The engine has a high coolant temperature, which could cause the engine to overheat in a shorter period than usual. A code 18 error is a little tricky to troubleshoot as it may involve several factors such as a low coolant level or a leak.
Check coolant temperature reading to make sure coolant sensor works. WT-1 and WT-2 readings should be 120-150F and shouldn't jump up or down rapidly (more than 2 degrees or so). Sometimes it's sporadic and hard to catch. WT-1 is behind water pump. WT-2 is in the harness outside of the unit.
Check coolant supply and air in coolant lines by draining coolant. Check thermostat. It may be defective or covered with dirt. Clean it off and check if the same error code is appearing. Next is to check if the water pump is working correctly.
If you’ve checked the thermostat and water pump and have determined they are working correctly, you may want to pressure wash the radiator. A dirty radiator may hinder the circulation of air, which may cause overheating.
Make sure evaporator fan works (by putting your hand next to it in Cool mode or run Fan only service test).
If the above steps did not work, try checking if the engine oil is within the usual level. While you are at it, check if the oil is in its normal consistency. The diesel can mix with the engine oil if there is leaking.
Check the pressure in the main coolant tank. If the black cap on the side (drain cap). If this csp is blocked (bad) which would also be indicated by collapsed big black tube next to the main radiator then circulation of coolant will be stopped which will cause overheat.