Air/Fuel Mixture Control and Emissions
What you must know
CNG burns cleaner than gasoline - lower CO, HC, particulates, and CO2 per unit energy - but still produces NOx at high combustion temperature, so EGR and precise mixture control matter. Stoichiometric AFR for natural gas is about 17.2:1 by mass (vs. ~14.7:1 gasoline), which is why the ECM's fuel tables and the wide-range O2 sensor interpretation differ. Closed-loop control trims fuel based on O2 feedback; long-term fuel trim drifting positive signals a lean condition (vacuum leak, low fuel pressure, dirty injector). Know the role of the catalytic converter, EGR, and the OBD monitor for catalyst efficiency.
Traps
A frequent distractor is applying gasoline stoichiometry to CNG. Another is blaming the catalyst for a P0420 when an exhaust leak or a lean misfire is the real cause. Remember that CNG's cleaner exhaust can mask rich conditions - the odorant smell is not a mixture indicator. Also know that fuel trim limits and monitor enable conditions vary, so a code may not set until specific conditions are met.
Rule of thumb
Positive fuel trims (adding fuel) = lean; negative trims = rich. If both banks are lean, look for a shared cause (intake leak, fuel supply); if one bank, look at that side's injector or exhaust leak.
Worked example
LTFT is +22% at cruise on a dedicated CNG bus. Smoke-test the intake; a cracked intake gasket is found. Repair, clear trims, and verify LTFT returns within ±10%.