Mitsubishi Heat Pump Not Heating: Troubleshooting and Fixes

The article explains common reasons a Mitsubishi heat pump not heating, clear troubleshooting steps, and guidance on when to contact a professional. The goal is to help homeowners identify issues quickly and take safe, effective action to restore heat.

Symptom	Likely Cause	Quick Action
No Warm Air	Thermostat settings, power, or defrost mode	Check mode/temperature and power cycle
Weak Warm Air	Clogged filter, low refrigerant, airflow restriction	Replace filter, inspect vents
Outdoor Unit Icing	Defrost cycle issue, sensor or refrigerant problem	Allow defrost, check for error codes
Unit Makes Unusual Noises	Compressor, fan motor, or loose parts	Turn off unit and inspect; call tech if unsure

How Heat Pumps Work And Why Mitsubishi Units May Fail To Heat

A heat pump transfers heat between indoors and outdoors using refrigerant, a compressor, an expansion device, and indoor/outdoor coils. In heating mode, a Mitsubishi heat pump extracts heat from outdoor air and moves it indoors. When any component fails—thermostat, reversing valve, compressor, sensors, or refrigerant charge—the system may stop delivering heat.

Modern Mitsubishi units include electronic controls and diagnostic codes; understanding these systems and the common failure points speeds troubleshooting and reduces unnecessary service calls.

Common Symptoms When A Mitsubishi Heat Pump Is Not Heating

Recognizing symptoms helps narrow causes quickly. Typical signs include: no warm air from registers, warm air that’s weaker than expected, the outdoor unit covered in ice, frequent short cycling, unusual noises, and visible error codes on the indoor controller or outdoor unit.

• No Warm Air: Fan runs but air stays near room temperature.
• Reduced Heat Output: System runs continuously but fails to reach setpoint.
• Outdoor Unit Icing: Excess ice on the outdoor coil while heating.
• Error Codes: Controller displays codes like H3, H5, or other fault messages (refer to the manual).

First-Line Troubleshooting Steps Homeowners Can Safely Perform

Before calling a technician, perform safe checks. These steps often resolve simple issues and confirm whether a professional is needed.

Check Power And Breakers

Verify the indoor controller and outdoor unit have power. Check circuit breakers and the outdoor disconnect switch. Reset any tripped breakers and ensure the unit is switched on at the thermostat.

Verify Thermostat/Controller Settings

Confirm the Mitsubishi controller is set to Heat mode and the temperature setpoint is higher than the current room temperature. Disable any timer or vacation mode and ensure the fan is set to Auto or On as needed.

Inspect Air Filters And Indoor Airflow

Dirty or clogged filters restrict airflow, reducing heat output. Remove and inspect filters; wash or replace them according to the manual. Also verify vents are open and not obstructed by furniture or drapes.

Check Outdoor Unit Condition

Look at the outdoor unit for ice buildup, heavy debris, or blocked airflow. Clear leaves and snow away from the unit. If the outdoor coil is iced, allow it to defrost—do not chip ice off violently; this can damage fins and coils.

Power Cycle The System

Turn the system off at the controller, then at the outdoor disconnect or breaker. Wait 5 minutes and restore power. This can clear minor electronic glitches or control lockups.

Diagnosing Common Mechanical And Refrigeration Problems

If basic checks do not restore heating, the cause may be mechanical or refrigerant-related. These issues often require a licensed HVAC technician with the right tools.

Refrigerant Leaks Or Low Charge

Low refrigerant reduces heat transfer and produces poor heating performance. Signs include long run times, ice on the outdoor coil, and low indoor discharge temperatures. Detecting and repairing leaks requires EPA-certified technicians and refrigerant recovery equipment.

Compressor Or Electrical Failures

The compressor is the heart of the heat pump. If it fails or the capacitors and contactors are faulty, the unit may not compress refrigerant and move heat. Electrical failures can also cause intermittent operation or no operation at all.

Reversing Valve Problems

The reversing valve changes the refrigerant flow for heating or cooling. If it sticks or fails to shift, the unit may stay in cooling mode or not transfer heat correctly. Technicians test valve operation electrically and mechanically.

Defrost Cycle Malfunctions

Heat pumps enter a defrost cycle to melt frost on the outdoor coil. If the defrost control or sensors malfunction, the unit may remain in defrost too long or not defrost, causing ice accumulation and loss of heating.

