The short answer
A Cosmo dishwasher that never gets hot usually has a supply problem rather than a failed element, because the machine takes hot water from the house supply and only tops it up. A long run from the water heater, a closed or throttled valve, or an ice-cold first fill all produce a cool wash that looks like a heating failure. Check the supply first.
What you are actually seeing
- The dishes and the tub are cool at the end of the cycle — Means: on-page
- The cycle runs much longer than it used to — Means: on-page
- Nothing dries at all, even with rinse aid — Means: on-page
- Greasy items come out with the grease unmoved — Means: on-page
- Heating failed after work was done on the kitchen plumbing — 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. The dishes and the tub are cool at the end of the cycle. The cycle runs much longer than it used to. Nothing dries at all, even with rinse aid. Greasy items come out with the grease unmoved. Heating failed after work was done on the kitchen plumbing. 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
- A cold first fill because the hot line has not been run before the cycle — Why this rank: First because it is free to fix, extremely common and produces exactly this symptom on a machine that is otherwise perfect
- A supply valve under the sink closed or barely open, or a hot line connected to cold — Why this rank: A machine plumbed to cold after kitchen work washes cold every time, and the fault dates from that work
- A water heater set low, or a very long run from the heater to the kitchen — Why this rank: This is a house problem rather than an appliance problem, and it is worth establishing before ordering a part
- An eco or low-temperature cycle selected — Why this rank: Eco cycles trade time for temperature and owners frequently leave them selected
- A failed heating element or thermostat — Why this rank: Last because it needs a part and the four causes above it are free to rule out
- 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: A cold first fill because the hot line has not been run before the cycle. First because it is free to fix, extremely common and produces exactly this symptom on a machine that is otherwise perfect. Next most likely: A supply valve under the sink closed or barely open, or a hot line connected to cold. A machine plumbed to cold after kitchen work washes cold every time, and the fault dates from that work. Third: A water heater set low, or a very long run from the heater to the kitchen. This is a house problem rather than an appliance problem, and it is worth establishing before ordering a part. Fourth: An eco or low-temperature cycle selected. Eco cycles trade time for temperature and owners frequently leave them selected. Fifth: A failed heating element or thermostat. Last because it needs a part and the four causes above it are free to rule out.
What to check, in order
- Run the kitchen hot tap until it is genuinely hot, then start the cycle straight away
- Check the supply valve under the sink is fully open and connected to the hot line
- Select a normal or heavy cycle with any eco option switched off
- Feel the door and the tub partway through a cycle to judge whether the machine heats at all
- Body: Each check rules out the causes above it. First, run the kitchen hot tap until it is genuinely hot, then start the cycle straight away. Second, check the supply valve under the sink is fully open and connected to the hot line. Third, select a normal or heavy cycle with any eco option switched off. Fourth, feel the door and the tub partway through a cycle to judge whether the machine heats at all.
Checks you can safely make yourself
- Run the kitchen hot tap until the water is hot to the hand, then start the dishwasher immediately — Tools: none — Time estimate: a few minutes
- Open the cabinet under the sink, find the dishwasher supply valve, confirm it is fully open and trace which line it comes from — Tools: torch — Time estimate: a quarter of an hour
- Check the setting on the household water heater, and time how long the kitchen tap takes to run hot — Tools: watch — Time estimate: a quarter of an hour
- Select a normal or heavy cycle, switch off any eco or quick option, and run it with the machine loaded normally — Tools: none — Time estimate: one cycle
- Partway through a hot cycle, feel the outside of the door: it should be noticeably warm if the machine is heating — Tools: none — Time estimate: one cycle
- Body: Checks an owner can make with the machine switched off at the breaker, or by watching a cycle. Step one: Run the kitchen hot tap until the water is hot to the hand, then start the dishwasher immediately. Tools: none. Time: a few minutes. Step two: Open the cabinet under the sink, find the dishwasher supply valve, confirm it is fully open and trace which line it comes from. Tools: torch. Time: a quarter of an hour. Step three: Check the setting on the household water heater, and time how long the kitchen tap takes to run hot. Tools: watch. Time: a quarter of an hour. Step four: Select a normal or heavy cycle, switch off any eco or quick option, and run it with the machine loaded normally. Tools: none. Time: one cycle. Step five: Partway through a hot cycle, feel the outside of the door: it should be noticeably warm if the machine is heating. Tools: none. Time: one cycle.
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. 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
Call a technician when the hot tap runs hot at the sink, the supply valve is open on the hot line, a normal cycle is selected with no eco option, and the tub is still cool partway through. That leaves the element, the thermostat or the control, all of which sit behind a panel with mains terminals.
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) - [COSMO Dishwasher: Does Not Clean or Dry](/blog/cosmo-dishwasher-cleaning-performance) - [COSMO Dishwasher: Controls, Display or Error Code](/blog/cosmo-dishwasher-control-fault)
Lead Appliance Repair Technician · 12 years experience
Experienced technician with 12 years specializing in washing machine and dryer repairs across all major brands.
