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Nefit Boiler Error Code 4P 221

Call a professionalReviewed November 2026Checked against manufacturer documentation
What this code means
4P 221: The contacts of the safety temperature sensor are interrupted

Code 4P 221 means the boiler can no longer electrically see the safety sensor. Without that safeguard the burner is not allowed to run. The cause is a loose connection, a broken wire or a faulty sensor; an installer measures that.

Also shown as: 4P

Applies to: Nefit ProLine and TrendLine condensing boilers

Fix level
Call a professional
The checks on this page are preliminary. The likely fix involves a part, wiring or the control board.
Time to run the checks
About 15–30 minutes
For the 3 checks on this page — not a full repair.
You may need
No special tools
The checks below use basic household items only.
Parts that may be involved
Safety temperature sensor
Parts are model-specific; see the note below before ordering.

Common causes

  1. Broken wire or loose plug at the safety sensor
  2. Safety sensor faulty
  3. Damaged sensor cable

What to check, in order

Safety first. A boiler works with gas, electricity and hot water under pressure. Never open the casing or touch anything inside it. In the Netherlands, work on a gas boiler may only be done by a certified company (CO-vrij). The checks on this page stay on the outside: water pressure, thermostat, radiators and the reset button. If you smell gas, leave the building and call the national gas emergency number 0800-9009 (Netherlands).
  1. Note the full code and reset the boiler once.
  2. Only check the water pressure; do not open the boiler.
  3. If the code returns, call a CO-certified installer; the sensor must be measured and usually replaced.

Clear the code and test

  1. Fix what the checks above found first — pressure topped up, a radiator bled, the thermostat set right. Resetting without fixing the cause only brings the code back.
  2. Make sure the boiler has power and the gas tap at the boiler is open.
  3. If the water pressure is below 1.5 bar, top it up to between 1.5 and 2 bar with the filling loop, as your manual shows.
  4. Press the reset button once and watch a full cycle: the boiler should start, heat the radiators or the tap water, and keep running without the code coming back.

When to call a professional

This code usually points to a faulty part, wiring or the control board on a Nefit boiler. Run the checks above to rule out the simple causes, then book a technician — and book one straight away if any of these apply:

  • The code comes back after you have run the checks and a test cycle.
  • You smell burning, see sparks or scorch marks, or the breaker trips.
  • Water is leaking and you cannot stop it.
  • The fix needs panels opened beyond what your manual describes, or a part tested with a meter, and you are not set up for that.

When you call, give the brand, the code 4P 221, your model number (on a label on the type plate, usually at the bottom or on the side of the boiler) and the checks you already ran. It shortens the visit.

Repair or replace?

This fault is in professional territory: the fix usually means a major part or board-level work, and the labour adds up. If your machine is already well into its service life, get the repair quote first, then compare it with the price of a current equivalent model before you decide. Paying a large share of a new machine's price to repair an old one is rarely the better deal; replacing a machine over one cheap fault is wasteful too.

Parts that may be involved

Safety temperature sensor

Parts are model-specific. Find your model number first — usually on a label on the type plate, usually at the bottom or on the side of the boiler — and match the part to that number before ordering.

Questions people ask about 4P 221

What does 4P 221 mean on a Nefit boiler?

4P 221 means: The contacts of the safety temperature sensor are interrupted. Code 4P 221 means the boiler can no longer electrically see the safety sensor. Without that safeguard the burner is not allowed to run. The cause is a loose connection, a broken wire or a faulty sensor; an installer measures that.

Can I fix Nefit 4P 221 myself?

Only the preliminary checks. Code 4P 221 usually means a part, wiring or the control board has failed, which is a technician job. Run the checks to rule out simple causes before you book.

How do I clear Nefit 4P 221?

Fix what the checks above found first — pressure topped up, a radiator bled, the thermostat set right. Resetting without fixing the cause only brings the code back. Make sure the boiler has power and the gas tap at the boiler is open. If the water pressure is below 1.5 bar, top it up to between 1.5 and 2 bar with the filling loop, as your manual shows.

Sources checked

The meaning and checks for this code were checked against the sources above in November 2026. Codes vary by model and region; if your manual lists a different meaning for your exact model, follow the manual.

