Before you touch any part of the dishwasher, you must do two things in this order. First, find the water isolation tap — it is usually a small lever or screw tap on the cold water pipe under the sink, or on the wall behind the machine — and turn it fully clockwise until it stops. Second, pull the dishwasher plug out of the mains socket at the wall. Both steps are essential. Water and electricity together are extremely dangerous, and the dishwasher's pump can activate without warning if the machine remains plugged in while you are working on it. Do not skip either of these steps, even if the machine appears to be off.

With the machine safely unplugged, look at the water isolation tap on the cold supply pipe. It should be turned fully anti-clockwise to the fully open position. If it is only partially open — even slightly — it restricts the flow rate enough to prevent the pressure switch from registering adequate pressure, which is a very common cause of i40. Turn it fully anti-clockwise and make sure it moves freely with no stiffness or resistance. If the tap is stiff and difficult to turn, do not force it — a stiff isolation tap may need replacement by a plumber.

The inlet hose is the flexible pipe that runs from the water tap to the back of the dishwasher. Gently ease the dishwasher forward from the cabinet slightly — ask someone to help if the machine is heavy — and run one hand along the full length of the hose from the tap connection all the way to where it enters the machine. Feel for any kink, sharp bend, or section that has been crushed between the machine body and the cabinet wall. Even a moderate kink can reduce water flow to the point where the pressure switch cannot detect adequate pressure. Straighten every section of the hose so it runs in smooth curves with no tight bends.

The inlet filter is a small mesh screen located inside the water inlet port at the back of the dishwasher — this is where the inlet hose screws in. Its job is to stop grit and debris from the water supply entering the machine, but it can become blocked with limescale and mineral deposits over time, restricting the water flow that the pressure switch needs to detect. To clean it: make sure the water supply tap is still turned off, then place a folded towel under the hose connection at the back of the machine. Unscrew the hose fitting by turning it anti-clockwise — water will drip out as you loosen it, which the towel will catch. Look inside the inlet port opening on the machine and you will see a small flat or cylindrical mesh screen. Remove it carefully using a pair of tweezers. Rinse it under running water and scrub it gently with an old toothbrush to remove limescale and debris. When it is clean, push it firmly back into position, reattach the inlet hose, and hand-tighten the fitting followed by a quarter turn with pliers.

If the inlet hose is clear and the filter is clean but i40 persists, the water pressure at your property may be too low for the dishwasher's pressure switch to register a normal fill. To check this simply: disconnect the inlet hose from the back of the dishwasher, point the open end into a bucket, and briefly turn the tap on for three seconds. You should see a strong, steady, uninterrupted flow of water. A weak trickle or intermittent flow indicates a pressure problem at the property rather than a fault with the dishwasher. In this case, contact your water supplier or a plumber — fitting a booster pump may be required.

The pressure switch monitors water level and pressure using a small, narrow hose — sometimes called a pressure tube — that connects the bottom of the tub or sump area to the pressure switch unit. With the dishwasher still unplugged, carefully pull it fully out from the cabinet so you can access the side or rear panels. The pressure switch is typically a small round or oval component mounted on the inner side panel or near the sump, connected by a thin rubber or plastic hose. Inspect the full length of this small hose for kinks, splits, or disconnection from either end. A blocked or kinked pressure hose prevents the switch from detecting water correctly even when the tub has filled normally. If the hose has come away at either connection point, push it firmly back on.

Refit the inlet hose securely, push the dishwasher back into position carefully, turn the water supply tap fully on, and plug the machine back in. Select a Quick Wash or short programme and remain present throughout the first five minutes. You should hear the machine begin to fill with water within 60 seconds of starting — a gentle rushing sound from the base of the machine confirms water is entering. The programme timer should then begin counting down. If i40 reappears during this test fill phase, the pressure switch itself has likely failed and will need to be replaced by a qualified engineer.