1998 Mitsubishi Eclipse stops working properly when hot?

Pressure wash not good idea, you need to check for any vacuum lines or connectors blown off by press washer, also check make sure air filter element didn't get wet. If spark plugs down in valve cover could have shorted out wires. Check engine light needs to have OBD2 computer scan, fastest way to find problem seeing it controls all sensors and various other engine components. Also could have got water in ignition module. After checking first part of answer if nothing found and check light still on take to reputable shop for computer scan which tech can tell you exact problem and really cheapest way to get fixed.