Hi this may or may not help you. I have an efi diagnostics and repair manual and you are right code 11 is a/c load signal. This is so that when the a/c is on the ecu can detect this and adjust the idle up to compensate for the extra load. the book also show an a/c amplifier but does not state what its purpose is
This book states 1.check that the a/c is operating properly 2.when the a/cis on check that 8-14 volts is available between terminals E1 and a/c at the ECU wiring connector. If 8-14 volts is available renew the ECU.
Does not sound too helpfull.
But that will not be the reason for high fuel consumption normally
The main reasons are 1. ignition system 2.airflow meter 3.injectors 4.Fuel supply 5.throttle position sensor 6.coolant temp sensor.
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