Run capacitor
Common no-start and weak-compressor failure point. Match the microfarad and voltage rating.
HVAC Error Code Troubleshooting — What Every Technician Needs to Know
HVAC error codes are how your heating and cooling equipment tells you what’s wrong. They appear as blinking LED patterns, numeric displays, or alphanumeric codes depending on the equipment type and age. This guide explains how to read every major type and what the most common codes mean across brands.
| Equipment Type | How Codes Display | Where to Look |
|---|---|---|
| Gas furnace | Blinking LED (flash count) | Lower access panel or sight glass |
| Mini-split | Alphanumeric display | Indoor unit LED or remote display |
| Heat pump | Flash codes or display | Air handler control board |
| Boiler | Alphanumeric display | Boiler control panel or display |
| VFD | Numeric/alphanumeric fault | Drive keypad display |
| CNC machine | Alarm number + message | Controller screen |
| Commercial refrigeration | E-codes or numeric | Controller display on unit |
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.
For field measurements, see the HVAC thermal imager buying guide before choosing a camera, infrared thermometer, or pipe clamp thermometer.
How to Read Flash Codes on a Furnace
- Turn the thermostat to Heat and set the setpoint above room temperature
- Open the lower furnace access panel
- Find the LED light on the control board (usually green or amber)
- Count the flashes in one complete cycle (the pattern repeats)
- Match the count to your furnace’s code chart (often printed on the inside of the access panel door)
Example: If the LED flashes 3 times, pauses, then flashes 3 times again — that’s a 3-flash code. On most brands, 3 flashes = pressure switch fault or limit switch fault.
Most Common HVAC Fault Types
Pressure Switch Faults
Nearly every 90%+ efficiency furnace generates pressure switch faults eventually. The inducer motor creates negative pressure; the pressure switch confirms that pressure before allowing ignition. Common causes: blocked condensate trap, cracked pressure hose, weak inducer motor, failed pressure switch.
Quick check: Disconnect the hose from the pressure switch and try to suck air through it — it should flow freely. If not, the hose or trap is blocked.
Ignition Lockouts
The furnace tried to light (usually 3 attempts) and gave up. This is always one of three things: (1) no gas reaching the burner, (2) igniter failed and can’t light the gas, or (3) gas lit but the flame sensor didn’t confirm it.
Quick check: Watch through the sight glass during a call for heat. If you see the igniter glow red but no flame appears, you have a gas supply or valve issue. If a flame briefly appears then goes out, the flame sensor needs cleaning.
High Limit Faults
The heat exchanger got too hot. Almost always caused by restricted airflow: dirty filter, closed registers, blocked return air. Less common: failed blower motor or blower wheel clogged with debris.
Quick check: Replace the filter. If the limit trips again within one heating cycle, check that all supply and return registers are fully open.
Flame Sensor Issues
A thin metal rod in the burner proves the flame is present. It works by passing a tiny DC current through the flame. After 3–5 years of use, oxidation on the rod surface reduces conductivity and causes false “no flame” signals.
Quick check: Remove the sensor (one screw), rub the metal rod with fine steel wool or 400-grit sandpaper for 10 seconds, reinstall. This clears most flame sensor faults.
How to Read Mini-Split Error Codes
Mini-splits display codes on the indoor unit’s digital panel or on the remote control LCD. The codes follow brand-specific formats:
- Mitsubishi: E-codes (communication/sensor), U-codes (system protection), P-codes (abnormal operation)
- Daikin: E-codes, U-codes, F-codes
- LG: CH (Check) codes followed by numbers
- Samsung: E-codes followed by two digits (e.g., E1-01)
- Fujitsu: E and P codes
Most mini-split codes require accessing the mode button or diagnostic mode — consult your model’s service manual for the procedure to retrieve codes when the display only shows limited information.
How to Read VFD Fault Codes
Variable frequency drives display fault numbers or abbreviated text on their keypads. Common patterns across brands:
- Overcurrent (OC, F001, Fault 7): Motor drawing too much current — check for mechanical jam, wrong motor settings, or acceleration ramp too fast
- Overvoltage (OV, F003, Fault 3): Bus voltage too high — check incoming power, add braking resistor if needed
- Undervoltage (UV, F004): Low supply voltage — check incoming power quality
- Overtemperature (OH, F004): Drive or motor too hot — check cooling fan and ambient temperature
When Error Codes Indicate Danger
Some fault codes require immediate action before attempting restart:
- Gas furnace roll-out switch trip — Do not restart; have heat exchanger inspected
- High-pressure switch fault on a heat pump — Could indicate refrigerant overcharge or blocked coil
- Ground fault on a VFD (GF fault) — Motor winding or cable insulation failure; do not re-energize without megger test
- Boiler lockout on ignition failure — Check for CO buildup before entering the mechanical room
When to Call a Pro
If the same code returns after basic cleaning and filter replacement, or if any safety device trips (roll-out switch, high-pressure switch, CO detector), contact a licensed HVAC or industrial technician. Codes that involve gas, refrigerant, or electrical safety are not DIY repairs.