Heat Pump Error Codes: What to Check Before You Call an Engineer

Quick answer: Every brand uses its own error codes, but nearly all heat pump faults fall into six categories: water pressure and flow, sensors, refrigerant and pressure, communication, defrost, and backup-heater faults. Check the simple things — system pressure, filters, power — restart once, and if the code comes back, call an MCS-certified engineer rather than resetting repeatedly.

Why there is no universal code list

A Mitsubishi Ecodan, a Vaillant aroTHERM plus, a Daikin Altherma, a NIBE and a Samsung EHS all speak different languages: some use letter–number codes, some plain-text messages, some blinking-light sequences on the outdoor unit. The same fault can be “E5” on one brand and a three-digit code on another, and codes even shift between models within a brand. So the authoritative source is always your unit’s manual or app — most manufacturers publish code lists online. What is consistent across brands is the categories of fault, and that is where a homeowner can usefully start.

The six fault categories (and what you can safely check)

Winter deserves a special mention: frost on the outdoor unit is normal and periodic defrosting is part of the design — see how the defrost cycle works. A defrost error code is the unit telling you the process is failing, which is different from the process merely happening.

The right way to handle any code

• Write the code down (or photograph the screen) before touching anything — codes clear on restart and the engineer will want it.
• Check the basics: power on at the isolator, system pressure in range, thermostat calling for heat, no tripped breakers.
• Restart once. Power the system down for a minute and back up. Transient glitches — after a power cut, for instance — clear this way.
• If it returns, stop. A recurring code is a message, not a nuisance. Repeated resetting can mask a developing fault and force the compressor to keep restarting into the problem.

What never to do

Do not open the refrigerant circuit, and be wary of anyone without proper certification who offers to: in the UK, work on the refrigerant loop legally requires an F-gas certified engineer (GOV.UK). Do not bypass safety cut-outs, run the unit with a known flow fault, or let a general handyman “have a look” — unauthorised work is one of the fastest ways to void your cover, as our warranty comparison explains. The economics rarely favour DIY here anyway: the diagnosis is usually the hard part, and engineers carry the tools and gauges to do it properly.

Make the engineer’s visit count

A little preparation shortens the visit — and call-outs are usually charged by time. Have ready: the code (and any history of codes with dates), the model and serial number from the unit’s data plate, your commissioning record if you have it, and a one-line timeline of what the system was doing when the fault appeared. Clear access to the outdoor unit and the cylinder cupboard before the engineer arrives. If the fault is weather-dependent, say so when booking: a refrigerant issue that only bites below 5°C is far easier to diagnose on the right day.

Who to call, and when

For anything under warranty, start with your installer or the manufacturer’s service line — using the manufacturer’s approved network keeps the warranty intact. Out of warranty, look for an MCS-certified heat pump engineer with F-gas certification. If the fault is intermittent, gather evidence first: dates, codes, what the weather was doing, whether hot water or heating was affected. Our heat pump troubleshooting guide covers the no-code problems — noises, poor output, high bills — and an annual service catches many of these faults before they trip a code at all. For the bigger ownership picture, see our heat pump guide.

FAQs

My heat pump shows an error after a power cut. Should I worry?

Usually not. Give it a single restart and time to resume its programme — some units sit in a delayed-start state for several minutes to protect the compressor. If the same code returns within a day or two, log it and call your engineer.

Is it safe to keep using the heating with an error code showing?

If the system is still heating and the code relates to a non-critical function, short-term use is generally fine — but treat it as running on borrowed time and get it looked at. If the code accompanies unusual noises, leaks or a tripping breaker, switch off at the isolator and call an engineer.

Will clearing error codes before a service hide anything?

Possibly from you, rarely from the engineer — most units keep an internal fault history that survives resets. Either way, do not bother: the stored history plus your notes gives the engineer the fastest route to a diagnosis.