The short answer
On an older Cosmo dishwasher, a heating failure ranks differently: the element has had years of scale deposited on it, the thermostat has aged, and the machine may be extending its cycle to compensate rather than reporting a fault. Check the supply as always, but expect scale and a tired element to be the real answer.
What you are actually seeing
- Cycles have grown steadily longer over a year or more — Means: on-page
- The tub is warm but never hot, where it used to be too hot to touch — Means: on-page
- Heavy scale is visible on the element in the base of the tub — Means: on-page
- Drying has fallen off gradually rather than stopping — Means: on-page
- The machine performs better on a long cycle than on a short one — Means: on-page
- The machine will not run a cycle at all — Means: different-fault — Go to: cosmo-dishwasher-wont-start
- Dishes are clean but wet, and the tub is warm — Means: different-fault — Go to: cosmo-dishwasher-cleaning-performance
- Body: Match what you see before you take anything apart. Cycles have grown steadily longer over a year or more. The tub is warm but never hot, where it used to be too hot to touch. Heavy scale is visible on the element in the base of the tub. Drying has fallen off gradually rather than stopping. The machine performs better on a long cycle than on a short one. Some near-miss symptoms belong on a different page: The machine will not run a cycle at all points at a start or control fault, covered on cosmo-dishwasher-wont-start; Dishes are clean but wet, and the tub is warm points at a drying rather than a heating problem, covered on cosmo-dishwasher-cleaning-performance.
Causes, ranked for this machine
- Scale insulating the heating element — Why this rank: First on an older machine because scale builds gradually on the hottest surface in the machine, which matches a slow decline rather than a sudden failure
- An element that has aged and no longer delivers its original output — Why this rank: Explains a machine that heats but never gets properly hot, with no fault code at all
- A thermostat or sensor that has drifted, so the control stops heating early — Why this rank: A drifted sensor produces a machine that thinks it is hot when it is not
- A hot supply that has degraded, such as a scaled water heater or a throttled valve — Why this rank: Worth checking on an older house as well as an older machine
- A control board that is no longer commanding the heat correctly — Why this rank: Last because it needs a part and the four above cost nothing to establish
- Body: Ranked for a built-in kitchen dishwasher with an electronic control, a manual filter and a drain hose running to a disposer or an air gap. Most likely: Scale insulating the heating element. First on an older machine because scale builds gradually on the hottest surface in the machine, which matches a slow decline rather than a sudden failure. Next most likely: An element that has aged and no longer delivers its original output. Explains a machine that heats but never gets properly hot, with no fault code at all. Third: A thermostat or sensor that has drifted, so the control stops heating early. A drifted sensor produces a machine that thinks it is hot when it is not. Fourth: A hot supply that has degraded, such as a scaled water heater or a throttled valve. Worth checking on an older house as well as an older machine. Fifth: A control board that is no longer commanding the heat correctly. Last because it needs a part and the four above cost nothing to establish.
What to check, in order
- Look at the element in the base of the tub for scale before anything else
- Run a descaling cycle and judge whether the tub temperature improves
- Check the hot supply at the tap as you would on any machine
- Compare a long cycle against a short one and note the difference
- Body: Each check rules out the causes above it. First, look at the element in the base of the tub for scale before anything else. Second, run a descaling cycle and judge whether the tub temperature improves. Third, check the hot supply at the tap as you would on any machine. Fourth, compare a long cycle against a short one and note the difference.
Checks you can safely make yourself
- Open the door and look at the element in the base of the tub with a torch, noting any white crust along its length — Tools: torch — Time estimate: a few minutes
- Run the hottest cycle with a proprietary descaler and an empty machine, then look at the element again — Tools: descaler — Time estimate: one long cycle
- Run the kitchen hot tap until hot, start a normal cycle immediately, and feel the door partway through — Tools: none — Time estimate: one cycle
- Run a long cycle and a short cycle on separate days and note whether the difference in result is large — Tools: notepad — Time estimate: two cycles
- Check the household water heater setting and how long the kitchen tap takes to run hot — Tools: watch — Time estimate: a quarter of an hour
- Body: Checks an owner can make with the machine switched off at the breaker, or by watching a cycle. Step one: Open the door and look at the element in the base of the tub with a torch, noting any white crust along its length. Tools: torch. Time: a few minutes. Step two: Run the hottest cycle with a proprietary descaler and an empty machine, then look at the element again. Tools: descaler. Time: one long cycle. Step three: Run the kitchen hot tap until hot, start a normal cycle immediately, and feel the door partway through. Tools: none. Time: one cycle. Step four: Run a long cycle and a short cycle on separate days and note whether the difference in result is large. Tools: notepad. Time: two cycles. Step five: Check the household water heater setting and how long the kitchen tap takes to run hot. Tools: watch. Time: a quarter of an hour.
Safety first
Safety
Switch off at the breaker before removing any panel. The element sits in the base of the tub with mains terminals beside it and it stays hot long after a cycle ends, so do not reach into the sump immediately after a wash. On a machine of this age, treat any connection under the sink as suspect before you touch it: corrosion and hardened insulation are both cumulative, and neither is visible until a panel comes off. Unplug the appliance or switch off its breaker before removing any panel. A capacitor can hold a charge after the power is off. unplug-before-service
Where the owner fix ends
On an older machine, call a technician once the element has been descaled, the hot supply is proven and the tub is still never properly hot. A tired element or a drifted thermostat is a parts job behind the lower panel, and on an aged machine it is the moment to weigh the cost against replacement.
Nearest pages of the same kind
Generalized information
This page is generalized across the Cosmo dishwasher line. Front-control and top-control machines, and panel-ready and stainless fronts, share the ranking above but not every part, and this page does not cover a specific model or control revision. Family- and model-specific pages, where they exist, are linked below.
Links: - [Common COSMO Dishwasher Problems](/blog/cosmo-dishwasher-problems) - [Cosmo Dishwasher: Abnormal Noise or Vibration](/blog/cosmo-dishwasher-abnormal-noise-2) - [Cosmo Dishwasher: Does Not Clean or Dry](/blog/cosmo-dishwasher-cleaning-performance-2) - [Cosmo Dishwasher: Controls, Display or Error Code](/blog/cosmo-dishwasher-control-fault-2)
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
