Chrysler
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    Fuel System Lean Conditions
Cause:
    Engine running in closed loop, altitude less than 8000 ft., BTS input over 20°F, then test started and PCM detected that a lean air/fuel condition was present in the test period.

Ford
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    Fuel System Too Lean Conditions
Cause:
    Engine running at Cruise speed for 3-4 minutes, then the PCM detected that the Adaptive Fuel Control System was at its rich correction limit (due to a lean A/F ratio)

Generic
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    System Too Lean

GM
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    Fuel Trim System Lean Conditions
Cause:
    No PCM codes set, ECT from 140-239°F, IAT from -22 to 140°F, Fuel Trim and closed loop enabled, BARO over 70 kPa, engine speed 500-4000 rpm, throttle angle less than 70%, MAP input from 20-90 kPa, MAF from 5-100 g/sec, VSS under 75 mph, EVAP duty cycle under 75%, then the PCM detected that the LONGFT was over 155, or that the SHRTFT was at 138 for 10 seconds

Nissan
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    Fuel trim - Bank 2 - mixture too lean
Cause:
    Wiring, exhaust system, HO2S front, fuel system, injector, MAF sensor, ECM