A P0420 code indicates a problem with the catalytic converter system, while a P0138 code points to an issue with the upstream oxygen sensor, specifically a slow response. These two codes can be related, as a malfunctioning upstream sensor can prevent the engine control unit (ECU) from accurately assessing the catalytic converter’s performance.
The P0420 diagnostic process involves the ECU comparing signals from the upstream (sensor before the catalytic converter) and downstream (sensor after the catalytic converter) oxygen sensors. By analyzing these signals, the ECU determines if the catalytic converter is effectively reducing emissions. The upstream sensor, also known as a wideband sensor, measures the air-fuel ratio in the exhaust gases, providing crucial data for fuel injection adjustments and emissions control.
However, if the upstream oxygen sensor is faulty, sending incorrect or delayed signals, the ECU might not even initiate the catalytic converter diagnostic sequence. In such cases, a P0138 code will likely appear first. This was precisely the scenario encountered with a personal vehicle: a P0138 code initially appeared due to a bad upstream oxygen sensor. After replacing both upstream and downstream oxygen sensors, the P0138 code was resolved. Subsequently, a P0420 code appeared because the now-functioning ECU could accurately assess the catalytic converter and determined it was underperforming.
In this instance, further investigation revealed an outstanding recall for the catalytic converter. The dealer replaced the entire assembly free of charge under the recall. Had the recall not been an option, a cost-effective alternative would have been a Walker replacement catalytic converter, although local regulations regarding aftermarket parts should always be considered. Replacing the faulty upstream oxygen sensor allowed the ECU to correctly diagnose the catalytic converter issue, leading to the proper repair. Therefore, understanding the critical role of the upstream oxygen sensor in the diagnostic process is essential for resolving both P0420 and P0138 codes effectively.
