Run a fingernail down the door, somewhere in the middle of the panel, and look at the nail.
If there is a soft brownish smear on it and the door underneath has a clean track where the nail went, that is grease and it can be dealt with. If the nail leaves a mark but takes nothing off, and the yellow is uniform across the whole panel and stops dead at the edge banding, the door is going. Those two things get confused constantly, they get charged for identically, and only one of them responds to anything a cleaner brings in the van.
How do you tell one from the other?
Location tells you most of it before you touch anything.
Grease has a geography. It is heaviest on the doors within about a metre of the hob, worse on the wall units than the base units because warm air rises, and worst on the door faces angled toward the pan rather than away from it. The corner unit at the far end of a galley kitchen will be noticeably cleaner than the one beside the extractor. Finish failure has no gradient like that, or it has one that runs the other way, along heat and light sources rather than out from the cooker.
Colour differs too. Grease film reads brown-amber and sits on top of a white that is still white underneath. Failed white lacquer or a degraded foil reads creamy, ivory, sometimes faintly pink, and it goes right through, which is why the inside face of the door still looks factory-fresh while the outside has aged a decade.
What the edges tell you
Look at the top edge of a wall unit door and the vertical edge nearest the dishwasher.
On a vinyl-wrapped MDF door, the wrap is a single sheet of PVC foil pressed over the front and folded round to a seam on the back. That seam is the weak point, and steam gets into it. Once it lifts, you get a fingernail-width of exposed MDF along one edge, sometimes swollen. That is a door past saving, whatever the front looks like.
On a lacquered or two-pack sprayed door, the edges stay sound and the flat of the panel yellows. On a laminate door, neither happens and the problem is nearly always grease.
What makes a finish yellow on its own?
Three separate mechanisms, and it pays to know which one you have, because two of them are somebody’s fault and one of them is nobody’s.
Heat is the commonest. PVC foil is thermoplastic, which means it softens and moves at temperatures a kitchen produces routinely. A door mounted immediately beside a built-in oven, particularly above a pyrolytic one, gets a slow cooking of its own every time the oven runs. The foil discolours, shrinks slightly, and pulls away at the nearest edge. A kitchen on Ermine Road in Lewisham had a housing unit where the two doors flanking the oven were visibly creamier than the eight identical doors elsewhere in the run, and the fitter had left no heat gap at all.
Light is the second. White finishes yellow under UV, and the ones that yellow fastest are the cheap solvent-based lacquers and older polyurethane systems. In a kitchen with a south-facing window or a set of bifolds, the doors nearest the glass will run ahead of the rest. Take a door off and hold it against one from the far end of the room and the difference is obvious, which is a test worth doing before anyone pays for a deep clean.
Third, and the one people find hardest to believe: the finish leaching its own softeners out. PVC foil contains plasticisers to keep it flexible, and over ten or fifteen years those migrate to the surface and sit there as a faintly tacky, faintly yellow layer. It feels exactly like grease. It wipes away and comes back within a week, because the source is inside the material rather than on it. Nothing stops it. A house in Pinner had this on every door of a kitchen fitted in 2009, and three cleans in eighteen months achieved nothing except a slow flattening of the sheen.
The door beside the oven
Take that one off its hinges and look at the back.
Heat damage shows there first: a faint scorch shadow, a foil edge lifting on the hinge side, sometimes a soft ripple in the surface where the board underneath has moved. A door that shows any of that is past saving and the rest of the kitchen probably has years left. Two replacement doors from the original supplier cost a fraction of a new kitchen and most people never consider it.
If it is grease, why is it sticky rather than oily?
Because it has been there long enough to stop behaving like oil.
Fresh cooking splatter is wet and comes off with washing-up liquid. Old deposit has oxidised into something tacky that grabs dust, and the dust is a large part of what makes the film look grey-brown rather than yellow. That is the whole explanation and it does not need more than that.
What removes it
An alkaline degreaser, applied warm, given four or five minutes to sit rather than sprayed and wiped, worked with a soft pad, and rinsed off with clean water on a separate cloth. The rinse is the part everyone skips and it is the part that decides whether the door stays clean or turns tacky again in a fortnight.
Do a test patch on the inside of a door first, always, because some finishes will not take an alkaline product. Matt lacquers in particular go patchy and there is no way back from that.
Budget about ninety seconds a door for the wipe and rinse on a wall unit, and more like three minutes on a base unit door beside the hob, where the deposit is thicker and the handle area needs separate attention. A run of eighteen doors is most of a morning done properly, which is why it gets skipped on a standard visit and why the film keeps building until somebody notices the colour.
What makes the problem worse?
Almost everything sold to fix it in a hurry.
Melamine foam pads are the worst thing in domestic cleaning and I will argue about it with anybody. They are a fine abrasive. What they do to a gloss door is remove the top few microns of the finish along with the marks, which looks like a triumph on the day and leaves a dull, slightly rough patch that then holds dirt faster than the surrounding surface. Six months later the door has a permanent cloudy oval where somebody was thorough. On a foil-wrapped door they can go through the print layer completely and take the wood-grain pattern off.
Solvents come next. Acetone, white spirit, nail varnish remover, and the citrus degreasers people reach for when the ordinary spray fails. All of them will soften PVC foil and several will lift lacquer. A tenant on Fentiman Road in Oval took the sheen off four doors in one afternoon with a solvent wipe intended for oven interiors.
Bleach does nothing useful on grease, and on a yellowed finish it does nothing at all, because the yellowing is not a stain. It is the material. People keep trying, and what they end up with is a door that is both past saving and faintly smelling of chlorine.
Then heat. Steam cleaners get pointed at kitchen doors constantly and they are a poor idea on anything wrapped, since the steam goes straight into the back seam and starts the delamination that was going to take another five years to arrive on its own.
The scouring pad question
Green scourers, wire wool and gritty cream cleaners belong on a pan and nowhere near a cupboard door. The one exception is a true laminate face, which is hard enough to take a cream cleaner on a bad mark, and even then a test patch first.
When is a door past saving, and what then?
When the yellowing is in the material rather than on it, and no clean is going to change that.
The options are narrower than people expect. Doors from a mainstream range fitted in the last decade can often be reordered individually, which is the cheapest answer by a distance if you can identify the range. Photograph a hinge plate, the back of a door, and any label inside a carcass, and a kitchen showroom will usually tell you what you have. A homeowner near Canbury Park Road in Kingston replaced eleven doors on a fifteen-year-old carcass run for less than a fifth of what three quotes for a new kitchen came to.
Spray refinishing is the second option, and it works properly on solid or MDF doors with a sound substrate. On a wrapped door it is a waste, because the wrap is the problem and paint over a lifting foil lifts with it.
Refoiling exists. At London labour rates it is rarely worth the money, and the finish is seldom as good as the factory one it replaces.
What to ask before anyone quotes
Ask whether the doors are foil-wrapped, sprayed, or laminate, and get the answer from somebody who has taken one off rather than looked at it. Ask what the heat gap beside the oven housing is, because if the original fitter left none, new doors will do exactly the same thing.
And ask whether the extractor works, since a hood that has not moved air properly in years is the reason the far doors are yellow as well as the near ones.
The doors in Chingford went back on with a two-millimetre packer beside the oven and a new carbon filter in the hood. Eight months, no change to the colour.