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P0420 Catalyst System Efficiency Below Threshold (Bank 1)

The sensor after the catalytic converter says it isn't cleaning the exhaust as well as it should — but the converter itself is only sometimes the culprit.

Emissions

There are two oxygen sensors around the catalytic converter. The upstream one swings rapidly as the ECU trims fuel; the downstream one should be comparatively flat, because a healthy converter stores and releases oxygen and smooths those swings out. P0420 means the downstream sensor started mimicking the upstream one, which is what a worn-out converter looks like. It is also what a lazy downstream sensor, or an exhaust leak, looks like — which is why replacing the converter on this code alone is one of the most common expensive mistakes in DIY repair.

Diagnose P0420 on your specific vehicle Add your year, make and model for ranked causes, repair videos and the exact parts you need

Can I keep driving?

Driveable

Yes. The car will usually drive completely normally. The cost of ignoring it is a failed inspection, not a breakdown — unless there are misfire or fuel-trim codes stored alongside, in which case fix those first and quickly.

How serious

Low urgency mechanically, but it is an automatic emissions-test failure, and if the real cause is an upstream problem like a misfire or rich running, that cause is actively destroying the converter while you wait.

Symptoms you'll notice

What causes P0420

Ordered by what to check first — how likely each cause is, weighed against what it costs to rule out. Working down the list in order is what stops you replacing an expensive part to fix a cheap problem.

01

Exhaust leak

A leak anywhere between the two sensors — a cracked flex pipe, a failed gasket, a rusted seam — lets outside air in and skews the downstream reading. Cheap to fix and easy to miss, which is exactly why it belongs at the top of the list rather than the bottom. Check the exhaust before buying anything.

02

Failing downstream oxygen sensor

A lazy or contaminated sensor after the converter reports swings that aren't really there. A fraction of the cost of a converter and a very common true cause. Compare the two sensors' waveforms on live data before deciding.

03

An upstream fault damaging the converter

Misfires and rich running dump raw fuel into the converter and cook the substrate. If P0300, P030x or P0172 are stored alongside P0420, those are the real problem — and a new converter fitted without fixing them will fail again.

04

Worn-out catalytic converter

The genuine cause, and the expensive one — but it sits here rather than at the top on purpose. Converters do degrade with age and mileage, and much faster if they've been fed unburned fuel or oil. Eliminate everything above first: a converter is often the single most expensive part on an emissions repair, and replacing one that was never the problem is the classic P0420 mistake.

05

Engine burning oil or coolant

Oil ash and coolant silicates poison the catalyst permanently. A head gasket leak or worn valve seals will kill a replacement converter the same way it killed the first.

06

Aftermarket or high-flow converter

Cheap replacement converters frequently have too little catalyst material to satisfy the ECU's monitor, so a P0420 that returns shortly after a converter replacement often means the new part, not a new fault.

How to diagnose it

  1. Check for any other stored codes first. Misfire or fuel-trim codes alongside P0420 change the diagnosis entirely — fix those and re-test before touching the converter.
  2. Inspect the exhaust from the manifold back for leaks, especially at flex pipes and gaskets. Listen with the engine cold, when leaks are loudest.
  3. Compare upstream and downstream oxygen sensor waveforms on live data. The downstream signal should be relatively flat; if it swings nearly in step with the upstream one, the converter is genuinely not storing oxygen.
  4. Check the downstream sensor's response by forcing a brief throttle change and watching how quickly it reacts — a sluggish sensor is the cheap fix.
  5. Check for oil consumption or coolant loss, which will poison a replacement converter too.
  6. A temperature comparison across the converter — inlet noticeably cooler than outlet on a working one — is a useful confirmation with an infrared thermometer.

Parts you might need

Tools for the job

Related trouble codes

Get the parts list for your vehicle Ranked causes, tutorials and live part prices for your exact year, make and model

Fitment matters. Causes and parts above are the common cases for P0420 across vehicles. The specific part numbers, procedures and torque specs vary by year, make, model and engine — always confirm against service data for your vehicle before buying parts or starting work.