Worcester Bosch heat pump alarms are short codes, such as E0, P0 or H6, where the letter points to the area at fault. E codes mostly cover temperature sensors and water flow, P codes are protections such as refrigerant low or high pressure, H codes cover communication, more sensors and the fan, and L codes are inverter and compressor drive faults. C and F codes cover heat sink temperature, master and slave setup and the DC supply.
These codes come from a single alarm table in a manufacturer's manual whose model range is not stated. They apply to some Worcester Bosch and Bosch heat pump ranges, not every model, so check your own manual before relying on a meaning.
If a code returns or you are unsure, book an engineer rather than guessing; you can book a heat pump repair or call 0333 041 5130. For other brands, see our heat pump error codes hub. Safety first: never open the unit or touch the electrics inside.
Please read first: this guide is here to help you understand what your Worcester Bosch heat pump is telling you. Every effort has been made to keep the information accurate, but manufacturers update their manuals and codes can differ between models and software versions, so always check the official Worcester Bosch manual for changes before acting on anything here. Do not attempt anything that could be dangerous, and avoid any work that could void your warranty. If in doubt, call a qualified heat pump engineer.
If your Worcester Bosch heat pump is not showing a code, or you want to rule out the basics first, our guide to common heat pump problems walks through the general checks and matches common faults to error codes across every major brand.
Is it safe to fix a Worcester Bosch heat pump fault yourself?
Basic checks are safe, but almost every repair behind these codes is engineer work. Checking the pressure gauge, opening valves and clearing snow or leaves from around the outdoor unit are fine for most homeowners.
Never open the casing or touch the wiring. Many codes involve the inverter module and compressor drive, which run at dangerous voltages.
Refrigerant work is legally restricted to F-Gas certified engineers, so codes such as P0, P1 and H8 that may involve the refrigerant circuit are not DIY. Unqualified work inside the unit can also affect your warranty, so check its terms first.
Which Worcester Bosch error codes can you check yourself?
A few codes have a realistic first check, mostly those linked to water flow, airflow and power. Do it once, then call us if the code returns. The checks below are general engineering advice, as the manual gives descriptions only.
| Code | What it means | Safe homeowner check | Stop and call an engineer if |
|---|---|---|---|
| E0, E8 | Water flow failure | Check isolation valves are open and the pressure gauge is in range. Top up only if you have a filling loop and your manual allows it | The code returns or pressure keeps falling |
| PP | Abnormal temperature difference between water in and out | The same flow checks as E0 | It repeats, as three PP faults in cold conditions trigger Hb |
| Pb | Antifreeze mode, not a fault | Keep the unit powered | Other alarms appear alongside it |
| P0 | Low pressure protection | Clear snow, leaves or debris from around the outdoor coil by hand, without tools | It returns once airflow is clear |
| Pd | Outdoor unit over-temperature | Clear anything blocking airflow around the unit | It returns once airflow is clear |
| H6, HH | DC fan failure | From outside, look for ice, leaves or debris blocking the fan. Do not reach inside | The fan is clear but the code stays |
| H7 | Abnormal power supply voltage | Check for a power cut or tripped breaker | It returns with normal power |
What are the most common Worcester Bosch heat pump error codes?
The manual does not rank codes by frequency, so these are the ones with clear descriptions and causes an engineer can check. The manual gives meanings only; all causes below are general engineering knowledge.
E0 and E8: water flow failure
E0 appears after three water flow failures. E8 is displayed three times and can reset after a few minutes.
Likely causes (general engineering knowledge): closed isolation valves, a blocked strainer, air in the circuit, a pump running at the wrong speed, sludge, scale in the plate heat exchanger and low system pressure.
What to do: Check valves and pressure. If it returns, an engineer will bleed the system, clean the strainer and check the pump, and heavy sludge may need a heat pump powerflush.
PP and Hb: temperature difference and freeze risk
PP means the difference between the water entering and leaving the unit is abnormal. Hb is raised after three PP faults in a row with leaving water below 7C, and the manual says it is reset by a power failure.
