Fault codeP0420

MODERATE

Catalyst System Efficiency Below Threshold (Bank 1)

What this code means

The catalytic converter no longer cleans the exhaust as well as expected. Often linked to an ageing converter or to untreated misfires.

“Bank 1” is the side of the engine that holds cylinder number 1; “Bank 2” only exists on V and flat engines. An in-line engine has a single bank.

P0420 is a generic code: it means the same thing on every make. P0420 belongs to family P04xx — Emission controls. Its page explains how to read a code and groups all its codes by part.

Catalytic converter

The catalytic converter turns the most harmful exhaust gases into less harmful ones. The control unit judges its efficiency by comparing the oxygen sensors before and after it: a converter in good condition smooths the signal of the downstream sensor.

Symptoms

Most of the time, nothing you can feel from the driver's seat: the check engine light comes on, and that is all. The control unit compares the oxygen sensor before the catalytic converter with the one after it; when the second starts swinging like the first, it concludes the converter no longer stores oxygen. A rotten-egg smell from the exhaust or a slight loss of power can accompany a converter that is genuinely dead.

Common causes

Worn catalytic converter — the most common cause past 150,000 km, but not the only one
Ageing downstream oxygen sensor (sensor 2) sending a false signal
Untreated misfires or a rich mixture (P0300, P0172) that have overheated the converter
Exhaust leak upstream of the downstream sensor, or a leaking manifold gasket
Burnt oil or coolant coating the substrate (head gasket, piston rings)

How to diagnose it

Before replacing the converter — the most expensive part of the exhaust — look at what surrounds it. First check that no other code is present: a misfire or an incorrect mixture often explains the P0420, and will bring it back on a new converter.

In live data, compare the two oxygen sensors on the same bank with the engine warm: sensor 1 should swing quickly between 0.1 and 0.9 V, sensor 2 should stay roughly steady around 0.6 to 0.7 V. If sensor 2 mirrors sensor 1, the converter is no longer working. If it is frozen or slow to react, it may be the sensor itself. Finally, inspect the exhaust for a leak between the manifold and the downstream sensor.

Watch it live in Klaro

With an OBD2 adapter plugged in, these live readings show the part at work, when your car reports them:

O2 sensor voltage, bank 1 sensor 1
O2 sensor voltage, bank 1 sensor 2
Catalyst temp. bank 1 sensor 1
Catalyst temp. bank 1 sensor 2

What to do

Have it checked this week. The car remains usable, but the fault will not clear itself and will fail the roadworthiness test.

Clearing a code switches the light off but repairs nothing: if the fault remains, the code comes back. Clearing a fault code yourself →

Repair cost

An oxygen sensor costs €80 to €200 fitted. An original converter runs from €400 to over €1,500 depending on the model, fitted; cheap aftermarket converters rarely last more than two years and do not always pass the emissions test.

How to read this code

PPowertrain
0standardised code
4Emission controls
20P0420