Catalytic Converter and EGR System Diagnosis
The catalytic converter is a three-way catalyst that simultaneously oxidizes hydrocarbons (HC) and carbon monoxide (CO) into CO2 and H2O, and reduces nitrogen oxides (NOx) into nitrogen and oxygen. Its effectiveness is monitored by comparing the oxygen sensor readings upstream (pre-catalyst) and downstream (post-catalyst). In a healthy system, the upstream sensor switches rapidly (1–3 times per second) as the ECU cycles the mixture lean and rich around stoichiometry. The catalyst stores and releases oxygen, buffering the downstream sensor — a healthy downstream sensor shows a slow, steady, slightly rich signal. P0420 (Catalyst System Efficiency Below Threshold, Bank 1) is set when the downstream sensor begins switching at a rate approaching the upstream sensor — indicating the catalyst's oxygen storage capacity is depleted and it is no longer processing exhaust gases effectively. A catalyst can fail from thermal damage (overheating from misfires feeding unburned fuel into the catalyst), physical damage (impact damage to the monolith substrate, heard as a metallic rattle), or poisoning (leaded fuel, coolant, or oil contamination of the exhaust stream). The EGR (Exhaust Gas Recirculation) valve recirculates a controlled amount of cooled exhaust gas into the intake manifold to reduce combustion temperatures, thereby reducing NOx formation. EGR faults: a stuck-open EGR valve allows exhaust gas into the intake at idle and low load, causing rough idle, hesitation, and stalling. A stuck-closed EGR valve prevents NOx reduction, triggering P0400–P0408 codes. Test the EGR valve by commanding it open with a scan tool (bi-directional control) at idle — a properly functioning valve causes a significant and immediate RPM drop as exhaust gas dilutes the intake charge. No RPM change indicates a stuck-closed valve or disconnected vacuum hose.