What to do: Treat repeated PP as a flow problem and fix it quickly, as low flow in cold weather risks freezing. An engineer should find the cause before Hb is cleared.
Pb: antifreeze mode
Pb is not a protection fault. The manual says the alarm light does not flash and an antifreeze icon shows instead.
What to do: Nothing, beyond keeping the unit powered. Our guide to a heat pump in cold weather explains what is normal in winter.
P0: low pressure protection
The refrigerant side is running at low pressure. Likely causes are a dirty or iced evaporator, a failed fan, very low outdoor temperatures, low refrigerant charge or a blocked expansion device.
What to do: Clear the area around the coil and check the fan is not blocked. Our guide to a frozen outdoor unit covers safe ice checks; beyond that, call an F-Gas engineer.
P1: discharge temperature or high pressure protection
Refrigerant pressure or discharge temperature has gone too high. Poor water flow, a high return temperature and an overcharged system are the usual reasons.
What to do: An engineer should restore flow and check the flow temperature setting before suspecting the refrigerant circuit.
H6 and HH: DC fan failure
H6 is a DC fan failure. HH is raised after ten H6 failures within 120 minutes and resets after shutdown.
What to do: Look for ice, leaves, debris or an animal blocking the fan before anyone replaces it. Our guide to a heat pump making noise covers fan symptoms.
E3 to E7, E9, EA to EC, H2, H3, H5, H9 and HA: sensor failures
These cover the leaving water, tank, unit, room, inertial tank, outdoor, refrigerant and plate exchanger sensors. The usual cause is an open or short-circuit sensor, a loose connector or damaged cable.
What to do: An engineer tests the sensor resistance and wiring before replacing any board.
E2, H0, H1, HE and Hd: communication faults
E2 is between the user interface and hydraulic module, H0 between the indoor and outdoor units and H1 between the unit and inverter module. HE is between the hydraulic module and its adapter board, and Hd between master and slave units in parallel systems.
What to do: An engineer checks cabling and terminations first.
L codes and H4: inverter and compressor drive
L0, L1, L2, L4, L5, L7, L8 and L9 cover DC bus voltage, compressor module errors, zero speed, phase sequence and speed variation. H4 appears after three L signals within an hour and cannot be reset; the manual says to check the last three L signals to find the cause.
What to do: These need an engineer with refrigeration and electrical competence.
Which error code might you see in common situations?
These are the codes from the alarm table most likely in everyday situations. Do the first check, then call us if the code stays.
| Scenario | Codes you may see on a Worcester Bosch | First check |
|---|---|---|
| Cold snap and iced outdoor unit | Pb, P0, H6, HH, Hb | Keep the unit powered and clear snow from around it. Do not chip ice off the coil |
| Losing pressure or low pressure | E0, E8, PP | There is no water pressure code in this table, so read the gauge and look for leaks |
| Radiators lukewarm | E0, E8, PP, P1, P5 | Check valves are open and pressure is correct |
| No hot water | E4, EC | Check the hot water schedule on your controller |
| After a power cut | H7, F1, L1, E2, H0 | Give the system a few minutes to restart and check the breaker |
| Unit noisy or fan not spinning | H6, HH | From outside, look for ice or debris around the fan |
| After a service or filter clean | E0, E8, PP | Check isolation valves were reopened and pressure is back up |
How do you reset a Worcester Bosch heat pump?
The alarm table gives reset behaviour for only a few codes, so check your manual or ask your installer for the exact steps on your model. E8 can reset after a few minutes, HH resets after shutdown, HP resets automatically and Hb is cleared by a power failure.
H4 cannot be reset at all. The manual directs engineers to the last three L signals to find the real fault.
Resetting repeatedly without fixing the cause is a bad idea. Codes like HH and H4 exist precisely because the same fault has repeated, and each forced restart adds strain to the fan or compressor.
