Low Fuel Pressure Indication

Nice aircraft!  And picking an emergency landing site looks easy ?.  I don’t think there is a ready made spot for a separate mechanical gauge.  I would put it where you existing sender is, and if you want them both installed at once you could install a temporary T fitting plus whatever adapter fittings you need. It will only be for ground testing.  

The Rotax adaptor fitting part number 841131 is installed by removing the banjo bolt at the regulator (item #11 in figure below)

(this position is the very end of the fuel rail, just before the fuel enters the regulator so it gives a accurate measurement of the rail pressure) 

Photo of adaptor and sensor is attached. This adaptor has M12x1.5 threads so the selector of sensors is limited. A possible solution is to add a 1/8NPT adaptor to the M12x1.5 port?

Thank you for this information. 

However being in Jamaica, we have no access to these special adaptors to hook up a simple pressure gage.

I shipped in a standard 1/8 npt pressure gage, but there is no way to use this tool without the special adaptors indicated.

I just received a Rotax fine filter and a new Rotax fuel pump assembly.

I'm praying that the fuel filter fixes the problem.

If not, then the pump assembly.

US Certified A&P mechanic coming tomorrow to fix it.

I will report back to the forum the results of our efforts.

Thank you to everyone who has replied to this post.

The "Other Input"  should be connected to the Airbox.  Verify that it still is !!!
Take note of the fuel pressure before the engine start.  ~3bar (44psi).
If it drops more than 0.2 bar (3psi) with the engine running and/or varies with the throttle, the Airbox Connection is suspect.
The constant displacement fuel pumps move the same amount of fuel at idle and WOT, so moving the throttle will not affect the pressure produced by the pumps.
This is a fuel-injected engine.  The rules for carburetors do not apply.

The mechanical gauge should be connected in place of the Pressure sender.
The fittings here are pipe threads and any "Special" fittings (Tees) will be readily available from any plumbing/Hardware store.

The engine ECU does NOT know what the fuel pressure is.
The ECU assumes the fuel pressure is correct.
If the pressure was truly low the engine would be running very lean.
This is most likely a Sensing/Display issue as indicated by the engine operating normally otherwise.
Do you see the fuel pressure decrease with Increasing altitudes and Decreasing throttle?

UPDATE 

I received a new Rotax fine fuel filter and a new fuel pump assembly from USA. Found a U.S. FAA certified mechanic to work on it. But still no solution. Checked the fuel pressure on the Garmin display with engine off pumps on. It was down to 19 psi. 
He replaced the fuel filter and we ran the pumps again. Now it was 18 psi. Started the engine and ran it for a few minutes. No change. 
Since there is no port on the fuel system to install a mechanical pressure gauge, we disconnected the AN fitting from the fine filter to the fuel rail. We attached a mechanical pressure gauge to the line from the filter. The engine side was not connected. Turned on the pumps and fuel pressure read on the Garmin display as high as 51 psi. On the mechanical gage it read about 90 psi. So the pumps were working. Called Super Petrel and they said replace pump assembly. The old pump assembly was contaminated with water but it seemed to be pumping fine. We had a bad odor of fuel for over a year but no one could pinpoint where it was coming from. Might have been the pump. 
He replaced the pump assembly and we ran it again. Garmin fuel pressure was now reading even lower than before. 16 psi. 
So we have exhausted our parts that were shipped in and still have no resolution. 
We are thinking it must be either the fuel pressure sensor or fuel pressure regulator. We took off the snap ring and inspected the fuel pressure regulator. All clean and no debris in the valve body. Since the engine was running fine, it’s probably a bad pressure sending unit. 
Any thoughts?