Your heat pump is flashing a code at you and you have no idea what it means. Maybe it’s blinking on the indoor unit display, maybe it showed up on your thermostat screen, maybe your app sent you a notification with a string of letters and numbers that might as well be in another language. Either way, your heat pump is trying to tell you something. Here’s how to actually understand what it’s saying.
First: Don’t Panic
Not every error code means your system is dying. Some codes are informational, some are minor faults the system recovers from on its own, and some do require a technician. The code itself is just your heat pump’s way of flagging something worth attention. The goal here is to help you figure out which category you’re in before you pick up the phone — or to confirm that yes, you should definitely pick up the phone.
How Error Codes Work
Heat pumps are essentially small computers running a heating and cooling system. The control board monitors everything — refrigerant pressure, temperatures at various points in the system, electrical inputs, fan speeds, defrost cycles — and when something falls outside of normal parameters, it logs a fault code and often displays it.
Codes vary significantly by manufacturer. A Mitsubishi E6 means something completely different than a Daikin E6. There’s no universal standard, which is genuinely annoying and one of the reasons homeowners struggle with this. The brand and model of your system is the starting point for interpreting any code you see.
Where to Find Your Error Code
Depending on your system, the code might show up in a few different places. The indoor unit often has a small display or a series of blinking LED lights that indicate a fault. Some systems communicate through a wall thermostat or control pad. Newer systems from brands like Mitsubishi, Daikin, and Bosch often connect to a smartphone app that will display the error and sometimes even describe it in plain language.
If you’re seeing a blinking light pattern rather than an alphanumeric code, count the blinks. Many manufacturers use blink sequences to communicate fault codes on units without a display — for example, two blinks, pause, four blinks might translate to a specific error in the manual. Your installation manual should have a blink code legend. If you don’t have the manual, the model number on the unit’s nameplate will get you to a PDF online.
Common Error Code Categories
While codes vary by brand, most fall into a handful of categories worth understanding.
Communication errors are among the most common. These indicate the indoor and outdoor unit have lost contact with each other. Sometimes this is a loose wire connection, sometimes it’s a control board issue, sometimes it’s caused by a power fluctuation. If your system just had a power outage or surge, this is worth checking first. We see this come up a lot after the kind of winter storms that roll through Worcester and Concord and knock out power for a few hours.
Refrigerant faults show up when the system detects pressure outside of normal operating range. This can indicate a refrigerant leak, but it can also indicate a sensor issue or a problem with the expansion valve. Either way, this one warrants a call to a technician. Refrigerant work requires EPA certification and isn’t a DIY fix.
Temperature sensor errors happen when a thermistor (a small temperature-sensing component) fails or reads outside its expected range. These are relatively common and often straightforward to diagnose and repair.
Defrost faults can appear during cold weather when the outdoor unit is having trouble completing its defrost cycle. Some of this is normal system behavior in icy conditions, but repeated defrost faults can indicate airflow restrictions, low refrigerant, or a defrost control issue. In towns like Carlisle, Sudbury, and Lexington where properties sit on larger lots with more exposure, outdoor units can ice up faster during prolonged cold snaps and trigger these codes more frequently.
Electrical and inverter faults cover a range of issues related to the compressor, power supply, or the inverter drive that controls compressor speed. These tend to be more serious and almost always require a technician.
What to Do When You See a Code
Start by writing down the exact code. If it’s a blink sequence, count carefully and write that down too. Note when it appeared, whether the system is still running or has shut down, and whether it’s been happening repeatedly or this is the first time.
Then look it up. Your installation manual is the best source. If you don’t have it, search your unit’s brand and model number along with the fault code. Manufacturer websites and HVAC forums often have solid documentation.
If the code is one the system recovered from on its own and heating seems normal, monitor it and see if it comes back. If the system has shut down, is blowing cold air when it should be heating, or the same code keeps reappearing, that’s when to call.
When to Call a Technician
Some things are worth attempting yourself, like checking that the outdoor unit isn’t buried in snow or ice, making sure the air filter isn’t clogged, or confirming the circuit breaker hasn’t tripped. Beyond that, heat pump diagnostics involve refrigerant pressure testing, electrical measurements, and component-level troubleshooting that requires proper tools and training.
We work on systems across central and eastern Massachusetts every day. In older homes throughout Newton, Brookline, Cambridge, and Watertown, we often find that error codes trace back to aging electrical panels or wiring that wasn’t originally designed for a heat pump load. In newer construction in Marlborough, Northborough, and Reading, it’s more often a commissioning issue or a communication fault between components. The point is that context matters, and an experienced technician will know what to look for based on your home and your system.
A Note on Newer Smart Systems
If your heat pump was installed in the last few years and connects to a smartphone app, take advantage of it. Mitsubishi’s Kumo Cloud, Daikin’s Residential Controller app, and similar platforms will often tell you not just the error code but a plain-language description of the fault. Some systems even log fault history so a technician can see what’s been happening before they arrive, which makes diagnosis faster and your service call shorter. For homeowners in Somerville, Waltham, and Cambridge who tend to be running newer systems in recently renovated multifamily buildings, this kind of remote visibility is especially useful.
Is Your Heat Pump Throwing a Code You Can’t Figure Out?
We work on heat pump systems across central and eastern Massachusetts every day, from Worcester to Boston and everywhere in between. Whether you’re in Natick or Needham, Dedham or Wakefield, Braintree or Lexington, we can help you sort out what your system is telling you. And if it does need a technician on site, we’ll tell you exactly what we found and what it will cost before we touch anything.
Heat Pump Error Codes Frequently Asked Questions
Can I reset my heat pump to clear an error code?
Sometimes. Many systems allow a reset by turning the unit off at the thermostat or control pad, waiting a few minutes, and turning it back on. Some require a breaker reset. If the underlying issue is still present, the code will come back. A reset that clears the code permanently is good news. One that clears it temporarily and keeps returning means something needs attention.
My heat pump shows an error code but is still running. Should I be worried?
It depends on the code. Some faults are logged but don’t interrupt operation. Others will eventually cause the system to shut down to protect itself. Look up the specific code for your system and model. If you can’t find clear information, give us a call and we can help you figure out whether it’s urgent.
The outdoor unit is covered in ice. Is that an error?
Light frost on the outdoor unit during cold weather is completely normal. Heat pumps go through periodic defrost cycles to clear this. A solid block of ice that isn’t clearing, or ice building up on the refrigerant lines, is a different story and can indicate an airflow problem, low refrigerant, or a defrost control issue worth having looked at. This comes up regularly during hard winters in inland towns like Sharon, Canton, Milton, and Concord where temperatures stay low for extended stretches.
I can’t find my installation manual. Where do I get error code information?
Find the model number on the nameplate of your outdoor unit (usually on a sticker on the side of the cabinet). Search that model number along with your brand name and “error codes” or “fault codes.” Most major manufacturers have documentation available online. You can also call us and we can often point you in the right direction.
How much does it cost to diagnose a heat pump error code?
Diagnostic service calls typically involve a trip charge plus time on site. The cost to repair depends entirely on what’s causing the fault. We always tell customers what we found and what it will cost before we do any repair work, so there are no surprises.
Will error codes void my warranty?
No, error codes themselves don’t affect your warranty. Having unauthorized repairs performed by someone without proper certification can affect it, though. If your system is still under manufacturer warranty, make sure any repair work is done by a licensed HVAC contractor.
