Short answer
The Saunier Duval Isofast F28 E1 It is a high-flow mixed boiler that, unlike the more recent condensing models, displays its faults with two-digit numerical codes (02, 05, 09…) on the display. In this guide, you have the meaning of each code, why it appears, and how to act, as well as the exact spare part when the fault corresponds to a specific component. Some checks, such as replenishing the water pressure, you can safely do yourself.
Error code table for the Isofast F28 E1
Quick summary of all fault codes. Below you have the details of each one with its cause and solution.
| Code | Meaning |
|---|---|
| 02 | Safety shutdown due to lack of air (sealed boiler) |
| 04 | Ignition fault: the boiler does not ignite |
| 05 | Safety shutdown due to overheating |
| 06 | Fault of the NTC2 heating sensor |
| 07 | Fault of the domestic hot water (DHW) NTC1 sensor |
| 08 | Fault of the NTC3 sensor |
| 09 | Water pressure sensor fault |
| 12 | Main PCB fault |
| 13 | Temperature limiter fault |
| 14 | Stepper motor fault of the gas valve |
| 15 | Gas leak through the EVS valve |
| 16 | Flame detection fault |
| 17 | Communication fault / low supply voltage |
| 20 | Communication fault with the PCB |
What can you check yourself?
Some alerts can be resolved with simple and safe checks: replenishing the water pressure, bleeding the air from the radiators, checking that the gas valve is open, or resetting the boiler after a temporary lockout. If the code reappears after doing this, there is an underlying cause that should be reviewed by a qualified technician. Do not manipulate internal gas components nor electronics.
How to replenish the pressure step by step
- Locate the filling valve under the boiler.
- Open it slowly while observing the pressure gauge.
- Close the valve upon arrival at between 1 and 1.5 bar in cold.
- Bleed the radiators starting with the furthest one and check the pressure again.
- Reset the boiler; if the pressure warning disappears, the problem was resolved.
All fault codes and their solution
02 — Safety shutdown due to lack of air (sealed boiler)
Why does it appear: The air differential pressure switch does not detect the correct flow: weak or faulty fan, blocked or disconnected pressure tubes, obstructed flue duct, or defective pressure switch.
What to do: Check that the flue outlet is not obstructed. The inspection of the fan, pressure tubes, and pressure switch should be carried out by a technician.
Related spare part: Fan / extractor
04 — Ignition fault: the boiler does not ignite
Why does it appear: Lack of gas or closed gas valve, dirty or mispositioned ignition electrode, defective gas valve, or insufficient ignition power.
What to do: Check that the gas valve is open and that there is supply, and reset the boiler. If the lockout repeats, a technician should inspect the electrodes and gas valve.
Related spare part: Gas valve
05 — Safety shutdown due to overheating
Why does it appear: A reset safety thermostat detects an excessive temperature in the circuit: lack of water circulation, blocked pump, air in the system, or low pressure.
What to do: Check the pressure and bleed the radiators. If it trips again, the circulation pump needs to be checked and the safety thermostat reset (technician).
Related spare part: Circulation pump
06 — Fault of the NTC2 heating sensor
Why does it appear: Heating temperature sensor in short circuit or open circuit, or with poor connection.
What to do: Check the wiring and, if necessary, replace the heating NTC sensor.
Affected component: Temperature sensor
07 — Fault of the NTC1 sensor for domestic hot water
Why does it appear: Domestic hot water temperature sensor faulty or poorly connected.
What to do: Check the wiring and replace the DHW NTC sensor if it is faulty.
Affected component: Temperature sensor
08 — Fault of the NTC3 sensor
Why does it appear: Third temperature sensor (according to configuration) in short circuit or open circuit.
What to do: Check the sensor wiring and replace it if necessary.
Affected component: Temperature sensor
09 — Water pressure sensor fault
Why does it appear: Incorrect circuit pressure, loose or faulty pressure sensor wiring, or excess water in the installation.
What to do: Replenish the pressure to 1-1.5 bar when cold using the filling key. If the warning persists, the water pressure sensor needs to be checked.
Related spare part: Pressure sensor / filling key
12 — Main PCB fault
Why does it appear: Internal fault of the PCB.
What to do: A technician must diagnose and, if necessary, replace the PCB.
Related spare part: PCB (control board)
13 — Temperature limiter fault
Why does it appear: Operation or fault of the temperature limiter, possibly due to a failure of the NTC2 sensor or lack of circulation.
What to do: Check the circulation and the heating sensor. The limiter inspection should be carried out by a technician.
Affected component: Temperature sensor
14 — Stepper motor fault of the gas valve
Why does it appear: Fault in the stepper motor that regulates the gas or its wiring.
What to do: Requires technical inspection of the gas mechanism and its regulation motor.
Related spare part: Gas valve
15 — Gas leak through the EVS valve
Why does it appear: The gas safety solenoid valve does not close properly, faulty connection or valve motor.
What to do: It is a gas circuit safety warning. The gas valve must be checked by a qualified technician immediately.
Related spare part: Gas valve
16 — Flame detection fault
Why does it appear: The ionisation electrode does not detect flame, faulty safety solenoid valve or burner, or poor combustion.
What to do: A technician must check the ionisation electrode, the burner, and the safety solenoid valve.
Related spare part: Ignition/Ionisation Electrode
17 — Communication fault / low supply voltage
Why does it appear: Communication fault with the PCB or mains voltage below the minimum operating level (175 V).
What to do: Check the mains voltage. If it is correct, a technician should inspect the PCB and communication wiring.
Affected component: Electrical installation
20 — Communication fault with the PCB
Why does it appear: Internal communication error of the control electronics.
What to do: A technician must check the PCB and its connections.
Related spare part: PCB (control board)
When to call a technician
Fault codes related to sensors, electronics, gas valve, fan, or flue gas evacuation require a qualified technician with the appropriate equipment. If the same fault code reappears after a reset, or if the pressure drops repeatedly, a professional inspection is advisable. In Uniclima do you find spare parts for this boiler.
Frequently Asked Questions
The 05 is a safety shutdown due to overheating. It is usually caused by a lack of water circulation (blocked pump, air in the circuit, or low pressure). Check the pressure and bleed the radiators; if it repeats, a technician should inspect it.
Indicates a safety shutdown due to lack of air in the sealed boiler: the pressure switch does not detect the correct air flow. Common causes are a weak fan, blocked pressure tubes, or a clogged flue.
No. The Isofast F28 E1 uses the old system of two-digit numerical codes (02, 04, 05…), different from the F.xx system of the more modern Saunier Duval condensing boilers.
Yes, it is a gas circuit safety warning and should not be ignored. The gas valve must be checked by a qualified technician as soon as possible; do not attempt to tamper with it.