Heat Pump Troubleshooting: Find Your Symptom, Get the Checklist

Start from the symptom you actually have. Each section gives you the two-minute version, then links to the full diagnosis.

Start with the symptom you actually have. Each entry below gives you the two-minute version and links to the full diagnosis. Work top to bottom within a section, because the items are ordered by how often each turns out to be the cause.

Before anything else, three things are true often enough to check first every single time: the thermostat batteries, the air filter, and both circuit breakers. A meaningful share of service calls end with one of those three.

Nothing happens at all

No air, no outdoor unit, no response to the thermostat.

1. Thermostat batteries, mode and setpoint. Set at least three degrees away from room temperature.
2. Both breakers, indoor and outdoor. Push fully off, then firmly on.
3. The outdoor service disconnect near the unit.
4. The condensate float switch. Standing water in the drain pan shuts the system down deliberately.
5. The air handler access panel, which has a door interlock switch on many systems.
6. Wait ten minutes for the anti-short-cycle delay before concluding anything.

Full sequence: heat pump not turning on.

It runs but the air is cold in heating

1. Is it in a defrost cycle? Outdoor fan stopped, steam rising, lasts 5 to 15 minutes. Normal.
2. Is emergency heat switched on at the thermostat? It disables the compressor.
3. Check the filter and registers for restricted airflow.
4. Check the O/B reversing valve setting if the thermostat was recently replaced.

Full ranking of causes: blowing cold air on heat mode. Defrost specifically: the defrost cycle explained.

Ice on the outdoor unit

1. Light frost in cold damp weather is expected and clears at the next defrost.
2. A solid block encasing the coil, or ice on top of the unit, is not normal.
3. Check that melt water can drain away rather than refreezing under the unit.
4. Never chip ice off with anything metal; a punctured coil turns this into a refrigerant leak.

Full guide: frozen outside, normal or not.

It starts and stops every few minutes

1. Confirm it is not the defrost cycle or the anti-short-cycle delay.
2. Replace the filter and open all closed registers.
3. Check where the thermostat is mounted: sun, lamps, drafts and supply registers all cause false readings.
4. Time the cycles for an hour. Repeated runs under about seven minutes are the threshold.

Full guide: short cycling. If it turns out to be an oversized system: oversized versus undersized problems.

Auxiliary heat runs constantly

1. Check the outdoor temperature against your balance point. Below it, aux running is correct.
2. Check the aux lockout setting in the thermostat.
3. Turn off deep setbacks, which trigger aux heat during recovery.
4. If aux runs on mild days, something is genuinely wrong rather than merely configured badly.

Full guide: aux heat that will not turn off.

There is a smell

1. Dry, dusty, like scorched dust, on the first heating cycles of the season: normal, fades within 30 minutes.
2. Sharp, acrid, like melting plastic, or getting stronger: cut power at the breaker and call.
3. Musty: mould or bacteria on the coil or in the drain pan.
4. Rotten eggs: leave the house and call the gas utility from outside.

Full guide: burning smells, normal or not.

There is an error code on the display

Write it down before cutting power, because a reset clears the stored fault. Then find your brand:

• Mitsubishi
• Daikin
• LG
• Fujitsu
• Carrier

If the indoor unit has no display, the fault is coming out as a blink pattern, and the method for reading it differs by brand.

The maintenance that prevents most of this

Task	How often	Why it matters
Check or replace the filter	Every 1 to 3 months	Restricted airflow causes a large share of all faults
Clear around the outdoor unit	Seasonally, and after storms	Prevents pressure faults and poor defrost
Flush the condensate drain	Annually, before cooling season	Prevents float switch shutdowns and water damage
Rinse the outdoor coil	Annually	A fouled coil cannot exchange heat
Confirm the aux lockout setting	Once, at the start of heating season	Directly controls your winter bill
Professional service	Annually	Charge, electrical connections and safety checks you cannot do yourself

When to stop and call

Call a technician when a breaker trips repeatedly, when you smell anything electrical, when the unit hums or clicks without starting, when a code returns after one correct reset, or when anything points at the refrigerant circuit. Refrigerant work legally requires EPA Section 608 certification.

Have the model numbers of both units, the exact code if there is one, and a description of what does and does not happen when you change the setpoint. If you are weighing repair against replacement, our cost breakdown and state rebate guide are where that decision gets made.

Common questions

What should I check before calling for any heat pump problem? Thermostat batteries and settings, the air filter, both breakers, the outdoor disconnect, and whether the outdoor unit is clear of debris and ice. Those five cover a large share of service calls and cost nothing.

How do I know if something is a fault or normal behaviour? The most commonly misread normal behaviours are the defrost cycle, the anti-short-cycle delay, dust burning off heat strips at the start of the season, and aux heat running below the balance point. All four look like faults and are not.

Is it worth paying for annual maintenance? For most systems yes, mainly for the things you cannot check yourself: refrigerant charge, electrical connections and capacitor condition. A failing capacitor found during a service visit is cheap; the same capacitor failing in a January cold snap is an emergency call.

How long should a heat pump last? Typically in the range of 15 years, varying widely with climate, maintenance and how well it was sized and installed. A system that has been short cycling for years will not reach the top of that range.

How this guide is put together

Diagnostic steps here are ordered by how often each cause turns out to be the answer, not by how serious it is. That is deliberate: working the common causes first is what saves you a service call, and the rare-but-severe ones are flagged where they apply.

• Sources. Manufacturer installation and service manuals, published fault-code tables, and the documentation linked at the foot of this page.
• The DIY line. Anything involving refrigerant is legally restricted to an EPA Section 608 certified technician. Sealed systems, capacitors and line-voltage wiring are called out as technician work rather than left ambiguous.
• Where brands differ. Code meanings and defrost behaviour vary by series and model year. Where published charts disagree with each other, we say so instead of picking one.

Sources

Primary references for the figures and procedures on this page. Where a number here is a range drawn from field experience rather than a published measurement, it is labelled as such in the text.

• EPA Section 608 Technician Certification — Who is legally permitted to handle refrigerant, and why.
• ENERGY STAR: Air-Source Heat Pumps — Current federal efficiency criteria and the qualifying product list.
• AHRI Directory of Certified Product Performance — Look up a specific model’s certified capacity and efficiency ratings.

Last checked September 2026. Rebate amounts and program status change frequently; always confirm against the official program page before you sign a contract.