1997 Honda Accord P0420 Check Engine Light: Catalytic Converter vs Oxygen Sensor Diagnosis

The likelihood is approximately 90% that the issue lies with the catalytic converter, with only 5% attributed to the oxygen sensor and 5% due to other factors. The sluggish performance strongly suggests a restricted converter. As for the intermittent stalling, if the problem persists after replacing the converter, it could point toward ignition system faults or EGR valve issues that are causing fuel trim imbalances during acceleration.

snowman420420 wrote: I own a 1997 Honda Accord with 140,000 miles featuring a 2.2L VTEC engine. I recently received a check engine light triggered by fault code P0420. Most sources suggest either a failing catalytic converter or a malfunctioning oxygen sensor as the cause. The car exhibits noticeable sluggishness during acceleration, especially when demanding more power—though it doesn't completely stall under normal driving conditions. Given that this issue appears to worsen with increased throttle input, I'm trying to determine whether to prioritize replacing the catalytic converter or the oxygen sensor. Is there a reliable way to diagnose which component is actually at fault? Or am I looking at a 50/50 chance between these two parts? I would strongly lean toward replacing the catalytic converter—especially if the vehicle runs smoothly otherwise. (My own Honda with a bad O2 sensor behind the cat still runs fine.)

P0420 is triggered when the difference between the pre-catalyst and post-catalyst oxygen sensors is too low. In nearly all cases, this indicates a failing catalytic converter. Oxygen sensor faults typically generate specific diagnostic codes (out of eight possible), but none are present here. The post-cat O2 sensor does not affect engine drivability—it only monitors the converter's performance through pre-cat readings.

You'll likely need to replace both the catalytic converter and the oxygen sensor located within the converter assembly. Catalytic converters can be expensive, so be prepared for a significant cost—consider purchasing genuine Honda parts as they tend to perform more reliably than aftermarket alternatives. Be aware that the exhaust system may be severely rusted and brittle; expect potential issues such as broken bolts (a torch with oxy-acetylene can help loosen rusted fasteners) or cracked exhaust components, so take appropriate precautions during replacement.