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An increasing number of zero emission powertrain technologies will be required for meeting future CO2 targets. While this demand will be met by battery and fuel cell electric vehicles in several markets, other solutions are needed for harder to electrify sectors. Hydrogen (H2) internal combustion engines (ICEs) have become an attractive option for high power, high duty cycle vehicles and are expected to play a strong role in achieving zero emission goals. A unique characteristic of H2 ICEs is their ability to operate extremely lean, with lambda (λ) greater than 2. At such conditions, a multitude of benefits are observed including higher thermal efficiency, lower engine-out nitrogen oxides (NOx) emissions, and mitigating common problems with H2 abnormal combustion such pre-ignition and knock. However, two critical issues arise during extreme enleanment of H2 ICEs which have practical implications on controls and calibration of these engines. The first is the ability to properly measure air fuel ratio (AFR); both in a test cell environment and on-vehicle. The second is the deteriorating combustion efficiency with enleanment despite relative engine stability. In this study, several sources of error when measuring AFR for H2 ICEs are discussed and quantified. A H2-specific AFR equation is derived and the sensitivity to various measured combustion products is explored. It is shown that among these, H2 fuel slip introduces the highest sensitivity to exhaust-measured AFR. The challenge this H2 slip AFR sensitivity poses for closed-loop transient controls is explored and the impact on NOx emissions is highlighted.