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If EGR function is confirmed but NOx remains high, the catalytic converter has lost its NOx reduction efficiency. O2 sensors monitor mixture but do not directly reduce NOx. Throttle and ignition faults affect HC or drivability, not NOx directly. ASE stresses linking NOx persistence to catalyst inefficiency after primary controls (EGR) are verified.

If the misfire moves when the coil is swapped, the coil is confirmed faulty. Crank sensors affect multiple cylinders, not one. Injectors or vacuum leaks do not move with coil swaps. ASE stresses systematic swap testing as a confirmation step for ignition faults.

If fuel pressure is stable but trims show lean under boost, the PCM is underfueling due to incorrect airflow reporting, often from a faulty MAF. EGR faults cause NOx, not detonation under boost. Ignition issues would create misfire but not lean trims. Exhaust restrictions reduce power, not cause detonation. ASE stresses verifying MAF accuracy under dynamic load.

A thermostat stuck open prevents coolant from reaching normal temperature quickly, keeping the PCM in open loop and reducing fuel economy. NOx, HC, or misfires are not the primary outcome. ASE emphasizes how coolant temp directly affects PCM loop readiness.

The secondary air system injects oxygen into the exhaust during cold start to promote rapid catalyst light-off. If the system fails, HC emissions remain high until the converter reaches operating temperature. CO and NOx are not directly affected. ASE stresses associating secondary air faults with cold-start HC levels.