Error code · Engine

P0421 — Warm Up Catalyst Efficiency Below Threshold (Bank 1)

Safe to drive

Open this code in the appDownload the app

A fault code doesn’t yet mean the unit or sensor itself is faulty: often it’s the plug and connector leading to it.

Possible causes

What you can do yourself

  1. The car pulls as usual — you can keep driving.
  2. Weak pull and the engine “chokes” uphill — the catalyst is clogged, see a mechanic right away.

Find the part on Kaspi ShopBefore buying, check the part number or match the part by VIN in the Kaspi “Garage”.

Check how the second O2 sensor works on the “Sensors” tab.

OA case from Oleg’s practice

  • Mitsubishi Outlander 2007 Check Engine: P0421 and P0431 — the catalytic converters. The owner suspected the oxygen sensors, but these codes don’t confirm a sensor fault: the converters and exhaust are checked first. How it went ↗

The explanation comes from the error code reference in the AvtoElektrik PRO app.

Need a mechanic’s help?

Couldn’t fix it yourself? Oleg, an auto electrician, does computer diagnostics and repairs: he comes out anywhere in Almaty or sees you at his workshop.

Car computer diagnostics in AlmatyCall: +7 701 121 11 10

Want to figure it out yourself?

Oleg’s course “Car computer diagnostics with a scan tool”: how to hook up a scanner, read codes and live data, and use them to find the cause. The first lessons are free.

Scan tool diagnostics course →

Nearby codes

All codes in P04xx →

Who keeps this reference

The reference is kept by Oleg Dmitriev, an auto electrician from Almaty with 26 years of experience: computer diagnostics of cars and trucks, repair of control units (ECUs). Can’t find the cause? Message him.

Oleg’s auto electrician websiteWhatsApp +7 701 121 11 10