P0420 (catalyst efficiency below threshold, bank 1) is the single most common reason a perfectly good catalyst is replaced. The ECU is comparing pre-cat O₂ activity to post-cat O₂ activity — so the code flags either a weak catalyst or a misreporting downstream sensor.
The logic behind the code
A healthy catalyst stores oxygen. The post-cat sensor should read relatively flat (steady voltage) while the pre-cat cycles. When the downstream activity increasingly mirrors the upstream, the catalyst has lost storage capacity — or the downstream sensor has drifted.
Gas-based evidence
At the tailpipe with a warm engine:
- HC and CO remain low: catalyst still oxidising — P0420 likely a sensor issue.
- HC and CO elevated past tailpipe limits: real catalyst failure. Compare pre-cat and post-cat where accessible.
- Temperature rise across cat: working catalysts raise exhaust temperature. A cold cat is a dead cat.
Sensor-side checks
- Compare upstream and downstream lambda activity under steady cruise — downstream should be notably flatter.
- Swap sensor (if supported) and see if the issue follows the sensor.
- Look for contaminants — silicone, coolant, oil — that killed upstream and may be lying in wait at downstream.
// before you replace a cat
Always verify HC and CO output at the tailpipe. If both are low, your cat is converting. Don't throw £500 of catalyst at a £40 sensor problem.