A faulty contactor is one of the most common reasons a heat pump outdoor unit stops working while the indoor unit appears to run normally. The good news is there are several visual and audible clues you can check safely — but because the outdoor unit contains high-voltage components, most electrical testing and all repairs must be carried out by a licensed professional.
What Is a Contactor and What Does It Do?
A contactor is an electrically operated switch inside your outdoor unit's electrical panel. When your thermostat or indoor unit calls for heating or cooling, a low-voltage signal energises a small coil inside the contactor, which pulls down a set of contacts to complete the high-voltage circuit powering the compressor and fan motor. If the contactor fails, the outdoor unit simply won't start — or may run continuously even when it shouldn't.
Signs Your Contactor May Be Faulty
Before opening any panels, watch and listen for these common symptoms:
- Outdoor unit won't start — the indoor unit runs and blows air, but the outdoor unit fan and compressor stay silent.
- Outdoor unit runs constantly — the unit won't switch off even when the set temperature is reached (contacts welded shut).
- Chattering or buzzing noise from the outdoor unit — the contactor coil is energising but the contacts aren't closing cleanly.
- Burnt smell or visible scorch marks around the electrical panel area.
- Frequent tripping of the circuit breaker supplying the outdoor unit.
- Intermittent operation — the unit starts and stops randomly without a clear reason.
If you notice any of these signs, it's worth doing a few safe visual checks before calling a tradesperson, as it helps you give them useful information.
Safe Visual Checks You Can Do Yourself
Safety first: Before opening the outdoor unit's electrical panel cover, switch the heat pump off at the indoor controller and turn off the dedicated isolator switch (usually a weatherproof switch mounted near the outdoor unit). Wait at least five minutes for capacitors to discharge. Even then, treat all wiring as live until confirmed otherwise by a licensed electrician.
1. Check the Isolator and Circuit Breaker
Make sure the outdoor isolator switch is in the ON position and the circuit breaker in your switchboard hasn't tripped. A tripped breaker that keeps tripping points to an electrical fault — stop here and call a professional.
2. Visual Inspection of the Contactor
With the unit fully isolated and powered off, carefully remove the outdoor unit's electrical panel cover (usually a few screws). Look for:
- Burnt, pitted, or blackened contact surfaces — a clear sign of arcing and wear.
- Melted or discoloured wiring around the contactor terminals.
- Broken or cracked plastic housing on the contactor body.
- Stuck or visibly corroded contact bridge — sometimes you can see the moving contact arm.
If you see any of these, the contactor almost certainly needs replacing. Photograph what you find — it will help your electrician.
3. Listen When the Unit Is Called to Run
With the panel cover replaced and the unit powered back on, set the indoor controller to a mode that should start the outdoor unit (e.g., cooling on a warm day). Stand near the outdoor unit and listen:
- A healthy contactor makes a single, firm click when it engages.
- A rapid chattering sound suggests a weak coil, low voltage, or dirty contacts.
- Complete silence from the outdoor unit when the indoor unit is running suggests the contactor coil isn't being energised, or the contacts are open-circuit.
What You Cannot Safely Do Yourself
Under the Electrical Workers Registration Board (EWRB) rules, testing live voltages, replacing a contactor, or any other electrical repair inside the outdoor unit is restricted electrical work in New Zealand. This must only be carried out by a registered electrician or a licensed heat pump technician. Attempting this work yourself is illegal, unsafe, and may void your home insurance.
A licensed technician will use a multimeter to test:
- Coil resistance (to check if the coil is open or shorted)
- Voltage across the coil terminals when energised
- Continuity across the main contacts
Contactor replacement is generally a straightforward job for a qualified technician and is usually completed in under an hour.
How Long Do Contactors Last?
Contactors typically last 5–15 years, depending on how often the unit cycles, local conditions (coastal salt air accelerates corrosion), and the quality of the original component. In New Zealand's varied climate — from humid Northland to cold, damp Southland winters — outdoor units work hard, and contactors in high-use systems may wear sooner. BRANZ research highlights that regular maintenance significantly extends heat pump component life.
Preventing Contactor Problems
- Annual servicing by a qualified heat pump technician includes checking electrical components including the contactor.
- Keep the outdoor unit clear of debris, and ensure good airflow around the unit.
- In coastal areas, consider more frequent servicing due to salt-laden air accelerating corrosion.
- Check that your home's supply voltage is stable — consistently low voltage causes contactors to chatter and wear prematurely.
When to Call a Professional
Call a licensed heat pump installer or registered electrician if:
- You observe any of the visual fault signs described above.
- The outdoor unit won't start or won't stop.
- Your circuit breaker trips repeatedly.
- You hear chattering or buzzing from the outdoor unit.
- You are unsure about anything you've found.
All electrical testing, diagnosis, and contactor replacement is restricted electrical work under NZ law — it must be done by a professional. You can find qualified local heat pump installers and electricians on tradedirectory.co.nz.
For more on your rights and what to expect from tradespeople, Consumer NZ is a useful resource.
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