Overview
E10 on Potterton Promax Combi / System / Store boilers indicates a fault with the outdoor sensor (external probe). The outdoor sensor reports the outside temperature to the boiler for weather-compensated control of the heating. When the boiler detects a fault with that sensor (open circuit, short, or out-of-range reading), it logs E10 so the control system knows the external temperature reading is invalid. This fault is typically low-to-medium severity. In many installations the boiler will continue to operate using internal controls but weather compensation will be lost (heating may run at incorrect temperatures). Some models or specific fault conditions can cause the boiler to lock out, preventing normal operation until the fault is cleared. Common causes are a physically damaged probe, water ingress/corrosion, rodent damage to the cable, a disconnected or loose connector, or a wiring/PCB fault. Basic checks can be done by a competent homeowner, but any work involving internal wiring, opening the boiler casing, or replacing components should be carried out by a qualified Gas Safe registered engineer or a competent electrician.
Possible Cause: Outdoor sensor error
Troubleshooting Steps
Safety precautions:
- If you are not confident working around electrical equipment or boilers, stop and call a qualified engineer. Gas and mains electricity are hazardous.
- Before doing any internal checks isolate the boiler from mains power at the external isolator switch and, where applicable, turn off the gas supply. Do not open the boiler casing while the unit is live.
- Never short out or bypass sensors or safety devices to make the boiler run.
- If you smell gas, leave the building immediately and call the gas emergency number. Do not attempt any checks.
Initial homeowner checks you can do (no tools or minimal tools):
1. Note the exact error code and whether the boiler has locked out. Take a photo of the display and the boiler label (model/serial) to give to an engineer.
2. Try a simple reset first: use the boiler reset/button or on-panel reset — hold for 3–5 seconds as per the manual. If the E10 clears and does not return, monitor for a while. If it returns immediately or repeatedly, continue with checks below.
3. Locate the outdoor sensor on the outside wall or where the probe is fitted. Inspect visually for obvious damage: broken or ripped cable, crushed probe, signs of water ingress, paint sealed over the head, or rodent chew damage. Ensure the sensor is not covered by insulation or debris and is mounted as the manufacturer intended.
4. Check external wiring run for obvious damage or disconnection where it enters the building. If the cable is visibly disconnected at an external junction, note that and consider calling an engineer — do not rewire gas or mains while live.
Basic diagnostic steps (for people comfortable with simple electrical checks):
1. Isolate electrical power to the boiler at the isolator before touching terminals or removing covers.
2. If you can access the outdoor sensor connector on the boiler's terminal block without removing components, check that the probe connector is fully seated and the terminal screws are not loose or corroded.
3. If you have a multimeter and are competent using it: with the sensor disconnected from the boiler, measure the sensor resistance at room temperature and then gently warm the sensor (hold in your hand or use warm water) and check that resistance changes. A functioning NTC thermistor will change resistance as temperature changes. If the resistance is open (infinite) or shorted (near zero) or does not change with temperature, the probe is faulty and needs replacement. Do not measure with the probe still connected to live circuitry.
4. If the sensor and cable appear intact but the fault persists, inspect the wiring continuity along the run (if accessible) for breaks or shorts. If the cable is internal and not visible, do not force access — call an engineer.
If the above steps do not clear the fault or you find a problem you cannot safely fix:
- Re-apply power and attempt one more reset. If E10 persists, stop and call a Gas Safe registered engineer. Provide them with the boiler model, serial number, the E10 code, what checks you have already done, and any photos you took.
- Do not attempt to replace the outdoor sensor or PCB yourself unless you are qualified. A competent engineer will correctly source the correct sensor type and fit it, test the system, and check for related wiring or PCB faults.
- If you found visible damaged cable outside, an electrician or heating engineer can repair or replace the cable/run and fit a new probe.
When to call a professional and what to expect:
- Call a Gas Safe registered engineer whenever the fault persists after a reset, if the boiler is locked out, if there are signs of internal wiring damage, or if you are not confident with the electrical checks described.
- The engineer will test the sensor with calibrated equipment, check wiring and terminal connections, and if required replace the outdoor probe or correct wiring faults. If the problem is on the PCB or other internal control, they will diagnose and advise the appropriate repair or replacement.
Additional tips:
- Do not bypass the sensor to make the boiler run; this can cause poor control and potentially unsafe operation.
- Keep records of faults, resets, and any physical findings to speed diagnosis when the engineer attends.
- If the outdoor sensor fault happens only in very cold weather and the probe is iced, thaw it gently and see if the error clears; persistent recurring faults still need professional attention.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Promax Combi, System, and Store.