Main (Baxi) Boiler Error Codes: Common Faults & Reset Procedures

Quick Answer: Main boilers (Main Eco Compact, Main Eco Elite, Main Combi) are manufactured by BDR Thermea UK — the same group as Baxi and Potterton. Fault codes are shared across the three brands: E133 (ignition failure), E119 (low pressure), E125 (flue/APS fault), E128 (fan fault), and E168 (internal fault). Reset: hold the reset button for 3 seconds. Parts and diagnostics are interchangeable with Baxi equivalents.

Summary

Main is a UK boiler brand manufactured by BDR Thermea UK, the company that also produces Baxi and Potterton boilers. The Main Eco Compact and Eco Elite ranges are compact combi boilers designed primarily for properties with limited space — smaller than equivalent Baxi models while using the same internal components and control systems.

Because Main, Baxi, and Potterton share the BDR Thermea platform, the fault code system is essentially identical across all three brands. An engineer who knows Baxi diagnostics will find Main boilers immediately familiar. Spare parts are cross-referenced between all three brands and are widely stocked by UK plumbing merchants.

The Main Eco Compact is particularly common in London flats and smaller properties where space is at a premium — the boiler is designed to fit inside a kitchen cupboard. This installation location affects maintenance access and makes it particularly important to ensure adequate air circulation around the boiler and easy access to the condensate trap.

Key Facts

Fault Code Reference Table

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Code Fault Description Common Causes Diagnosis Steps Typical Fix
E133 Ignition failure / no flame Electrodes, gas supply, condensate freeze, gas valve Check gas; inspect electrodes; check condensate Clean electrodes; clear condensate; check gas valve
E119 Low water pressure System leak; expansion vessel failure Check pressure gauge; inspect for leaks Repressurise to 1.0–1.5 bar; fix leak
E125 Flue / air pressure switch (APS) Condensate blockage; flue obstruction; APS failure Clear condensate; check flue terminal Clear blockage; replace APS
E128 Fan circuit fault Fan bearing; PCB fan control; wiring Check fan operation; listen for noise Replace fan; check wiring
E131 Main board pressure switch fault APS, condensate, flue As E125 As E125
E160 Primary circuit overheat Poor circulation; pump failure; closed TRVs Check pump; check all valves open Fix circulation; replace pump
E168 PCB / control board fault Board failure; wiring loom Power cycle; check connectors Replace PCB
E167 Flow temperature sensor Sensor wiring; sensor failure Check resistance (NTC: ~10kΩ at 25°C) Replace sensor
E166 Return temperature sensor As E167 As E167 Replace return sensor
E32 Thermal fuse blown Severe overheat event Check fuse continuity; investigate overheat cause Replace thermal fuse; fix root cause
E00 Memory fault (some models) Power supply issue; PCB Power cycle Replace PCB if persistent

Detailed Guidance

Reset Procedure

Main Eco Compact and Eco Elite:

  1. The fault code appears in the digital display window on the facia.
  2. Locate the reset button — on Main boilers it is typically clearly labelled on the right side of the controls panel.
  3. Press and hold for approximately 3 seconds until the display changes and the boiler attempts ignition.
  4. If the boiler restarts and runs normally, monitor for recurrence.

For repeated lockouts, do not reset more than twice before investigating — repeated resets without resolution indicate the fault has not cleared.

E133 — Ignition Failure (Main Boilers)

E133 on Main boilers is diagnostically identical to E133 on Baxi and Potterton boilers. The key steps:

MAIN BOILER E133
     |
     v
Gas supply check
- Other gas appliances working?
- PAYG meter topped up?
     |
     v
Cold weather: condensate pipe?
- Main Eco Compact fitted in kitchen cupboards often have condensate
  running to an internal drain -- less freeze risk than external pipe
- But check if the condensate trap inside the boiler is blocked
- Access the trap (typically at the base of the boiler, behind the
  bottom panel): remove, clean, and refit
     |
     v
Electrode inspection:
- Access via front panel (isolate if required)
- Ionisation probe: clean if contaminated
- Spark electrode: check gap (3–4mm), check HT lead
     |
     v
Reset and test
     |
     v
Still E133?
-- Gas Safe engineer: gas pressure, gas valve, combustion analysis

E119 — Low Pressure (Main Compact specifics)

The Main Eco Compact's filling loop is inside the boiler casing, at the base. Access by opening the front bottom panel. The loop has two black-handled isolation valves. The procedure is the same as all sealed system boilers — open valves, fill to 1.0–1.5 bar, close valves.

Compact installation expansion vessel: In the Main Eco Compact, the expansion vessel is internal and accessible when the front panel is removed. The Schrader valve is typically on the right side of the vessel. Test with system at 0 bar pressure.

E125 — APS Fault (Main Compact condensate specifics)

The Main Eco Compact is particularly susceptible to E125 caused by condensate trap blockage. The compact design places the condensate trap in a location where access is more restricted than in standard-sized boilers. When attending for E125 on a Main Compact:

  1. Remove the lower front panel.
  2. Locate the condensate trap (a plastic U-tube or trap assembly, typically transparent or translucent).
  3. If the trap is filled with debris, algae, or staining, it is likely blocked.
  4. Remove the trap (it usually clips or screws off). Clean under running water with a small brush.
  5. Refit and reset.

E160 — Overheat

Main Eco Compact overheat diagnosis focuses on three areas:

For flats with all-TRV systems and a Main Eco Compact, ensure at least one radiator in the property does not have a TRV — a towel rail or hallway radiator is ideal. This ensures there is always a circulation path even when the room thermostat is satisfied.

Frequently Asked Questions

Main vs Baxi — should I prefer one over the other?

They are the same boiler internally. The Main Eco Compact and Eco Elite were designed for specific market segments (small properties, space-constrained installations). If size is not a constraint, a standard Baxi or Potterton with the same output is a direct equivalent. Spare parts prices are identical.

My Main Eco Compact is making a clicking sound and showing E133 — what is this?

Clicking during an E133 lockout is the ignition spark firing repeatedly as the boiler tries to light. This is normal during an ignition attempt. If clicking stops but E133 remains, the spark circuit itself has stopped working (check the ignition electrode and HT lead). If clicking continues but no flame appears, the gas valve is not opening or gas supply is absent.

How do I find the service manual for my Main boiler?

Visit www.main-boilers.co.uk and enter your boiler model number. Alternatively, call the BDR Thermea technical helpline: 0344 871 1525. The installation and servicing manual for your specific model contains the full fault code list and diagnostic flowcharts.

The Main Eco Compact is installed in a cupboard — does this affect servicing?

All boilers installed in cupboards require adequate ventilation as specified in the installation manual. For routine servicing, the front panel must be removable — typically the cupboard door provides enough access if the boiler is installed correctly. If the boiler was installed without adequate clearance and the engineer cannot safely access the front and top panels, the installation is non-compliant and may need to be addressed.

What is the typical lifespan of a Main Eco Compact?

With annual servicing, 12–15 years is typical for modern condensing combis. Beyond 15 years, heat exchanger deterioration, PCB age, and parts availability make replacement increasingly cost-effective compared to repair.

Regulations & Standards