Halstead 4 Fault Code: Causes, Fixes & Repair Costs

Common causes

• Faulty or worn flue gas sensor Common

  The flue gas sensor is an NTC thermistor that feeds temperature data back to the PCB. On an ageing Halstead boiler these sensors deteriorate over time, causing them to report inflated temperatures even when the flue is not actually overheating. This is the most commonly identified root cause — several owners have resolved code 4 simply by replacing the flue sensor, which is a relatively straightforward swap for an engineer once the sensor is confirmed faulty by resistance testing.

• Scaled or partially blocked heat exchanger Common

  Limescale build-up inside the primary heat exchanger restricts water flow, causing heat to accumulate rather than transfer efficiently. This can drive flue gas temperatures above the 95°C trip point as excess heat escapes up the flue rather than into the heating circuit. Hard water areas accelerate this problem, and on an older Halstead boiler that has never been chemically cleaned, scaling is a realistic culprit.

• Blocked or restricted flue Sometimes

  Any obstruction in the flue — debris, a bird's nest, or a condensate build-up in a poorly pitched section — prevents combustion gases from venting freely. Back-pressure causes gases to linger and heat up, pushing the sensor reading above 95°C. You may notice the boiler briefly igniting before cutting out, which can look similar to an air pressure switch fault.

• PCB fault causing false sensor reading Rare

  The printed circuit board processes all sensor signals. A failing PCB can misinterpret or fabricate a high-temperature reading from the flue sensor circuit, triggering code 4 even when the flue is operating normally. This is regarded as a last-resort diagnosis and should only be reached after all sensors and the heat exchanger have been physically tested and ruled out. The PCB was notably the most common underlying fault on Ace High models in particular.

How to fix it

1. Reset the boiler DIY safe

   Move the boiler's mode switch to the RESET/STANDBY position, hold it there briefly, then return it to ON within two seconds. This is the correct reset procedure for Halstead Ace HE models. If the boiler fires up and runs normally, monitor it over the next few hours. If fault code 4 returns immediately or after a short time, do not keep resetting — further resets will not cure an underlying fault and you should move to the next step.

2. Check that your gas supply is working DIY safe

   Confirm that other gas appliances in your home (hob, gas fire) are working normally. If they are not, contact your gas supplier rather than the boiler engineer, as the issue lies with the supply rather than the boiler itself.

3. Visually inspect the external flue terminal DIY safe

   From outside your property, take a look at where the flue exits the building. Check for any obvious blockages such as leaves, a bird's nest, or ice. Do not attempt to dismantle any flue components — simply note what you can see and report it to your engineer. Never block or cover a flue terminal.

4. Engineer tests the flue gas sensor Gas Safe engineer

   A Gas Safe engineer should trace the sensor circuit from the PCB using a voltmeter and check the sensor's resistance against manufacturer specifications. If the readings are out of range, the flue sensor should be replaced. Owners who have had this repair done report it is relatively straightforward once the correct sensor is sourced — Halstead parts are available through specialist stockists such as PHC Parts, though lead times can be longer than for current-production boilers.

5. Engineer inspects and cleans or replaces the heat exchanger Gas Safe engineer

   If the sensor tests within tolerance, the engineer should assess the heat exchanger for scale. A chemical descale may restore adequate flow, but if the exchanger is heavily scaled or damaged, replacement is the more reliable long-term solution. New seals should always be fitted alongside a replacement plate heat exchanger.

6. Engineer inspects the flue system for blockages or installation issues Gas Safe engineer

   The engineer should check the full flue run for obstructions, incorrect pitch (which can trap condensate), and any joints or sections that may have deteriorated over the boiler's lifetime. Hoses leading to the air pressure switch should also be checked for kinks or perforations at the same time.

7. Engineer considers PCB replacement as a last resort Gas Safe engineer

   Only after all sensors, the heat exchanger, and the flue have been assessed and cleared should the PCB be considered. The engineer should confirm the diagnosis by voltage testing at the board before committing to replacement. Given the age of Halstead boilers and the cost of a PCB, a like-for-like replacement should be weighed carefully against the cost of a new boiler — see the FAQ below for more on this.

8. Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer

   If a reset has not resolved the fault, you need a qualified Gas Safe engineer to diagnose and repair the boiler. Working on gas appliances without Gas Safe registration is illegal in the UK. When booking, mention the boiler make and model (e.g. Halstead Ace HE 24) and the fault code, so the engineer can bring likely replacement parts.

Parts you may need

• Halstead flue gas sensor (NTC thermistor) · from £35
• Halstead flow/return water temperature sensors (pair) · from £45
• Primary plate heat exchanger · from £120
• Halstead PCB (printed circuit board) · from £175

The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.

Typical repair cost

Expect to pay roughly £120–£350, depending on the underlying cause.

Frequently asked questions