Poor Fuel Mileage

Ensure fuel pump is operating. Check fuel pressure and pressure regulator. Check for leaks or restrictions.
Check injector operation one at a time. Engine RPM should drop noticeably. Check opening duration. Check injectors for leaks.
Check coolant temperature sensor operation.
Check throttle position sensor for proper adjustment and operation.
CO too high. Ensure mass airflow meter filament is not broken and burn-off function is working.
On 900 Series vehicles equipped with catalytic convertors, check idle speed and pulse ratio.
Ensure electrical harness, connectors and wires are not broken or loose. Check all input signals to ECU.