2007 Vauxhall Zafira engine light code 171 and 1112 coad

P0171 - Fuel Trim System Lean Bank 1 - 10.4%
A P0171 or P0174 trouble code tells you the fuel mixture is running lean (not enough fuel and/or too much air). This type of problem can be confirmed by looking at the Short Term Fuel Trim (STFT) and Long Term Fuel Trim (LTFT) values with a scan tool. Normally, STFT and LTFT should be plus or minus 5 to 10 from zero. If STFT and/or LTFT are more than about +12, it indicates the engine is running lean. Negative numbers (-12 or more) indicates the engine is running rich.
A lean fuel condition can be caused by:
* Low fuel pressure due to a weak pump or leaky fuel pressure regulator. Use a fuel pressure gauge to check fuel pressure at idle. If fuel pressure is less than specifications, your fuel filter may be plugged, your fuel pump may be failing or have a bad wiring connection, or the fuel pressure regulator may be leaking.
* Dirty fuel injectors. Cleaning the injectors with a fuel system additive, or having the injectors professionally cleaned may solve your problem.
* Vacuum leaks at the intake manifold, vacuum hose connections or throttle body.
* Leaky EGR valve. Check the operation of EGR valve and system, and for a buildup of carbon under the valve.
* Leaky PCV Valve or hose. (Check valve and hose connections)
* Dirty or defective Mass Airflow Sensor (MAF). Try cleaning the MAF sensor wires or filament with aerosol electronics cleaner. Do NOT use anything else to clean the sensor, and do not touch the sensor wires.

z16xe1 engine there is a EGR distributor plate on the cylinder head which is underneath the inlet manifold. This plate gets blocked with carbon from the exhaust, also blocked are the four small holes which goes from the egr plate into the inlets off the cylinder block.

I think when the EGR valve opens, the fuel is increased because more air has just gone into the cylinders from the exhaust via the egr valve and distributed to the cylinders through this plate. But with the blockage the engine is now too rich, so the rear o2 sensor senses the engine is rich, the ecu then wrongly thinks the engine must be lean [air leak] to need to run rich. So adds fuel to the long term fuel trim. when this trim gets to 21% the ecu sets a system too lean code.

The inlet manifold needs to be removed. The plate needs to be removed from the cylinder head and cleaned and the holes in the cylinder block need to be cleared.