outdoor unit for debris — leaves, grass clippings, cottonwood, and dirt restrict airflow and cause refrigerant pressure to rise. Rinse the coil with a garden hose from inside the unit outward (spray from inside through the coil fins — not from outside in, which pushes debris deeper). For heavy buildup, use a commercial coil cleaner. Ensure the outdoor unit has at least 18–24 inches of clearance on all sides. Restore power and observe whether P2 returns. If the coil is clean and P2 persists, the cause is refrigerant overcharge or a failed expansion valve — both require a certified HVAC technician.
Step 5: Call a Certified HVAC Technician for Refrigerant and Compressor Errors
Contact a certified HVAC technician if: the error involves refrigerant pressure (P1, P2) and the condenser coil is clean; the compressor protection fault (E5, H6) recurs after a power reset and coil cleaning; the error code persists after checking all connections and replacing the indicated sensor; or you are not comfortable working inside the electrical compartment of the indoor or outdoor unit. Provide the brand name, model number, and exact error code. For refrigerant errors, confirm EPA Section 608 certification before authorizing work.
Frequently Asked Questions About Common AC Error Codes
What are the most common AC error codes?
The most common AC error codes are E1 (indoor temperature sensor fault), E2 (indoor coil sensor fault), E3 (outdoor temperature sensor fault), E4 (outdoor coil sensor fault), E5 (compressor overload or protection fault), E6 (communication error between indoor and outdoor units), P1 (low refrigerant pressure), and P2 (high refrigerant pressure). Exact codes vary by brand — Daikin, Mitsubishi, LG, Carrier, and Trane each use different code numbering systems. Always verify with your specific brand and model number.
What does E1 mean on an air conditioner?
E1 on most air conditioners means the indoor ambient temperature sensor (thermistor) has failed or is reading outside its expected range. Test with a multimeter — a good thermistor reads 10kΩ–15kΩ at room temperature. An open circuit or far-out-of-range reading indicates a failed thermistor. Replacement thermistors cost $5–$20. Check the connector first — a loose connector produces the same E1 error as a failed thermistor.
What does E6 mean on an air conditioner?
E6 on most split-system air conditioners means a communication error between the indoor and outdoor unit control boards. Start with a power reset. If E6 returns, check the communication wire connections at both the indoor and outdoor unit terminal blocks. If connections are secure and the wire is undamaged, the control board requires professional diagnosis.
Can I reset an AC error code myself?
Yes — turn off the unit at the remote or thermostat, turn off the circuit breaker for both indoor and outdoor units, wait 5 minutes, restore power, and wait 10 minutes before calling for cooling. If the error returns after the reset, the underlying fault is active. Sensor errors (E1–E4) that return after a reset indicate a failed thermistor. Refrigerant errors (P1, P2) that return after a reset require a certified HVAC technician.
When should I call a technician for an AC error code?
Call a certified HVAC technician if: the error involves refrigerant pressure (P1, P2) and the condenser coil is clean; the compressor protection fault (E5, H6) recurs after a power reset; the error persists after replacing the indicated sensor and checking all connections; or you are not comfortable working inside the unit’s electrical compartment. For refrigerant errors, EPA Section 608 certification is legally required.
Common AC Error Code Repair Costs
The table below provides estimated repair cost ranges for the most common AC error code causes in the United States. DIY costs reflect the part only; professional costs include a typical service call and labor.
| Error Code | Common Cause | DIY Part Cost (USD) | Professional Repair Cost (USD) |
|---|---|---|---|
| E1 — Indoor temperature sensor | Indoor ambient thermistor failure or loose connector | $5 – $20 | $80 – $200 |
| E2 — Indoor coil sensor | Evaporator coil thermistor failure or loose connector | $5 – $20 | $80 – $200 |
| E3 — Outdoor temperature sensor | Outdoor ambient thermistor failure or wire damage | $10 – $30 | $100 – $250 |
| E4 — Outdoor coil sensor | Outdoor coil thermistor failure or wire damage | $10 – $30 | $100 – $250 |
| E6 — Communication error | Loose terminal, damaged wire, or failed control board | $5 – $50 (wire/connector) | $100 – $500 |
| P1 — Low refrigerant pressure | Refrigerant leak or undercharge | Not applicable | $200 – $800+ |
| P2 — High refrigerant pressure | Blocked condenser coil, failed fan, or overcharge | $0 (coil cleaning) | $100 – $500 |
| E5 / H6 — Compressor protection | Compressor overload, locked rotor, or discharge temp fault | $15 – $50 (start capacitor) | $300 – $2,000+ |
Note: Costs are estimates based on typical U.S. market rates as of 2024–2025. Compressor replacement ($1,500–$2,500+ installed) often approaches the cost of a new system — for units older than 8–10 years with a compressor error, full system replacement is frequently more cost-effective. Always obtain a written estimate before authorizing repairs. Refrigerant handling requires EPA Section 608 certification.
Conclusion: Most Common AC Error Codes Start with a Sensor or a Connection
The E2 error that woke me up every morning for a week was caused by a loose thermistor connector — a 10-second fix that cost nothing. That outcome is more common than most homeowners expect: the majority of sensor error codes (E1, E2, E3, E4) are caused by loose connectors or failed thermistors, both of which are DIY-accessible repairs that cost $5–$30 in parts.
Common AC error codes divide into two groups: codes that point to a specific, replaceable component (thermistors for E1–E4, capacitors for fan motor errors, communication wire for E6) and codes that indicate a refrigerant system or compressor fault (P1, P2, E5) that require professional service. The first group rewards methodical diagnosis — record the code, perform a power reset, check connections, test the indicated sensor with a multimeter, and replace if failed. The second group is non-negotiable: refrigerant handling is federally regulated, and compressor diagnosis requires specialized equipment.
Annual maintenance — cleaning the air filter, rinsing the condenser coil, and checking the condensate drain — prevents the majority of AC error codes that homeowners encounter. A clean filter prevents evaporator coil freezing (which triggers E2 coil temperature errors). A clean condenser coil prevents high pressure faults (P2). A clear condensate drain prevents water overflow shutdowns. These three steps, done once per season, eliminate most common AC error codes before they occur.
Sources
- U.S. Environmental Protection Agency (EPA) — Section 608 Refrigerant Management Regulations. Available at: www.epa.gov/section608
- Air-Conditioning, Heating, and Refrigeration Institute (AHRI) — Industry Standards for Residential Split-System Air Conditioners. Available at: www.ahrinet.org
- North American Technician Excellence (NATE) — HVAC Service Technician Certification Standards and Diagnostic Guidelines. Available at: www.natex.org
- U.S. Department of Energy (DOE) — Central Air Conditioner Efficiency Standards and Homeowner Maintenance Guidelines. Available at: www.energy.gov
- Consumer Reports — Room and Central Air Conditioner Reliability and Repair Cost Survey, 2023. Available at: www.consumerreports.org
