Toyota 4runner · Diagnostic Trouble Code
P0420 — Catalyst System Efficiency Below Threshold (Bank 1)
What it means, how urgent it is, the most likely causes, and the exact order to diagnose it — with the repair guides that fix it.
What P0420 means
The PCM judges the converter by comparing the upstream and downstream oxygen sensors. The upstream sensor switches rapidly between rich and lean as the computer fine-tunes the mixture; a healthy converter acts as an oxygen buffer, so the downstream sensor's signal should be slow and flat. When the downstream signal starts switching almost as fast as the upstream one, the buffer is gone — the converter's efficiency has dropped below the threshold — and P0420 sets.
This code is about emissions performance, not an immediate breakdown. But it almost always means the converter is genuinely worn out, especially past 100,000 miles.
⚠️ How urgent is it?
It becomes urgent if you notice a rotten-egg sulfur smell, rattling from under the car, or a major loss of power: those point to a converter that is breaking apart or clogging, which can overheat the exhaust and choke the engine. Also fix any accompanying misfire codes immediately — misfires are the fastest way to finish off a converter.
Most likely causes
- A worn-out catalytic converter that can no longer store oxygen efficiently — the most common cause on higher-mileage vehicles.
- An engine that has been running rich or misfiring, overheating and degrading the converter's internals over time.
- Oil or coolant contamination from burning oil or a leaking head gasket, coating the catalyst and killing its efficiency.
- Physical damage to the converter's substrate from impacts or severe misfires.
- A faulty downstream oxygen sensor giving a false efficiency reading (less common — verify before replacing the cat).
- An exhaust leak near the converter skewing the downstream sensor's readings.
How to diagnose it — in order
-
1
Fix misfire, lean, or rich codes FIRST. A new converter will quickly die again if the underlying problem that killed the old one is still present.
-
2
Compare the upstream and downstream O2 sensor waveforms. A healthy converter flattens the downstream signal; if the downstream sensor mirrors the upstream's rapid switching, the converter is not doing its job.
-
3
Do the temperature test. At operating temperature, the converter's outlet should be noticeably hotter than its inlet (the chemical reaction generates heat). Similar temperatures in and out mean a dead cat.
-
4
Check for contamination. Blue exhaust smoke (oil) or sweet-smelling white smoke (coolant) means the new converter is at risk until that is fixed.
-
5
Rule out the downstream O2 sensor. If its heater circuit is bad or its signal is stuck, replace the sensor before condemning the converter.
-
6
Listen for rattling from the converter housing — broken substrate material bouncing inside confirms internal failure.
Reading the code is step zero
A basic OBD2 scanner plugs into the port under your dash and tells you exactly which codes are stored — plus freeze-frame data showing what the engine was doing when the code set.
View OBD2 scanners on AmazonAs an Amazon Associate we earn from qualifying purchases.
Repair guides that fix P0420
General diagnostic information, not a factory service procedure. If you're unsure, have a qualified technician confirm the diagnosis before buying parts.