The shower is cold and the heater light is on: five checks before anyone quotes you

An electric water heater is a kettle with ambition: a tank, an element that is a stove coil in disguise, a thermostat that is a switch on a spring, and a cut-out that gives up before the fire does. Cold water means one of five things failed, and the ladder from simplest to final is worth knowing - because half of "element failures" are the cheap switch having done exactly its job.

The five, in order

• Isolation and breaker first, always. The heater has its own switch on the wall or its own breaker; both get looked at before both get reset, and the panel gets sniffed for the warm-fishy smell from this desk's breaker article before any reset happens at all. Power off at the wall before touching anything else in this list - the honest version of "unplug it" for a hardwired box is the breaker, with the neighbours' lights off so you can see the tell-tale glow die.
• The high-limit cut-out. Inside the thermostat cover sits a reset button on most units: it fires when the tank overheats - which is what a dying thermostat does before it dies, and it is also what a heater does when it ran empty during a supply cut. With power OFF, remove the access cover (screwdriver, one panel), find the little red or black button, press it. Heater lives, hot water returns in an hour: the fault was the safety, and you still owe the unit a thermostat check - a cut-out that fires once was brave, a cut-out that fires monthly is a witness.
• The thermostat itself. Set to maximum as a test - a thermostat stuck below the cut-out never lights the element, and the dial being down is the cheapest cold shower in the trade. A multimeter continuity check across its terminals, power off, is five minutes for anyone who owns the meter; if the reading contradicts the dial, the part is the invoice.
• The element. The classic: continuity through the element should be a low, real number, and continuity from either terminal to the copper tank should be nothing at all. A reading between a terminal and the tank means the element's sheath has cracked and it is leaking current into the water - which is the failure mode that makes "the RCD trips in the bathroom" the last warning you get before the shower becomes a conductor. That one is not a DIY swap; that one is the electrician's call, tonight.
• The tank, and the year it is at. A heater past its element's designed life in hard water area throws the element, the anode and eventually the seam all in the same season. A weep at the flange, rust bloom at the seam, a tank that has clearly never been descaled in the house's ownership: quote the whole unit, not the part - this desk has watched people buy elements for a coffin.

The two minutes of prevention that cost nothing

Set the thermostat to what the manual calls its lower band - a hotter tank scales faster and scalds the children's bath, and no comfort is purchased above "properly hot"; and if the area is hard-water, a drain-and-flush habit at the annual maintenance day moves the grit out before it becomes the element's burial shroud. The gas instant heaters get their own short line here too: no spark means the batteries first - the water-flow switch second - never the gas - and a flame that lifts off its burner is a shut-down-and-call fault, not a tune-up.

What this desk will not pretend

Tank draining, element replacement in a wet cupboard, and any work on a leaking energised vessel are jobs for the qualified person with the right tools and the insurance - the value of this page is that you hand them the diagnosis instead of the shower-curtain rumour, and pay for the repair, not the guess.