Summary

Pressure washers are common equipment for groundworkers, landscapers, roofers, render/cladding cleaners, and general builders doing their own site/vehicle/driveway cleaning, and "won't start" is one of the most common callouts or workshop drop-offs for both domestic and trade-grade units. The diagnostic approach differs meaningfully between mains-electric units (the large majority of domestic and light-trade machines) and petrol-engine units (more common on larger trade/commercial pressure washers where mains power isn't available on site).

A detail that catches out a lot of first-time troubleshooters: most pressure washers have a water-flow safety interlock that prevents the motor from running — or runs it briefly then cuts it — if there isn't water flowing through the pump when the trigger is pulled. This protects the pump from running dry, which damages seals within seconds. A customer reporting "it won't start" is quite often describing a machine that is actually working correctly but has no water supply connected, a kinked supply hose, or a closed tap — not a genuine electrical or mechanical fault.

This guide covers both mains-electric and petrol pressure washers, since the fault categories and first checks differ.

Key Facts

Quick Reference Table

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Symptom Likely cause First check
No response at all when switched on (electric) No power at socket/RCD tripped Test socket with another device, check RCD
Motor hums briefly then cuts out Thermal overload trip, or insufficient voltage (long/thin extension lead) Check lead length/gauge, allow cool-down, retest
Motor runs but no water comes out Water-flow interlock not satisfied — no supply connected or flowing Check tap on, hose not kinked, inlet filter clear
Motor runs then cuts out within seconds Water-flow interlock cutting out due to no/low flow Confirm supply flow rate meets machine's minimum requirement
Trigger has no effect, motor won't engage Trigger safety lock engaged Release the gun's safety catch
Repeated trip on restart attempts Thermal cutout not yet cooled, or genuine motor/pump fault Wait full cool-down period (10–15 min typical) before retrying
Petrol: pull cord resistance but no fire No spark, fouled plug, old fuel Check plug condition and fuel freshness
Petrol: starts briefly then dies Choke left on too long, blocked fuel filter, stale fuel Check choke operation, fuel system
Motor runs, low or no pressure build Unloader valve fault, or detergent mode left engaged Check injector mode switch, inspect unloader valve
Works on test bench, fails on 110V site supply Site transformer/generator supply fault, not the machine Test machine on a confirmed-good supply before condemning it

Detailed Guidance

The water-flow interlock — the most commonly missed first check

Before any electrical or mechanical diagnosis, confirm: is the water supply actually connected, is the tap open, and is water genuinely flowing to the machine's inlet (not just connected but blocked by a kinked hose, closed isolation valve, or a clogged inlet filter)? On many models, the pump's internal pressure switch or flow sensor will prevent the motor running — or cut it within a second or two of starting — specifically to protect the pump from dry-running damage. This is a deliberate safety feature working correctly, not a fault, and it is very easily mistaken for "the machine won't start" by someone who hasn't connected the water supply first or who has a kinked supply hose out of sight behind the machine.

Diagnosing an electric model systematically

Start with the supply: test the socket/extension lead with another appliance, check whether an RCD (on the extension lead, the socket, or the consumer unit) has tripped, and — if using an extension lead — confirm its length and cable cross-sectional area against the manufacturer's maximum recommended specification for that load. Long, thin domestic extension leads are a very common cause of nuisance tripping or failure to start under load, because the voltage drop under the motor's startup current draw is enough to either prevent starting or trigger a thermal/overload protection response. Once supply is confirmed good, move to the water-flow interlock checks above, then to the trigger safety lock, before considering a genuine motor or pump fault.

Diagnosing a petrol model systematically

Work through the standard small-engine sequence: confirm fuel is present and not stale (petrol more than a month or two old, especially from off-season storage, is a very common cause of hard-starting or no-start on infrequently-used equipment — recommend fuel stabiliser or running the tank dry before storage to avoid this recurring), check spark plug condition and gap, confirm the choke/primer bulb is operating correctly for a cold start, and check the air filter isn't heavily clogged (restricting air intake enough to prevent starting). If the engine turns over with normal compression resistance but produces no spark, or produces spark but won't catch with confirmed fresh fuel, this points to an ignition system or carburettor fault requiring more detailed small-engine diagnosis beyond the scope of routine site troubleshooting.

Distinguishing a genuine motor/pump fault from the common false alarms

If the supply is confirmed good, the water-flow interlock is confirmed satisfied (water genuinely flowing to the inlet), and the trigger lock is released, but the motor still won't run or trips immediately — this points towards a genuine internal fault: a seized or binding pump (often from a long period of non-use allowing seals to stick), a failed thermal cutout that trips prematurely even when the motor isn't actually overheating, or a wiring/switch fault. At this point the job moves from on-site troubleshooting to workshop diagnosis or a decision on repair-vs-replace economics, particularly for lower-cost domestic units where parts and labour can easily exceed replacement cost.

Frequently Asked Questions

My customer says the pressure washer worked fine last time but won't start now — what's the most likely cause?

If it's an electric model that's been stored since last use, check the water-flow interlock first (a surprisingly common "it just won't start" cause is simply forgetting to connect the water supply before testing), then check for a tripped RCD on whatever socket/lead is being used. If it's a petrol model that's been stored over winter, stale fuel is overwhelmingly the most likely cause — this is the single most common reason a petrol pressure washer that worked fine at the end of last season won't start at the beginning of the next one.

Is it worth repairing a cheap domestic electric pressure washer, or should I recommend replacement?

For budget domestic units (roughly under £150 new), a confirmed motor or pump fault is often uneconomic to repair once parts and labour are factored in — a new unit is frequently cheaper. For mid-range and trade-grade units, repair (particularly of wear items like unloader valves, seals, and thermal cutouts) is usually worthwhile and should be the default recommendation before suggesting replacement.

Can I use a pressure washer on a 110V site supply, or does it need to be a specific model?

Many trade-grade pressure washers are available in 110V site versions specifically for this reason — using a 230V domestic unit via a step-up transformer is not standard practice and voltage mismatch between the machine and supply is a genuine (if less common) cause of no-start or poor-performance faults. Always confirm the machine's rated voltage matches the site supply before troubleshooting further.

Why does the machine run but build no pressure at all?

This is usually not a "won't start" fault but a separate pressure-build issue — check the detergent/chemical injector hasn't been left in low-pressure mode (common on dual-mode nozzles/lances), check the nozzle isn't partially blocked, and consider a faulty unloader valve bypassing water internally rather than building pressure at the gun.

Regulations & Standards