A boiler monitors a set of sensors and safety devices, and a fault code reports which check did not pass. That narrows the cause to a handful of possibilities but rarely names the faulty component. Replacing parts on a code alone is how people pay twice.
What a code is actually reporting
A modern boiler’s control board runs a sequence and monitors a set of inputs throughout. At each stage it expects a particular condition: the fan to reach speed, the air pressure switch to close, the flame sensing electrode to detect a flame within a safety time, the flow and return thermistors to read plausible temperatures, the pressure sensor to be above minimum.
When one of those expectations is not met, the board stops and reports which check failed. That is genuinely useful information – it narrows the search enormously – and it is not a diagnosis.
A code saying ‘no flame detected’ is reporting exactly that. It could mean no gas, a blocked condensate trap backing up, a failed ignition electrode, a dirty flame sensing electrode, a gas valve fault, or a flue obstruction. All of those produce the same code, because from the board’s point of view the same thing happened.
Most boilers show the current fault and clear it on reset. Some keep a fault history and some do not. The code, written down exactly, is the single most useful thing you can tell an engineer, and it is lost the moment you press reset.
The common code families
| Family | What the boiler noticed | Common actual causes |
|---|---|---|
| Ignition lockout / no flame | It ran the sequence and did not prove a flame | Gas supply, blocked condensate trap, ignition or sensing electrode, gas valve, flue obstruction |
| Flame loss during operation | It lit, then lost the flame | Gas pressure under load, condensate blockage, flame sensing, or recirculation at the terminal |
| Overheat | A temperature exceeded the safe limit | Poor circulation – pump, airlock, sludge, closed valve – far more often than a faulty sensor |
| Low water pressure | System pressure below minimum | A leak, or a failed internal expansion vessel. Not a boiler fault as such |
| Fan / air pressure | The fan is not running, or the flue is obstructed | Fan, pressure switch, blocked flue or terminal, or condensate backing up |
| Flow or return sensor | A thermistor is reading implausibly | Sensor, wiring, or genuinely no flow |
| Condensate | It cannot drain | Frozen or blocked condensate pipe, or a blocked trap |
| Gas valve | The valve is not responding as expected | Valve, wiring, or the control board |
| Communication | A control or module is not responding | Wiring, a thermostat, or a module |
| PCB / internal | The board has detected an internal fault | Sometimes the board. Frequently something else the board is misreading |
Notice how often condensate appears in that table. A partially blocked condensate trap causes several completely different-looking codes, which is exactly why a code is a starting point rather than an answer.
The checks to do before calling
- Boiler pressure. Below the minimum, most boilers will not fire at all. If it needs topping up regularly, that is the actual fault.
- The condensate pipe, particularly in cold weather and particularly any external section. Frozen condensate is seasonal, extremely common, and frequently resolved in ten minutes – see frozen condensate pipes.
- Other gas appliances. If the hob does not work either, it is a supply problem rather than a boiler one. Check the meter, including prepayment credit.
- The programmer and thermostat. Flat batteries present exactly like a dead boiler, and schedules reset after power cuts.
- The electrical supply. Fused spur, RCD, boiler isolator.
- Whether the flue terminal is obstructed – snow, a nest, or something stacked against it outside.
- Reset once, maybe twice. If it locks out again, stop.
Why parts get replaced unnecessarily
The pattern is consistent and it is worth understanding as a customer.
An intermittent fault with a code that could have several causes gets diagnosed by substitution – replace the cheapest likely part, see if it recurs. If it does, replace the next. Eventually the control board gets replaced, because that is the thing you reach for when nothing else explains it.
Sometimes the board genuinely is the fault. Frequently the real cause was a partially blocked condensate trap, marginal gas pressure under load, or a sensor that only misreads when hot – none of which show up on a cold visit with a meter.
- Ask what evidence points at the part being replaced, rather than accepting the code as the diagnosis.
- Ask whether the condensate trap has been cleaned, because it is cheap and it explains a lot.
- Ask whether gas pressure was measured under full load, not just standing.
- Ask whether the fault is intermittent, and if so what would confirm the diagnosis rather than just possibly resolving it.
None of that is adversarial. A good engineer will welcome it, because those are the questions they are asking themselves.
Codes that are not really boiler faults
Several common codes report a condition outside the appliance:
- Low pressure means the system is losing water. The boiler is reporting it correctly. Find the leak, or check the expansion vessel – see expansion vessel failure.
- Overheat very often means poor circulation – a failing pump, an airlock, sludge, or a closed valve. The boiler is overheating because water is not moving through it, which is a system fault.
- Condensate means a pipe outside is frozen or blocked. Nothing in the boiler has failed.
- Flame loss under load can mean the gas supply cannot deliver the required rate – a pipe sizing problem – see gas supply and pipe sizing.
- Air pressure faults can mean an obstructed flue terminal rather than a failed switch.
In each case, replacing a boiler component would be replacing a healthy part and the fault would return.
Boiler showing a code? Write it down exactly before you reset, and check the pressure gauge and the condensate pipe.
Questions we get asked about this
What does my boiler fault code mean?
It tells you which internal check failed, which narrows the cause to a handful of possibilities. It does not name the faulty part. The manufacturer’s manual lists the codes for your specific appliance.
Should I keep resetting the boiler?
Once, maybe twice. If it locks out repeatedly there is a genuine fault and further resets achieve nothing. Note the code before resetting, because most boilers clear it.
The boiler says low pressure. Is that serious?
It means the system is losing water, and a sealed system should not. Either there is a leak, or the expansion vessel has failed and water is being pushed out of the relief valve each cycle. Check the outside discharge pipe first.
Can a frozen condensate pipe cause other fault codes?
Yes, and it is one of the most useful things to know. Condensate backing up produces ignition faults, flame loss and air pressure codes on different appliances – all of which look like something else entirely.
My engineer wants to replace the PCB. Should I agree?
Ask what evidence points at it, and whether the condensate trap has been cleaned and the gas pressure measured under full load. The board is sometimes genuinely the fault and it is also the part people reach for when nothing else explains it.
Why does the fault only happen sometimes?
Intermittent faults are usually marginal rather than failed – a sensor that misreads only when hot, gas pressure that is adequate standing and not under load, or a trap that is partly blocked. They are harder to diagnose and worth being patient about.
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