Other Nefit boiler codes

Nefit 0Y 276The flow temperature sensor has measured a temperature above 95 °CNefit 0Y 277The safety temperature sensor has measured a temperature above 95 °CNefit 0Y 285The return temperature sensor has measured a temperature above 95 °CNefit 2C 348During hot water operation the flow temperature is above 85 °CNefit 2E 207The heating water pressure is too low (below 0.2 bar)Nefit 2F 260The flow temperature sensor measures no temperature rise after a burner startNefit 2F 271The temperature difference between the flow and safety temperature sensors is too largeNefit 2F 345The flow temperature sensor measures no temperature rise after a burner startNefit 2L 266The pump test has failed: the pressure sensor sees the pump build up no pressureNefit 2L 329The pressure sensor measures no water flowNefit 2P 212The temperature measured by the flow or safety temperature sensor rises too quicklyNefit 2U 213The measured temperature difference between the flow and return temperature sensors is too largeNefit 2U 349The temperature difference between the flow and return sensors measured at low load is too largeNefit 2Y 281The pump is stuck or is running in airNefit 2Y 282The control signal to the pump is missingNefit 3A 264The fan control signal or supply voltage dropped out during operationNefit 3C 217The fan runs irregularly during start-upNefit 3L 214The fan does not run during the start-up phaseNefit 3P 216The fan runs too slowlyNefit 3Y 215The fan runs too fastNefit 4A 218The flow temperature sensor has measured a temperature above 105 °CNefit 4A 332The flow temperature sensor has measured a temperature above 105 °CNefit 4C 224An appliance thermostat (maximum or burner thermostat) has measured too high a temperature and is openNefit 4E 278The boiler's sensor test has failedNefit 4E 347The return temperature sensor has measured a higher heating water temperature than the flow temperature sensor; a restart follows after 10 minutesNefit 4E 375The contacts of the external sensor (for example a solar sensor) are short-circuitedNefit 4E 376The contacts of the external sensor (for example a solar sensor) are interruptedNefit 4F 219The safety temperature sensor has measured a temperature above 105 °CNefit 4L 220The contacts of the safety temperature sensor are short-circuited, or the sensor has measured a temperature above 130 °CNefit 4U 222The contacts of the flow temperature sensor are short-circuitedNefit 4Y 223The contacts of the flow temperature sensor are interruptedNefit 6A 227Insufficient ionisation current was measured after the burner ignited: the boiler does not see a proper flameNefit 6C 306An ionisation current was measured after the burner had gone out: the boiler still sees a flame that should not be thereNefit 6L 229Insufficient ionisation current was measured while burning: the flame drops out or is not seen properlyNefit 7C 231The mains voltage was interrupted during a locking faultNefit 7H 328There has been a brief interruption of the mains voltageNefit 7L 261The burner control unit is faultyNefit 7L 280The burner control unit is faultyNefit 8U 364The leak test carried out on the gas valve has failedNefit 8U 365The leak test carried out on the gas valve has failedNefit 8Y 232The external switching contact is openNefit 9A 235The KIM is too new for the burner control unitNefit 9H 237The burner control unit or the KIM is faultyNefit 9H 272The burner control unit or the KIM is faultyNefit 9L 234The contacts of the gas control block are interruptedNefit 9L 238The burner control unit or the KIM is faultyNefit 9P 239The burner control unit or the KIM is faultyNefit 9U 233The burner control unit or the KIM is faultyNefit C0 288The water pressure is too high, or the contacts of the pressure sensor are interruptedNefit C0 289The contacts of the pressure sensor are short-circuitedNefit CA 286The return temperature sensor has measured a return temperature above 105 °CNefit CU 240The contacts of the return temperature sensor are short-circuitedNefit CY 241The contacts of the return temperature sensor are interruptedNefit E1 242The burner control unit or the KIM is faultyNefit E1 243The burner control unit or the KIM is faultyNefit E1 244The burner control unit or the KIM is faultyNefit E1 245The burner control unit or the KIM is faultyNefit E1 247The burner control unit or the KIM is faultyNefit E1 248The burner control unit or the KIM is faultyNefit E1 249The burner control unit or the KIM is faultyNefit E1 255The burner control unit or the KIM is faultyNefit E1 257The burner control unit or the KIM is faultyNefit EA 246The burner control unit or the KIM is faultyNefit EA 252The burner control unit or the KIM is faultyNefit EA 253The burner control unit or the KIM is faultyNefit EC 251The burner control unit or the KIM is faultyNefit EC 256The burner control unit or the KIM is faultyNefit EF 254The burner control unit or the KIM is faultyNefit EH 250The burner control unit or the KIM is faultyNefit EH 258The burner control unit or the KIM is faultyNefit EH 262The burner control unit or the KIM is faultyNefit EL 259The burner control unit or the KIM is faultyNefit EL 279The burner control unit or the KIM is faultyNefit EL 290The burner control unit or the KIM is faultyNefit EY 263The burner control unit or the KIM is faultyNefit H07The measured heating water pressure is too low; the output for heating and hot water operation is limitedNefit H11The hot water outlet temperature sensor is faulty; its function is taken over by the boiler softwareNefit H13The set maintenance period has expired; maintenance is dueNefit H25The contacts of the pressure sensor are interrupted

See all Nefit boiler error codes