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Secondary air injection promotes catalyst light-off by adding oxygen during cold start. Failure leads to high HC emissions until the converter heats up. CO and NOx are not directly affected by this system. ASE stresses associating air injection faults with cold-start HC failures.

Low MAF readings at WOT, combined with lack of power, strongly suggest an airflow restriction before the sensor measures intake flow. A partially collapsed duct or clogged filter reduces airflow and prevents the sensor from registering expected volumes. A vacuum leak after the throttle body would increase unmetered air, usually producing lean trim codes rather than low airflow. A stuck-open EGR valve would cause rough idle but not consistent low WOT readings. An oxygen sensor fault would not directly affect MAF readings. ASE stresses correlating sensor data with mechanical system inspection. Verifying airflow restrictions by checking ducts, filters, and intake obstructions is a critical step in diagnosing low-power complaints.

Idle-only lean conditions that normalize under load indicate unmetered air from an intake gasket leak. Dirty injectors or pumps affect load. O2 faults wouldn’t normalize under load. ASE stresses analyzing trim patterns at idle versus load.

A wide spark plug gap requires higher voltage but does not affect burn time if mixture and compression are correct. Lean mixtures shorten burn. Rich mixtures lower voltage. Weak coils reduce voltage output. ASE stresses waveform use for detecting plug gap faults.

When EGR operation is confirmed and passages are clear, but NOx remains high, the catalytic converter is not performing. The converter provides the secondary NOx reduction after combustion. O2 sensors and vacuum leaks affect mixture, not direct NOx conversion. Throttle plates affect idle airflow, not NOx. ASE stresses knowing which system controls which pollutant: NOx is primarily controlled by EGR (combustion temps) and catalysts (after-treatment).