Worcester Bosch 2948 Fault Code: Causes, Fixes & Repair Costs
What does the Worcester Bosch 2948 fault code mean?
Fault code 2948 appears on Worcester Bosch Greenstar boilers when the burner control unit fires at high output but cannot confirm a sustained flame. In plain terms, the boiler is attempting to ignite, something is preventing the flame from being reliably detected, and the appliance locks out as a safety measure. The boiler may attempt an automatic purge and restart cycle before displaying the code permanently. Because this fault sits in the combustion system, a Gas Safe registered engineer must investigate and clear it — homeowner resets alone rarely resolve the underlying problem.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Worn or failed burner gasket Common
The burner gasket seals the junction between the burner and the venturi assembly that feeds mixed air and gas to the burner. On Greenstar Junior, Si, and Ri models in particular, this seal tends to degrade after roughly five years of service. Once it starts to fail, combustion air can escape or become unbalanced, preventing a clean, consistent flame from forming at high output. This is the single most frequently diagnosed cause of code 2948.
- CO₂ / gas-to-air ratio out of range Common
Every boiler has a factory-set combustion ratio that must fall within tight limits for efficient and safe burning. Drift in this ratio — whether caused by a degraded gasket, a partial blockage, or component wear — means the burner cannot sustain an adequate flame at high fire. The boiler senses the shortfall and locks out. Correcting the ratio requires a calibrated flue-gas analyser and adjustment of the gas valve, tasks that only a Gas Safe engineer should carry out.
- Faulty ignition electrode or damaged lead Sometimes
The ignition electrode both sparks to light the burner and acts as the flame-sensing probe via ionisation current. If the electrode tip is cracked, heavily carboned, or the connecting lead is chafed or has a poor connector, the control board receives no reliable flame confirmation even when gas is burning. This is a relatively straightforward component swap for an engineer but must not be attempted by a homeowner.
- Gas valve fault or irregular gas supply Sometimes
A gas valve that is not fully opening, sticking, or leaking internally can restrict flow to the burner at high demand, resulting in a weak or absent flame. In rarer cases a low gas supply pressure — for example during periods of high local demand on the network — can produce a similar symptom. An engineer will check inlet gas pressure and valve operation as part of a full diagnosis.
How to fix it
- Check your gas supply is working DIY safe
Before calling anyone out, confirm that other gas appliances in the property (hob, gas fire) are working normally. If you have no gas at all, contact your gas network emergency line (0800 111 999) immediately. Do not reset the boiler if you suspect a gas supply problem.
- Reset the boiler once or twice DIY safe
Press and hold the reset button as described in your boiler's user guide. The boiler may attempt an automatic purge first — allow it to complete. If the fault clears and stays clear after one or two resets, monitor the boiler closely over the next few hours. If the fault returns, do not keep resetting, as repeated cycling without resolving the root cause can cause further damage.
- Arrange a Gas Safe engineer to inspect the burner gasket Gas Safe engineer
Given that a degraded burner gasket is the most common trigger for this code, the engineer should remove the burner assembly and examine the gasket closely. A genuine Worcester Bosch burner seal kit (part reference 8718600040) is relatively inexpensive at around £35–£45 for parts; labour brings the typical total to £150–£300. Ask the engineer to replace the electrode at the same service visit if either component shows wear, as the labour overlap makes this cost-effective.
- Have the CO₂ combustion settings checked and adjusted Gas Safe engineer
Using a calibrated flue-gas analyser, the engineer must verify that the CO₂ percentage at both minimum and maximum output falls within Worcester Bosch's specified range for your model. If the ratio has drifted, they will adjust the gas valve trim settings accordingly. This step cannot be skipped — an out-of-range combustion ratio is both a performance problem and a safety concern.
- Request inspection of the ignition electrode and connecting lead Gas Safe engineer
The engineer should check the electrode gap, condition of the ceramic insulator, and continuity of the high-tension lead. If the electrode is fouled or cracked, it should be renewed as part of the service kit. This is typically carried out alongside the gasket inspection and adds minimal extra time to the visit.
- If faults persist, have the gas valve assessed Gas Safe engineer
Should the above steps not resolve the fault, the engineer will need to test gas inlet pressure and gas valve operation. A faulty gas valve will need professional replacement — this is not a DIY task under any circumstances. Gas valve replacement typically costs £150–£300 including labour.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
Fault code 2948 is not something that can be permanently fixed by a homeowner. If you have not already booked an engineer, do so now. You can find registered engineers at gassaferegister.co.uk. Always ask to see an engineer's Gas Safe ID card before allowing work to begin.
Parts you may need
- Worcester Bosch burner seal / gasket kit (e.g. 8718600040) · from £40
- Ignition and flame-sensing electrode · from £35
- High-tension ignition lead · from £25
- Gas valve (model-specific) · from £120
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£350, depending on the underlying cause.
Frequently asked questions
Is fault code 2948 dangerous — can I keep using my boiler?
You should not attempt to keep the boiler running through repeated resets once this fault has appeared. Code 2948 means the boiler cannot confirm a stable flame at high output, which is a combustion safety issue. The lockout is the boiler protecting itself and your home. Stop resetting it after two attempts and book a Gas Safe engineer promptly.
How much does it typically cost to fix a Worcester Bosch 2948 fault?
For the most common repair — burner gasket replacement, electrode renewal, and a CO₂ combustion check — most homeowners pay between £150 and £350 including parts and labour. A standard Gas Safe callout is usually £60–£100 during normal hours. If a gas valve also needs replacing, costs can reach £400–£450. In rare cases where a PCB is involved, that component alone can cost £300–£500 before labour, but PCB replacement is not the typical outcome for this specific code.
Why does the burner gasket cause so many problems on Greenstar boilers?
The burner gasket on Greenstar Junior, Si, and Ri models is subject to repeated heating and cooling cycles every time the boiler fires. Over approximately five years this thermal stress causes the material to harden, shrink, or crack. Once the seal is imperfect, the air-gas mixture reaching the burner changes, making it difficult or impossible to sustain a clean, detectable flame — particularly at high output when demand is greatest.
My boiler is over ten years old and keeps showing 2948. Should I repair or replace?
As a rough guide, if your boiler is more than 10–12 years old and the repair quote exceeds roughly half the cost of a new boiler, replacement is usually the better long-term investment. A new A-rated condensing boiler will also be more efficient, reducing your gas bills. If the boiler is younger and in otherwise good condition, repair is almost always the more economical choice. Ask your engineer for an honest assessment of the boiler's overall condition before committing to expensive parts.