Baxi E.04.08 fault codewhat it means and how to fix it
"Maximum safe temperature value reached": the safety thermostat has opened on a Baxi 200/400 Combi or Combi 2. Circulation first, the thermostat second, in Baxi's own order. One reset, then an engineer.
Stop — this needs an engineer
Get a Gas Safe registered engineer to look at your Baxi E.04.08
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
Has it happened before?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
First time, and the boiler otherwise runs well
Possibly air after a top-up or a brief circulation stall. Baxi's reset is allowed once.
Check the pressure, bleed any cold-topped radiator, reset once (one second on a Combi 2, three on a 200/400). Mention it at the next service.
It has come back, or the display showed H.01.05, H.01.08 or H.01.14 beforehand
The circulation is failing and the thermostat is catching it. Pump, air or a fouled exchanger, in Baxi's order.
Stop resetting and book a Gas Safe registered engineer.
What E.04.08 means
E.04.08 is the overheat lockout on Baxi's two-part and dotted displays. The 200 and 400 Combi manual defines E04 08 as "Maximum safe temperature reached (Safety thermostat open-circuited)"; the Combi 2 manual has "Maximum safe temperature value reached" and Baxi's professional page the same. A thermostat on the heat exchanger has seen the water go past its limit and cut the burner, exactly as E110 does on the three-digit Baxis.
What the manuals do that the older ones did not is print the diagnostic order. The Combi 2 table heads its remedy "INSUFFICIENT CIRCULATION — check the pressure in the installation, switch on the manual degassing function, check that the pump is working, check the circulation in the boiler/installation", and only then "OTHER POSSIBLE CAUSES — check the safety thermostat connection, check that the safety thermostat is working correctly". The 200/400 table is the same list in different words, adding "check the cleanliness of the heat exchanger". So Baxi's own position is that an overheat is a circulation problem until proven otherwise: the burner heats water the pump is not moving, or a heat exchanger full of air or sludge cannot pass the heat on, and the thermostat trips because it is the last line.
It is a permanent fault, and the reset is the button — one second on a Combi 2, at least three seconds on a 200 or 400 — but this page is rated danger because a boiler that reaches its safety limit more than once is using its safety device as a control, and both the 200/400 and Combi 2 manuals end the householder's instruction the same way: if the fault cannot be reset or recurs, contact your installer, annual service provider or Baxi Customer Support. Related codes tell you the trouble was building: H.01.05 (flow and return too far apart), H.01.08 (flow temperature rising too fast), H.01.14 (maximum flow temperature) and E.04.03 (maximum flow temperature exceeded) are all circulation warnings the board shows before it gets to E.04.08.
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump not moving water | Baxi's first heading, "insufficient circulation — check that the pump is working". A pump that has stuck or lost its supply leaves water sitting in the exchanger while the burner heats it. | Common |
| Air in the boiler or system | Both manuals say to run the manual de-aeration (degassing) cycle and check the installation pressure. Air pockets after a refill or a pressure loss let part of the exchanger run dry. | Common |
| Heat exchanger fouled | The 200/400 table's "check the cleanliness of the heat exchanger". Sludge inside or deposit on the flue side slows the heat transfer and the metal runs hot. | Occasional |
| Safety thermostat or its connection | Baxi's "other possible causes": the thermostat itself drifted or its connector loose, so it opens on a boiler that is not overheating. Cheap, and checked after the circulation. | Occasional |
Can you fix it yourself?
Stop — this needs an engineer
What it costs to fix
Typical repair cost
£0–£600
Time on site
1h 30m
One reset costs nothing. Safety thermostat fitted £90–£150. Pump £200–£320. System clean £300–£500. Heat exchanger £450–£600.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Baxi / Potterton / Main safety (overheat) thermostat
7207653£15–£40Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main pump assembly
7684006£120–£240Authorised Baxi parts distributor
Price checked 28 September 2026
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Related Baxi fault codes
- E110Stop — call an engineerThe safety thermostat has tripped on overheat — "overheat of the primary water" in Baxi's words. One reset is allowed on every generation. If it trips again, the water is not moving.Related: overheat and circulation
- E125Get it checkedBaxi's primary circulation fault, decided by two temperature tests the manuals spell out. Pump, air or a blocked filter; the pressure is usually fine.Related: overheat and circulation
- E.01.17Get it checked"No water circulation (permanent)" on the Baxi 200/400 Combi and Combi 2, after H.01.18 has shown as a temporary block. Pressure, venting and the pump, in Baxi's order; reset once.Related: overheat and circulation
- E109Get it checkedPre-circulation fault on the digital-display Baxis: the board could not confirm water moving before it would light the burner. Vent the system; Baxi allow a reset from the button.Related: overheat and circulation
- E270Get it checkedBaxi's dry-fire circulation fault: the heat exchanger was heating with too little water passing through it. Air, a stopped pump or a closed circuit; not a code to reset repeatedly.Related: overheat and circulation
- E193Get it checkedPre-circulation fault on the selector-switch Baxis (Duo-tec Combi HE, Platinum, Megaflo HE A), shown as E1 then 93. Baxi offer no reset for it; the pump or air is the cause.Related: overheat and circulation
Own a Baxi boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
- Manufacturer support portal
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.