When diagnosing a vehicle with a P0420 trouble code, what should be done first?

When diagnosing a vehicle with a P0420 trouble code, the first step should be to test the upstream and downstream oxygen sensors. This is because the P0420 code indicates that the catalytic converter's efficiency is below the threshold, but before concluding that the catalytic converter itself is malfunctioning, it is essential to verify the performance of the oxygen sensors involved in monitoring the exhaust stream.

The upstream oxygen sensor measures the amount of oxygen in the exhaust before it passes through the catalytic converter, while the downstream sensor checks the exhaust after it has exited the converter. If either of these sensors is faulty or not functioning correctly, it could lead to an incorrect reading, making it seem as though the catalytic converter is failing when it may not be.

By testing these sensors first, a technician can determine if they are giving the correct readings and ensure that the root cause of the issue is accurately identified. This approach helps avoid unnecessary replacements or repairs, focusing on what may be the actual area of concern rather than prematurely changing components without verifying their condition through proper diagnostics.