An RV air conditioner that shows a code is telling you which of its parts it stopped trusting: a sensor, the incoming voltage, or the compressor. The makers publish the keys in their manuals, and the same letter can mean different things on different brands, so the tables below are kept separate by maker and quoted from the manual. Find your unit’s brand on the shroud or the thermostat, then read only that block.
Newer units report faults as a code on the wall thermostat, and the makers publish the key. The codes below are copied from their manuals; if yours shows one that is not here, the manual for your exact model is the place to look.
Dometic FreshJet (Series 4 and 7): P1 is “Under voltage protection,” P2 is “Unit AC over-current protection,” P6 is “Condenser coil protection (cooling mode),” and P9 is “Compressor drive abnormal or compressor does not start.” E0 is “Communication failure,” E1 “Room temperature sensor fault,” E2 “Evaporator coil temperature sensor fault,” E3 “Condenser coil temperature sensor fault.” A P1 on a hot afternoon points back to the voltage section above before it points at the unit; the main AC guide covers that check.
Furrion Chill (multizone thermostat): E1 “Indoor temperature sensor out of order. System will shut down.” E2 “Evaporator temperature sensor out of order. System will shut down.” E3 “Loss of communication between the thermostat and control box. System will shut down.” E4 and E5 are the condenser and outdoor temperature sensors. A lowercase “lo” is “Insufficient low level of +12V power supply. System will shut down,” which means the house battery, not the roof unit. The Chill Cube single-zone thermostat uses E3 for communication loss and E6 for its own indoor sensor.
Houghton (RecPro RP-AC2801): E0 “Communication fault,” E1 “Room temperature sensor fault,” E2 “Indoor coil temperature sensor fault,” E3 “Outdoor coil temperature sensor fault,” E4 “No cooling effect,” E5 “Outdoor coil temperature too high.” An E5 in Arizona usually means a dirty or blocked condenser coil, the same coil Coleman-Mach and Dometic tell you to clear first in the main AC guide.
Coleman-Mach has no code display on most units, but its thermostats and safety circuits still talk. On the 6536A thermostat, “If the light is blinking slowly, the unit is in a 3 minute time delay,” and “If the light is blinking fast, there is either no line voltage to Circuit #1 or one of the thermistors are open.” The digital wall thermostat shows “LO” below 33 degrees F and “HI” above 99 degrees F, which is a reading, not a fault. On the R410A rooftop units there is a high pressure switch: “Once the high pressure switch has tripped, this safety circuit will “Lock Out” the compressor preventing it from trying to restart or run until the 115 VAC supply power has been turned off and then back on to reset the High Pressure Switch Safety Circuit.” A unit that blows warm air with the fan running and will not restart until you cycle the breaker has usually tripped that switch, and on a 112-degree roof the usual cause is the condenser coil again.
Sensor codes (E1, E2, E3 and their cousins) are technician work. Voltage codes, “lo,” and the high pressure lockout are yours to rule out first.
What to tell the tech
Give the tech the brand, the model on the roof unit’s data plate, the code, and what was running when it appeared. A sensor code with the model number lets a mobile tech bring the part on the first visit. And if the code cleared after you cycled the power, say so; on a Coleman-Mach that is the high pressure switch resetting, which is a symptom, not a cure.