Overview
E10 on a Potterton Assure (and many Potterton/Promax/Promac models) indicates a fault with an external probe—commonly the outdoor temperature sensor or another externally mounted NTC probe. The boiler uses this probe for weather compensation or to monitor an external temperature; when the control detects a missing, shorted, open, or out-of-range signal it flags E10. Causes are usually a failed sensor, a broken or corroded cable, a loose or water-damaged connector, or occasionally a PCB/input-wiring fault. Severity is generally low-to-moderate. In most cases the boiler will either continue to run using default temperature control or revert to basic control mode, but you may lose weather compensation or see incorrect heating/DHW behaviour. If the fault causes the boiler to lock out repeatedly or other error codes appear, that increases urgency because it affects reliability and comfort. This is usually not an immediate safety emergency, but persistent faults should be fixed promptly to prevent boiler mis-control and further strain on components. Some basic checks are suitable for a competent homeowner (reset, visual inspection, check connector seating). Anything involving internal PCB work, gas components, or replacing internal wiring or the sensor inside the appliance should be done by a qualified, Gas Safe registered engineer. If you are unsure at any point, stop and call a professional.
Possible Cause: External probe fault
Troubleshooting Steps
Safety precautions:
- Before you start, turn off the boiler and isolate electrical power at the fused spur or consumer unit. Do not work on live equipment. If you are not comfortable isolating power, stop and call a pro.
- Do not attempt to work on or alter gas valves, burner assemblies, or sealed internal components. Those tasks require a Gas Safe registered engineer.
- Wear basic PPE (gloves, eye protection) and avoid working in wet conditions. If you need to access external wiring outdoors, ensure it is dry and safe.
Initial homeowner checks (safe, simple steps):
1. Note the exact code and any other indicators (lockout, flashing lights). Try a normal reset of the boiler using the reset button (hold 3–5 seconds) and observe whether E10 returns. If it clears and stays cleared, monitor for recurrence.
2. Locate the outdoor/external sensor physically (usually mounted on an outside wall, often near the boiler flue or in an attic/external junction box). Do a visual check: is the sensor intact? Is the cable damaged, chewed, or disconnected? Look for water ingress or corrosion in any external junction box.
3. Check the cable route back to the boiler: ensure the cable is not pinched, cut, or loose where it enters the boiler or terminal block. At the boiler, check that the connector labelled for the outdoor or external probe is fully seated. Make sure the boiler power is off while you move or reseat connectors.
Simple diagnostics for competent DIYers with a multimeter (only if you are comfortable and the boiler power is isolated):
4. With power isolated, open the boiler access cover only if you are confident and the cover is designed for user access (many do not require removing sealed parts). Identify the two-pin or three-pin terminals labelled OUTDOOR, EXT PROBE, or similar. Check the wiring is secure and free from corrosion.
5. If you can access the sensor at the outside end, disconnect it and measure resistance across its two wires with a multimeter. An NTC sensor should show a finite resistance that changes if you warm or cool the sensor slightly (for example, hold it in your hand and the resistance should change). If the meter reads open circuit (infinite / OL) or a short (near 0 ohms) or the value does not change at all with temperature, the sensor is likely failed.
6. If the sensor resistance looks OK but the boiler still shows E10, check continuity of the cable back to the boiler terminals. A broken cable will show open circuit. Also inspect connectors for corrosion or water damage.
If you find a clear fault (broken cable, corroded connector, open-circuit sensor):
7. For a damaged external sensor: it is reasonable to replace the external probe with the correct Potterton/approved replacement sensor. Replacements are normally simple to fit—power isolated, disconnect old sensor at both ends, connect new sensor, reseal any external box to keep water out. If you are not confident, let an engineer do it.
8. For wiring faults visible and local (e.g., a damaged external run between sensor and boiler): a competent electrician or heating engineer should repair or replace the cable. Ensure outdoor cable glands and seals are reinstated to prevent future ingress.
If checks above do not resolve E10 or you see other fault codes, intermittent behaviour, or lockouts:
9. Do not attempt internal PCB repairs, soldering, or gas-system work. Persistent errors after sensor/cable replacement, or faults that point to the boiler control board or internal wiring, require a Gas Safe registered Potterton-competent engineer.
10. When calling a professional, tell them: model, exact error code (E10), whether a reset was tried, what visual checks you performed, and whether you measured the sensor. This speeds diagnosis.
When to call a professional immediately:
- The boiler locks out repeatedly or will not operate after simple checks and reset.
- You find signs of water ingress into connectors or the boiler wiring area.
- You are unsure how to safely isolate power or test the sensor.
- Any work would involve internal wiring, the gas supply, burner, PCB, or altering sealed parts.
Summary note: E10 usually points to a failed or disconnected external/outdoor probe or its wiring and can often be solved by checking connectors, replacing a damaged probe, or repairing the cable. However, replacing sensors inside the appliance, repairing internal wiring, or any gas-related work must be carried out by a qualified Gas Safe engineer—call a professional if the fault persists or if you are unsure.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Assure Combi and system.