Chrysler
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    Fuel System Lean Conditions
Cause:
    ECT sensor over 170°F, engine running in closed loop, BTS ambient air input over 20°F, altitude less than 8000 ft., then the PCM detected an injector command that reduced pulse-width by 25% long term memory along with a 12% short term compensation command (a lean A/F condition).

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 input from 176-230°F, engine speed 1000-4000 rpm (0-4000 rpm on VIN 1), throttle angle steady below 91%, VSS under 70 mph, purge duty cycle below 80%, then the PCM detected that the LONGFT was over 100, or that the SHRTFT was below 94, either condition met for over 25 seconds

Nissan
Type:
    Powertrain - Fuel and Air Metering - ISO/SAE Controlled
Description:
    Fuel trim - Bank 1 - mixture too lean
Cause:
    Wiring, exhaust system, HO2S front, fuel system, injector, MAF sensor, ECM