Inspect the O-ring before replacing the whole hose. A small seal that is cut, flattened, dry or missing can create a surprisingly dramatic leak at a connection.
Clean the connection
Disconnect the machine safely, relieve pressure and inspect both sides of the fitting. Dirt caught against a seal can keep it from seating.
Look closely at the O-ring
Westinghouse specifically tells ePX3500 owners to clean the high-pressure-hose O-rings regularly and lubricate them with a non-water-soluble grease such as petroleum jelly.
Check whether the leak follows the hose
If you have another known-compatible hose, swapping it can help separate a hose-end problem from a machine fitting problem. Do not use that test with a hose that merely looks similar.
If the fitting is cracked, stop
An O-ring cannot repair a cracked plastic or metal fitting. Replace the correct component rather than trying to compensate with tape or excessive tightening.
Where exactly is the water coming from?
Dry the connection, turn the water back on and watch carefully. Water from between the mating surfaces points toward the seal. Water coming through a crack or from behind a crimp points toward a damaged fitting or hose end.
Do not solve every leak with thread tape
Many pressure-washer connections seal with an O-ring or a designed face seal rather than tapered pipe threads. Adding tape where it does not belong can make the fit worse. Identify the fitting first.
When replacing the hose is smarter
Replace a hose that is cut, blistered, badly kinked, leaking through the hose body or damaged at the crimp. A high-pressure hose is not a good place for improvised patches.
Sources used for this answer
Back to class: Electric Pressure Washer Class.