There is no simple answer as to why ECUs occasionally fail but we can make guesses for some of them.
The vast majority of Eberspacher heaters work for many years without faults, just needing servicing to keep them running. If ECUs were cheap to replace they would be considered simply as a part that very occasionally needs replacing. ECUs are painfully expensive so when one does fail most users want to know why.
A few ECUs fail because a component inside does not last as long as it normally should. All equipment suffers from failures of this type, such failures are inevitable and are one of the reasons for a manufacturers guarantee.
However Eberspacher say they are getting a higher ECU failure rate with heaters fitted on boats than on vehicles. Factors external to the heater must be the cause otherwise vehicle heaters would suffer the same failures. Two general causes of ECU failure are voltage spikes and overvoltage (longer duration than spikes) on the battery supply. Voltage spikes can be caused when connecting and disconnecting the battery and by electric welding. Overvoltage can be caused by switching on an older type charger when the battery is fully charged, the voltage rises for a very short period before it drops to normal. Modern boat electronics are said to be better so this should not occur. Eberspacher are investigating a problem if modern battery charges are connected with too thin cables, it can cause a ripple on the battery voltage which damages the capacitors. I have no further information on this problem at this time so cannot provide more advice.
From our own knowledge and contacts with users we can guess some other likely reasons for failures.
We do know that some definitely fail due to overheating conditions in the heater. Items placed where they completely block the heating outlet cause many failures. The heating and combustion air drawn into the heater help to cool components inside the heater including the ECU. Obstructions or installation errors to either of these air paths can be serious. Air ducts on boats are generally longer with more bends, increasing the back pressure which reduces air flow. It is not widely known that Eberspacher have specific limits on combinations of duct lengths and bends for each heater and these can easily be inadvertently exceeded. (See the 'Eberspacher starts normally but stops whilst running. It does not try to restart' section on Faults 2 page for more details). Heating ducts can be obstructed by items placed in front of outlets or by crushing / melting, see faults 2 page. Heaters are often installed in lockers with poor or no ventilation or in hot engine compartments raising the temperature of the case. There are a number of other reasons for overheating listed on the Faults 2 page. Truck heaters are usually professionally installed and maintained so generally not affected in this way.
Users often wonder why the overheat sensor fails to prevent failures. This is my best guess as to why it happens.
If the heating airflow is partially blocked there will still be some transfer of heat to the airflow, temperatures inside the heat exchanger will build up relatively slowly and the overheat sensor will shut off the heater with fault code 12, the fan will then continue running allowing some cooling after fuel is stopped. However if the heating airflow is almost completely blocked there will be very little heat transferred by the airflow. It takes time for the heat to transfer through the casing of the heat exchanger and the base of the overheat sensor so there will be a delay in detecting the overheating. Once the sensor does reach the normal overheat temperature limit the ECU shuts down the heater.
During that delay the heater continues burning fuel and much more heat is transferred to the heat exchanger inner surfaces. That heat then flows through the heat exchanger to the outside of the casing and with no cooling airflow the temperature rises rapidly until the overheat sensor is too hot for the system to measure. At that point the ECU is locked as a safety precaution to prevent the heater being restarted . A diagnostic readout would then give fault codes, 12 Overheat, followed by 17 or 15 Excessive overheating. If the heat level rises further permanent damage to the ECU can occur.
2) Battery problems
Leisure batteries do not have to supply the very heavy engine starting currents so deterioration is often not noticed.
Since we pointed this out on the website we discovered far more users had poor batteries than we expected. One user of our website had an ECU with a code 96 internal sensor fault and the external sensor input had become very insensitive so would not shut down the heat at normal temperatures. The ECU also showed fault code 99, too many resets - short term voltage drops, so the dealer asked if he had any battery problems. He had replaced the engine battery but believed the leisure battery condition was good so he had assumed it was ok. He was shore based with the leisure battery continuously charged by a CTEK charger. He purchased a new ECU before doing tests. With the charger switched off the voltage at the Eberspacher dropped to 9.5 volts during a starting test. When he replaced the battery with his engine battery and re-tested it was about 12v during starting. The Eberspacher was being powered mainly by the charger with the battery failing to smooth out the charger ripple and that had damaged the ECU. I also had three old leisure batteries which I used with an inverter to power electric drills during show demonstrations. When they finally let me down I did a capacity test on each of them, the best of the three had dropped from 110 Ah to less than 30, the other two even worse. The sulphation was caused by the batteries not being regularly charged and would have dropped the voltage during discharge. A similar problem can occur with leisure batteries in occasionally used boats. I suspect these two factors cause a lot of failures. Trucks are normally in regular use which charges the battery and would have the battery replaced if it failed to start the engine.
3) Unlike road vehicles nearly all boats have a battery isolation switch. Power can be disconnected before the heater has gone into or completed the 4 minute cool down sequence. This can cause the ECU to become very hot severely stressing the components. I guess this might happen fairly often especially if users are in a hurry to leave the boat.
4) DIY own goal failures.
Some ECUs are damaged by DIY repairs and poor quality installations.
DIYers have varying skills ranging from highly competent down to those who really should have used a dealer. Mistakes like shorting live wires or testing the fuel pump by connecting it direct to the battery without disconnecting the feed from the ECU are typical events. In one case someone disconnected the pump then applied the battery to the wiring back to the ECU instead of to the pump. And these are just some of ones who are brave enough to admit to us they caused the ECU failure. DIYers tend to wait if the heater has a problem that does not completely disable it. If it works again after switching it off and restarting they will usually continue to do so. They will also try things many times in the hope the fault will miraculously solve itself. Eventually the ECU dies which is when they start looking on the web for solutions and find our advice pages. Too late! One seller said he was repairing blown parts and users were fitting them without fixing the original fault, so blowing it a second time. He also has had second hand heaters he refurbished and sold returned as non working which he could tell were damaged by connecting the battery leads the wrong way round. He is finding several people damaged ECUs by mistaking the grey/red wire for either grey or red as the wire is striped along its length so looks a single colour if viewed from the wrong angle. He quoted fitting an 801 controller as a typical time it occurs. Truckers use trained mechanics so there is a better chance wiring and installation is done properly.
5) Obvious but often overlooked is the fact boats and water go together. Damp and corrosion may cause intermittent problems. Total immersion is definitely not good!