The newer Kohler generators can be a bit picky and code for many different reasons. Lets discus the most common ones for the 5ECD/7.3ECD.
LOC - Loss of coolant. Many people think this is a engine coolant/antifreeze side issue. It is not! LOC is loss of raw water coolant flow(sea, river, or lake water). This could be triggered by the pressure switch or the exhaust temp switch. Make sure to check your impeller https://www.seafarerstore.com/Kohler-359978-OEM-Impeller-Kit_p_10.html If the impeller is intact and the pump is not locked up, check the drive sleeve https://www.seafarerstore.com/Kohler-359448-Drive-Sleeve-_p_13.html behind the pump by removing the 2 allen bolts. TECH TIP: Do not remove all 4 bolts when checking/changing the impeller, leave on bolt in loosened just enough to flip the cover plate out of the way. Splitting the pump in half cam damage the seal and cause the pump to leak.
You pressure switch may also be clogged. https://www.seafarerstore.com/Kohler-GM30263-OEM-Water-Pressure-Switch_p_29.html This happens when in high salt/mineral waters and the system in shut of without cooling down enough, We highly recommend some king of flush system be used in these areas. The salt/mineral build up will also cause the high pressure fuel pup module to become blocked and cause the LOC code. When the fuel pump module (Also cools the fuel) become blocked, in can cause water to leak into the fuel side causing many other issues.
UU - Under Voltage. This is looking at the AC voltage. Many things can cause this error also. Start with the basics after verifying AC output. Check the spark plugs. The the engine used is these ECD units are known to go through spark plugs. Many times on plug will drop out but the engine will still run, RPM is just low so the gen will not produce enough AC voltage and UU will code. Verify and adjust valve clearance (with engine COLD!) .010 Check your fuel. Do you have a water separator installed? If not... it is highly recommended. Many boat builders did not install a separator. Water in the fuel can damage the fuel injector and cause the UU code. Use non ethanol/rec fuel. These small engines can not handle ethanol issues like the main engines may. Many places now have higher that E10 and can cause issues. This bring us back to the fuel module. If the module plugs with salt/mineral build up and the pressure get high enough, water will breach the seal to the fuel side. This water is then ran through the injector causing damage to the injector and the UU code.
Many of the item discussed are part of the service schedule. If you follow the schedule and check for leaks at start up and shut down, you will reduce maintenance cost an failures.
These codes are similar for the EKD units