Faulty Sensors Or Thermistors

Indoor or outdoor temperature sensors that read incorrectly cause poor control and incorrect cycling. Replacing sensors or recalibrating control boards often resolves these issues.

Interpreting Mitsubishi Error Codes And Diagnostics

Mitsubishi systems display codes on the indoor controller or communicate faults on the outdoor unit. Document any codes shown before resetting power—this helps the technician diagnose the issue. Common codes may indicate communication errors, low pressure, high pressure, defrost errors, or sensor faults.

Always consult the specific model manual for exact code meanings. If the code returns after a reset, schedule professional service and provide the code details.

When To Call A Professional HVAC Technician

Some issues require a trained, licensed technician. Contact professional service if any of the following apply: no power to the outdoor unit after breaker reset, visible refrigerant leak, compressor not running with power present, persistent error codes, frequent freeze-ups, or unusual burning smells.

Request a technician with Mitsubishi experience or one certified to service heat pumps because familiarity with inverter-driven compressors and electronics speeds repair and reduces risk of misdiagnosis.

What To Expect From A Service Visit

• Safety inspection and verification of proper electrical supply.
• System diagnostics, reading error codes, and performance testing.
• Measure refrigerant pressures and temperatures when safe and legal.
• Replace faulty sensors, capacitors, contactors, or control boards as needed.
• Provide a written estimate for major repairs or refrigerant recovery and recharge.

Cost Ranges For Common Repairs

Repair costs vary by region and complexity. Typical ranges below offer a general idea:

Repair Type	Estimated Cost Range
Filter Or Simple Cleaning	$0–$150 (DIY or basic service)
Sensor Or Control Board Replacement	$150–$800
Fan Motor Or Capacitor Replacement	$200–$700
Refrigerant Leak Repair & Recharge	$300–$1,500+ (depending on leak difficulty and refrigerant type)
Compressor Replacement	$1,000–$4,000+ (often costly on older systems)

Preventive Maintenance To Keep A Mitsubishi Heat Pump Heating Efficiently

Regular maintenance extends system life and reduces breakdowns. Recommended tasks include filter cleaning every 1–3 months, annual professional inspections, and seasonal checks before heavy use.

• Keep outdoor unit clear of debris, snow, and vegetation.
• Clean indoor coils and ensure grilles and vents are unobstructed.
• Schedule annual service to check refrigerant levels, electrical connections, and defrost operation.
• Install a programmable or smart thermostat to optimize runtime and reduce strain.

Safety Tips And Best Practices

Safety is essential. Before inspecting any electrical components, turn off power at the breaker and the outdoor disconnect. Avoid attempting refrigerant or compressor repairs unless certified and equipped. If the homeowner smells burning or sees smoke, turn off the system, leave the area, and call emergency services if needed.

Frequently Asked Questions About Mitsubishi Heat Pumps Not Heating

Why Is The Outdoor Unit Icing During Heating?

Some ice is normal in cold weather; the unit should enter defrost cycles to clear it. Excessive or persistent ice suggests a defrost control, sensor issue, or low refrigerant. If ice builds rapidly or does not clear, call a technician.

Is It Normal For Heat Pumps To Produce Lukewarm Air On Extremely Cold Days?

Yes, heat pump efficiency drops at very low outdoor temperatures. Many Mitsubishi systems include auxiliary heat (electric or heat strip) to maintain indoor temperatures during extreme cold. If auxiliary heat fails or is not active when needed, the home may feel underheated.

Can A Simple Reset Fix A Heat Pump That’s Not Heating?

Power cycling the system can clear transient electronic faults and restore normal operation. If the problem recurs after reset, underlying mechanical or refrigerant issues likely exist and require professional attention.

How Long Should A Heat Pump Run To Heat A Room?

Run time varies by outdoor temperature, insulation, and setpoint. Short cycling (rapid on/off cycles) indicates a problem and reduces efficiency. Proper cycling should gradually bring the home to setpoint without frequent short cycles.

Useful Links And Resources

Owners should reference the specific Mitsubishi model manual for error codes and recommended maintenance intervals. For refrigerant and major repairs, hire an EPA-certified technician. Many manufacturers maintain support pages with wiring diagrams and controller guides to aid diagnostics.

If the Mitsubishi heat pump not heating issue persists after basic troubleshooting, documented fault codes and observations (ice, noises, odors) help technicians diagnose and repair efficiently.