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)