If you run a search on the Internet for E1 errors on Dometic thermostats, particularly their CCC2 system, you will note finding E1 on your thermostat’s LED screen is all too common. So you may be wondering what it means and how to troubleshoot it, if the reset procedure doesn’t work.
Of course, you know an E1 error means the thermostat has lost its ability to talk to the control board mounted in the ceiling or mounted on the side of the air conditioning unit on the roof. Unless a board or thermostat has failed, an E1 error on a single zone system is relatively uncommon. On the other hand an E1 error is routinely found in the CCC2 thermostat, which is capable of controlling a number of air conditioners using the same thermostat.
The CCC2 system talks to the control boards by way of an RJ11 communication cable. This cable looks just like a standard telephone line with one important difference, when you position the ends of the cable in the same orientation, the order of the wires is exactly the same. Looks at a standard phone line and you will see the order is reversed. RV manufacturers provide the RJ11 cables for their RVs. Technicians like me make their own test cables. Communication via RJ11 cables is old technology for Dometic thermostats first appearing in the old four and then five button Comfort Controls. It was a problem then. It is a problem now.
So why use a single thermostat? For the manufacturers, it can make air conditioner installation faster and simpler. For owners, it saves them all that time that is required to set two thermostats, perhaps as much as 30 seconds per day. The downside is communication with RJ11 cables has always been problematic, but now it is worse.
Here is a link to the reset procedure for the CCC2 system
These same instructions are in multiple sites on the Internet. The issue no one addresses is why the RV manufacturers are installing a system that has resulted in so many posts on the Internet asking what to do when it doesn’t work.
So what do you see on the thermostat along with the E1 error code? You should see a flashing number telling which zone is not communicating. Now if you call into Dometic, the person on the phone will read you the manual and tell you to just keep trying the reset and I have had some customers who have tried the reset as many as 30 or 40 times . You do not need to, by the way, unplug your shoreline or disconnect your batteries. Just flip the AC breaker and pull the fuse for the thermostat so the screen is dead.
So let’s say you have no luck. What to do next? The next step is to take the problem unit offline. In other words, unplug the RJ11 cable to the problem child unit. The RJ11 cables are either plugged directly into the board or to a “connector” made of plastic. They are easily unplugged and plugged back in, so try that first. If the problem unit was any unit other than the first, find the RJ11 for that unit (often the cable that is a different color than the others) and unplug. If it is the first unit, unplug from the board. Doing that, you have removed the problem from the system and unit #1 may immediately start back up. Likewise, plugging the cable back in may again kill the system, but you know you are on the right track.
Now, disconnect the cable again and simply take the RJ11 tool that you can buy at any Lowes or Home Depot and gently press on the end as though you were installing a new end and see if tightening it works. If it does, when you plug it back in and go through the reset procedure, you will be back in business. You may have to do this at both the master and the problem unit.
If that doesn’t work, you will have to make a test cable. You can find instructions for making RJ11 cables on the Internet, but here is the fastest way to do it. Purchase a telephone cable long enough to reach from the control unit to the problem unit, 25-feet is usually enough. Cut one end off the cable and prepare the cable with your tool just like you were going to put a new end on.
Now, take the new end and orient it exactly the same way as end still in place on the cable. That is, match the black wire on the same side of the end, when you slide the new end on. Now crimp it down with your tool. If you have done this step correctly, your cable ends will look exactly the same with the colors of each wire in the same order.
Unplug from the connector just like you initially did and plug the new cable in the connector. Now take the cover off the unit that is not working and unplug the cable end there at the connector so your test cable takes the place of the original cable. Run the reset procedure. If it works, you have a bad cable and your task is how to run a new RJ11. Yes that is often impossible or nearly so, but you bought this system, not me. If the test cable doesn’t work, you have a bad board on the problem unit and it has to be replaced.
Now if these troubleshooting procedures sound like bad news, you are on target. My own suggestion would be not to purchase any RV with a CCC2 system in place. Having a thermostat for each air conditioning unit is a much more reliable system and you will never have to deal with losing all your air conditioning at the same time due to a control issue. If you just have to have a single thermostat, make sure you do not buy an RV with the little return airs in the ceiling. Without the large ceiling return, there is no way to get to anything without going up onto the roof of the RV and removing the cover and the evaporator coil cover on the air conditioner.
As a finally note, make sure you are not operating fluorescent or LED lights in close proximity to your multi-zone thermostat. Those types of lights with upset the electronics in the CCC2 as well as other multi-zone systems.
Steve (Mobility RV Service)
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