Close the oven door on a paper towel. Pull sharply. If the towel slides out easily, the gasket is compressed or perished. Heat escapes constantly, probe reads false high temps, and the element never catches up. Replace gasket: £80-£200 depending on model. Order from manufacturer with oven serial number.

Visible white or brown scale inside the chamber = element surface is insulated. In very hard water, scale thickness reduces heat transfer by 40%. Descale cycle with correct chemical (Rational EasyDescale, Convotherm ChemiClean, or generic commercial descaler rated 0-80°C). If scale is thick and brown (years of neglect), element may be damaged internally and require engineer replacement (£400-£700).

Electric combis require three-phase 400V + neutral. If one phase is lost, the element fires at 50% power. Kitchen electrician can verify with a multimeter. Call your site electrician or distribution board installer. This is a site issue, not oven fault. Cost: electrician call, typically £100-£300.

Pilot must be lit (blue flame visible when you peer into the viewing slot). If not, relighting procedure is in manual — some pilots auto-relight, others require manual reset after a power failure. If pilot won’t light, check gas supply at the inlet (hiss test with soapy water). Regulator pressure must be checked by Gas Safe engineer. Low pressure (below 20 mbar for gas line) = poor heating. This is a Gas Safe job only — do not adjust regulators yourself.

After 5+ years, thermostat probes can read 5-15°C higher or lower than actual. Your oven believes it’s at 200°C when it’s actually at 185°C. Probe recalibration or replacement requires engineer (£150-£300 for callout + parts). Rational SelfCookingCenter has software re-calibration; Convotherm and Lincat are manual adjustments inside the control board.

Hard water (>200 ppm calcium) deposits calcite inside the steam boiler. After 100-150 operating hours without descale, steam output drops 30-50%. Worst case: boiler blocks completely and steam function becomes inoperative. Manufacturer descale cycles use branded chemical tablets or liquid kits. Rational EasyDescale: 45 minutes. Convotherm ChemiClean: 60 minutes. Cost per cycle: £30-£50. If you’ve never descaled, run a cycle immediately — nine times out of ten, weak steam resolves in one pass.

Steam generation requires 1.5 to 6 bar incoming mains water pressure. Below 1.5 bar, boiler fills slowly and can’t reach steam pressure. Check water pressure at mains inlet with a pressure gauge (cost: £20-£40, available at Screwfix or plumber supply). If pressure is below 1.5 bar, contact your water authority (rare in UK urban areas, more common in rural/well-fed properties). Some operators fit a pressure-boosting pump (£800-£1,500) to stabilise low-pressure buildings.

Every combi has an inlet water filter cartridge (5 or 10 micron). After 6-12 months (depending on water quality), it can clog with sediment and restrict flow. Oven display may show error code E2 (Rational) or equivalent. Filter location: usually behind a valve on the water inlet line. Replacement cartridge: £15-£35. Replacement takes 10 minutes and a wrench. When you change the cartridge, run a descale cycle immediately afterward — the new filter traps contaminants that would otherwise build scale inside.

If weak steam persists after descale + filter change, the boiler element heating surface may be permanently damaged. This requires boiler replacement: £1,500-£2,500 parts + labour. Order new boiler from manufacturer (3-7 day lead time). Some operators fit a water softener at this point to prevent recurrence; ROI is typically 18-24 months.

Descale cycle flushes loose scale out the drain line. If drain is partially blocked (sediment, debris, hard water buildup), water backs up into the boiler and steam pressure drops. Clear the drain line with a pipe brush or plumbing snake (DIY, £20-£50 tool). Also verify the condensate trap isn’t blocked — that’s a separate line that routes excess humidity to the drain.

Rational SelfCookingCenter and Convotherm 4 use direct steam generators (faster response, less scale risk) instead of traditional boilers. Different descale chemistry. Verify your model in the manual — steam generator combis may require a different maintenance schedule. Convotherm steam generators use a separate descale circuit and schedule.

Every oven has a drain outlet at the base. During operation and cool-down, condensate and boiler bleed cycles route water out the drain. If the drain line is blocked (scale buildup, dirt, or debris), water backs up internally and leaks from the door seal or base pan. Clear the external drain hose with a plumbing snake or pipe brush. Pour hot water through it to flush sediment. Cost: DIY with a £20 brush, or £80-£150 for a plumber to rod it.

A perished or displaced gasket allows condensate to drip from the door hinge or seal line onto the floor. If you replaced the gasket recently, it may not be seated correctly — remove and reseat it. If it’s old (5+ years), replace it. Cost: £80-£200 + 30 minutes labour. Always check the groove on the door frame for debris before fitting a new gasket.

Inside the cabinet, rubber hoses carry water from boiler to steam nozzle and from condenser to drain. Age and heat cycling can crack them. Signs: water dripping inside the cabinet, pooling in the base pan, or leaking into the electronics. This requires engineer opening the oven and replacing the hose. Cost: £300-£700 depending on which hose and how accessible it is.