Worcester Bosch error code list
This list covers every code in the alarm table we used. It applies to some Worcester Bosch and Bosch heat pump ranges only, so check your manual if a description does not match. From H8 onwards the manual also lists a Modbus code for each alarm.
| Code | Meaning | Who should fix |
|---|---|---|
| E0, E8 | Water flow failure | Homeowner check, then engineer |
| E1 | Phase error (three-phase models) | Engineer |
| E2 | User interface to hydraulic module communication | Engineer |
| E3, E4, E5, E6, E7, E9, EA, Eb, EC | Water, tank, unit, room, inertial tank, solar and air temperature sensors | Engineer |
| Ed, EE | Twin sensor board replacement failure, hydraulic module EEPROM | Engineer |
| P0 | Low pressure protection | Homeowner check, then F-Gas engineer |
| P1, P4 | High pressure, discharge or exhaust temperature protection | F-Gas engineer |
| P3, P6 | Compressor overcurrent, inverter module protection | Engineer |
| P5 | Water temperature difference or supply temperature too high | Engineer |
| Pb | Antifreeze mode, not a fault | No action |
| Pd | Outdoor unit over-temperature | Homeowner check, then engineer |
| PP | Abnormal water temperature difference | Homeowner check, then engineer |
| H0, H1, HE, Hd | Communication faults | Engineer |
| H2, H3, H5, H9, HA | Refrigerant, Ta, Tw2 and plate exchanger sensors | Engineer |
| H4 | Three L signals in one hour, cannot be reset | Engineer |
| H6, HH | DC fan failure, repeated fan failure | Homeowner check, then engineer |
| H7 | Abnormal supply voltage | Homeowner check, then engineer |
| H8, HP | High pressure sensor, cooling mode low pressure protection | F-Gas engineer |
| Hb | Three PP faults with leaving water below 7C | Engineer |
| HF, bH | Unit EEPROM, PED board failure | Engineer |
| C7 | Heat sink over-temperature | Engineer |
| C0 | More than one unit set as master | Engineer |
| F1, L1 | DC bus low voltage | Engineer |
| L0, L2, L4, L5, L7, L8, L9 | Compressor module, high voltage, drive, speed and phase faults | Engineer |
Where can you find the official Worcester Bosch manual?
The alarm table in this guide comes from a Worcester Bosch heat pump installation manual, and Worcester Bosch publishes its heat pump installation and user manuals in its official library. Find the manual for your exact model there, as alarm lists differ between ranges.
Need help with a Worcester Bosch heat pump fault?
We repair Worcester Bosch and Bosch heat pumps across the UK, from a single alarm to a system that has shut down. Our F-Gas certified engineers carry out proper fault diagnosis and Worcester Bosch heat pump repairs, with a 12-month guarantee on the work.
No heating in a cold snap? Our emergency heat pump repair service offers same-day visits where possible. Find out more on our Worcester Bosch heat pump page, our Bosch heat pump page or in our Worcester Bosch heat pump review.
Flow, fan and sensor alarms are often caught early at an annual heat pump service, and our cover plans combine servicing with repairs. Get in touch or call 0333 041 5130.
Frequently asked questions
Why do the codes in my manual not match this list?
Worcester Bosch and Bosch have sold several heat pump ranges, and alarm lists differ between them. Always go by the manual for your exact model; the model name is on the unit's rating label.
What is the Modbus number next to some codes?
From H8 onwards the manual lists a Modbus code beside each alarm, presumably the number reported over a Modbus link to building controls. Most homes will never see it, but an engineer may use it on larger or connected systems.
Will an alarm code void my warranty?
An alarm on its own does not affect your warranty; it is the heat pump protecting itself. Warranty problems usually come from unqualified repairs or missed servicing, so check your terms.
Does an L code mean the compressor has failed?
Not necessarily. L codes also cover DC bus voltage, phase sequence and drive faults, which are often supply or board issues. Our guide to heat pump compressor problems explains the signs of a genuine failure.
What can I check before paying for a call-out?
Confirm the unit has power, airflow around it is clear and the controller settings have not changed. Check the pressure gauge and valves, then note the code and the model name before you call.
