Code 71:
AFRTrimValveOutput
FTV modulation driver signal fault

Check FTV for an open wire or FTV connector being disconnected
FTV PIN A (SIGNAL) TO PIN B (PWR)
Check FTV for an open coil by disconnecting the FTV connector and measuring the resistance (~26Ω +/-2Ω)
FTV PIN A (SIGNAL) TO PIN B (PWR)

Code 73:
AFRTrimValveUpperDC
FTV duty cycle at high (rich) limit
(Expected fault when FTV connector is unplugged)

Engine measured Air/Fuel ratio at the O2 sensor is excessively rich
Check FTV connector wiring for an open circuit
FTV PIN A (SIGNAL) TO PIN B (PWR)
Check FTV for an open coil by disconnecting the FTV connector and measuring the resistance (~26Ω +/-2Ω)
FTV PIN A (SIGNAL) TO PIN B (PWR)
* Check N-CA55-500-TR mixer for heavy end build- up and operation (see mixer section)
* Check N2001 secondary for operation (see N2001 Regulator section)

Code 74:
O2SensorSwitching
O2 sensor is not switching across the reference AFR voltage

Check the FTV for proper operation
* Check FTV Hose Connections
Check FTV for an open coil by disconnecting the FTV connector and measuring the resistance (~26Ω +/-2Ω)
FTV PIN A (SIGNAL) TO PIN B (PWR)

Could you tell me what the codes mean?
71, 73, and 74.

Thanx for your help.

There are two fuel trim valves that control the converter mix,if you follow the hose from the converter vacuum you should find them,can try cleaning them with diesel fuel but it's best to just replace them,also in one of the Y fittings that go to/from the valves is a restriction/orifice,make sure this is clear.