Rational SelfCookingCenter and some Convotherm 4 models use electric condensate pumps to lift water from the drain pan up to the waste line. If the pump fails, water cannot drain and pools inside. Pump replacement: £400-£600. Symptoms: error code E3 (Rational) or pump fault indicator. Engineer required.

If the oven is tilted, condensate pools on the low side instead of draining. Adjust levelling feet at the base. Oven should be 1-2 degrees toward the drain outlet. This is a 10-minute DIY with an adjustable wrench and a spirit level.

Heavy daily use (50+ cycles per day) can wear hinges after 3-5 years. Door hangs slightly skewed and doesn’t seat properly in the frame. Replacement hinges cost £100-£300 (depending on model) and require engineer fitting. Check for visible gaps between door and frame when closed. If one side is wider than the other, hinges are worn.

Solenoid locks or mechanical cam locks can stick with age or debris inside. Try opening and closing the door 10-15 times with firm pressure to work out the mechanism. If it still resists, lubricate with a light machine oil (WD-40) around the locking cam. If jamming persists, the solenoid coil is likely failed and requires replacement (£150-£300).

A swollen or out-of-seat gasket prevents the door from closing fully. Remove the gasket, inspect for cracks or swelling, and reseat it firmly in the groove. If swollen, it’s perished and needs replacement. A displaced gasket is often just poorly seated — remove and refit carefully, checking all four corners.

After years of intense service, the frame can shift slightly, especially if the oven has been moved or installed unevenly. Check for visible gaps or tilting. Adjustment requires partial disassembly and is engineer work. Cost: £200-£400.

Rational combis require EasyClean tablets (3 per cycle). Convotherm uses specific ChemiClean tablets. Generic drain cleaners or wrong-brand tablets will not work and may damage the boiler. Always stock the manufacturer’s tablets. Cost: £4-£8 per tablet. If you don’t have the right one, order from the manufacturer or distributor — never substitute.

Cleaning cycle requires 1.5+ bar inlet pressure to fill and spray. Check mains pressure (same as step 2 of Fault 2). If below 1.5 bar, cycle will abort. Solution: boost pump or check for a blocked water filter (see Fault 2, step 3).

If the drain is blocked, the oven fills with water during cleaning and cannot empty. Cycle stops, error code appears. Clear the drain (see Fault 3, step 1).

A brief power loss or someone opening the door mid-cycle will abort the clean. Restart from the menu. If interruptions are frequent, check your site’s electrical supply stability or install a small UPS (Uninterruptible Power Supply, £200-£400) to ride out brief outages.

Rarely, a humidity or temperature sensor can fail and trigger a false abort condition. You’ll see error codes like E1 (Rational) or equivalent on your brand. Engineer diagnostic and sensor replacement: £300-£600.

The cooling fan inside the cabinet can wear after 5+ years of continuous cycling. If you hear a grinding or squealing noise when the oven runs, the bearing is failing. Fan replacement: £300-£500 + labour. Engineer required; do not attempt DIY.

Check that your tray racks are seated fully and the rails are not bent. A rattle during preheating is often a loose rack. Remove and reseat it. Also check that any interior dividers or steam nozzles are tight.

Water and steam hoses can vibrate at certain pressures and make a rhythmic knock or rattle. If the sound correlates with steam injection, it’s likely hose resonance. Tightening hose clamps (DIY, wrench) usually solves this. If hose is cracked or split, it requires replacement (engineer work).

A small number of combis have integrated refrigeration for chilled base or prep surface. Compressor mountings can loosen over time and cause vibration and noise. Tighten mounting bolts (if accessible) or contact engineer. Refrigerated combi servicing also requires F-Gas competency — see PSSR section.

Temperature probe readings become inaccurate after 5+ years. Your oven thinks it’s at 200°C when it’s actually at 185°C. Recalibration or replacement is required. Cost: £150-£300 (engineer). Rational ovens have software recalibration via the menu; Convotherm and Lincat require manual adjustment inside the control board.

If the gasket is worn, warm air leaks continuously. Oven temperature oscillates more than usual as the element works harder to maintain set point. This causes uneven baking and colour variation. Test with paper towel (see Fault 1). Replace gasket if it fails the test.

If trays are overcrowded or stacked unevenly, hot air cannot circulate uniformly. Top trays cook faster than bottom ones. Reduce load and space trays 50 mm apart. This is a technique issue, not a fault, but it’s the most common cause of inconsistent results.

If steam injection times vary (weak steam some cycles, strong others), the boiler pressure is unstable. This happens when scale is building or water inlet pressure is fluctuating. Run descale cycle and check mains pressure. If variance persists, engineer boiler pressure test required (£200-£300).

If your saved recipes have been accidentally edited, results will drift. Check your program list in the oven menu. If times or temperatures look wrong, reset them from your reference manual or archived settings. If your oven has a “recall factory defaults” option, this is a good reset point.