Intergas Fault Code 5: Weak Flame Signal Explained
Intergas fault code 5 tells you that the boiler's flame sensor is picking up a signal that is weaker than expected — not necessarily no flame at all, but one that isn't strong enough to satisfy the boiler's safety parameters. As a precaution, the boiler locks itself out to prevent the risk of incomplete combustion or harmful gases building up. The underlying cause can range from a straightforward gas supply issue through to a faulty internal component, so a Gas Safe registered engineer will need to diagnose and resolve the fault before the boiler can return to normal operation.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Intergas 5 fault code mean?
Intergas fault code 5 tells you that the boiler's flame sensor is picking up a signal that is weaker than expected — not necessarily no flame at all, but one that isn't strong enough to satisfy the boiler's safety parameters. As a precaution, the boiler locks itself out to prevent the risk of incomplete combustion or harmful gases building up. The underlying cause can range from a straightforward gas supply issue through to a faulty internal component, so a Gas Safe registered engineer will need to diagnose and resolve the fault before the boiler can return to normal operation.
Common causes
- Stuck or restricted gas valve Common
The gas valve controls how much gas reaches the burner. Because it has moving parts, it can seize or become sluggish over time, preventing it from opening fully. A partially open valve delivers less gas than required, which is a textbook cause of a weak flame signal. This is the most frequently identified culprit when engineers investigate fault code 5 on Intergas boilers.
- Incorrect gas supply pressure Common
Both low and high gas pressure at the boiler can result in a flame that is too weak or unstable for the sensor to read correctly. Low pressure starves the burner of fuel, while high pressure can disrupt the flame pattern. The engineer will check the gas meter, the incoming supply pressure, and the run of pipework between the meter and the boiler for any restrictions or debris.
- Blocked pilot jet or burner ports Sometimes
The pilot jet is a very small component within the burner assembly, and even a modest accumulation of debris can partially obstruct it. This restricts gas flow precisely where it needs to be strongest, producing a weak or unstable flame. Older burners can also accumulate general debris across the ports, which compounds the problem.
- Faulty flame sensor (ionisation electrode) Sometimes
The flame sensor monitors the quality of combustion by detecting the electrical current conducted through the flame. If the sensor itself is dirty, corroded, or degraded, it may underreport a flame that is actually burning adequately. An engineer can test the sensor's output with a meter and replace it if necessary.
- Frozen or blocked condensate pipe Sometimes
Although a condensate blockage more commonly triggers its own dedicated fault code, a severely blocked condensate pipe can affect the boiler's ability to vent combustion gases properly, which in turn influences the quality of the flame. In cold weather, always check whether the condensate pipe is frozen before assuming a more serious fault.
- Faulty PCB (printed circuit board) Rare
The PCB processes the signal from the flame sensor and controls the gas valve. If it is malfunctioning, it may misread a perfectly adequate flame signal or fail to open the gas valve correctly. PCB faults tend to be diagnosed by elimination once other components have been ruled out, as they are relatively expensive to replace.
How to fix it
- Check that your gas supply is working DIY safe
Try lighting a gas hob or checking whether other gas appliances in the property are working normally. If there is no gas at all, contact your gas supplier — the problem may be external to the boiler and no engineer visit will resolve it until supply is restored.
- Check for a frozen condensate pipe DIY safe
In cold weather, locate the white plastic condensate pipe that runs outside (usually along an external wall and into a drain). If it feels solid or you can hear gurgling from the boiler, it is likely frozen. You can carefully thaw it by pouring warm — not boiling — water along its length. A proprietary pipe-thawing product also works well. Once clear, reset the boiler.
- Reset the boiler once or twice DIY safe
Press and hold the reset button on the boiler's control panel as described in your user manual. Allow the boiler a full ignition cycle before deciding whether the fault has cleared. If fault code 5 returns after one or two resets, stop — repeated resetting without fixing the underlying cause is not safe and will not resolve the fault.
- Check system pressure is within range DIY safe
Look at the pressure gauge on the boiler (or the app if your model supports it). Intergas boilers typically run between 1 and 1.5 bar when cold. If it is below 0.8 bar, top it up using the filling loop as described in your user manual. Note that low system pressure alone is unlikely to cause fault code 5 specifically, but it is worth ruling out.
- Arrange a visit from a Gas Safe registered engineer Gas Safe engineer
If the fault persists after the homeowner checks above, a qualified engineer must take over. They will carry out a gas pressure test at the boiler, inspect and test the gas valve, check the pilot jet and burner for blockages, measure the flame sensor's ionisation current, and test the PCB if required. Do not attempt to access internal gas components yourself.
Parts you may need
- Gas valve · from £150
- Flame sensor / ionisation electrode · from £30
- Pilot jet · from £15
- Burner assembly · from £80
- PCB (printed circuit board) · from £220
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–£380, depending on the underlying cause.
Frequently asked questions
Can I reset an Intergas fault code 5 myself to get the heating back on?
You can try resetting the boiler once or twice — this is safe for a homeowner to do and occasionally clears a transient fault. However, fault code 5 indicates a genuine problem with the flame signal, so if the lockout returns after a reset or two, stop resetting and call a Gas Safe engineer. Repeatedly resetting a boiler with an unresolved fault is not recommended.
How much does it cost to fix an Intergas fault code 5?
For most people, the repair falls somewhere between £120 and £380, depending on what the engineer finds. A simple fix such as cleaning a blocked pilot jet or adjusting gas pressure at the lower end; replacing a gas valve — parts plus labour — typically comes to around £300. A PCB replacement is less common but can cost £400–£500 including fitting. These figures assume a standard domestic call-out; prices vary by region and engineer.
Could a frozen condensate pipe cause fault code 5 on an Intergas boiler?
It is possible but not the most likely cause. A frozen condensate pipe more often triggers a different fault code on Intergas boilers, but if back-pressure from a blocked pipe affects combustion, a weak flame signal can follow. In cold weather it is always worth checking the condensate pipe first, as thawing it is something a homeowner can do safely and at no cost.
What is the most common repair needed for Intergas fault code 5?
In most cases an engineer will focus on the gas valve first, as it is the component most prone to sticking or restricting gas flow on Intergas boilers. If the valve can be freed and cleaned rather than replaced, the repair cost is relatively modest. A blocked pilot jet or dirty flame sensor are the next most frequent findings and are both straightforward and inexpensive to rectify.