P0431: Bank 2 catalytic converter warm-up efficiency is below the limit

Warm-up catalyst efficiency below threshold, bank 2

Severity: Monitor

The code mostly means poorer emissions and sometimes lower fuel economy, and the bulletins describe only a lit light, so it can be driven while you book a check, but it will fail an emissions test.

It gets more serious if:

Inspect soon: Inspect soon if the engine loses power or runs rough, because a failing converter or another fault is then getting worse.

Source: how we rate severity

P0431 in short

P0431 means the bank 2 catalytic converter is not cleaning the exhaust well enough soon after a cold start. A worn or damaged converter is the usual cause, though oxygen sensors, exhaust leaks and wiring can also set it. It is usually safe to drive, but will fail an emissions test.

The engine computer judges the catalytic converter by comparing the oxygen sensor ahead of it with the one behind it. When the two read too alike soon after the engine reaches closed loop, it stores P0431, a warm-up efficiency fault on bank 2, the side without cylinder 1.

TroubleCodes.net and the Delaware DMV list both give the same generic title: warm up catalyst efficiency below threshold, bank 2. P0421 is the bank 1 version.

The converter is the usual suspect, and Mitsubishi found a converter layer separation problem on 2006-2008 Eclipse and Galant models. But replacing oxygen sensors does not always help: a Wrangler owner reports P0431 returning after all four sensors were replaced.

Most likely causes

These are general causes for this code across vehicles. None is specific to one make or model.

Cause 1: Catalytic converter worn out or damaged
Common general cause
Why it ranks here
TroubleCodes.net lists a faulty converter first. Mitsubishi bulletin TSB-13-15-001 says some exhaust-manifold-mounted converters on 2006-2008 Eclipse and Galant can have catalyst layer separation that sets P0421 or P0431.
What would change this
Rises if the downstream sensor mirrors the upstream sensor at warm idle. Falls if the downstream signal stays steady.
How to test the catalytic converter
Cause 2: Failed or slow oxygen sensors
Common general cause
Why it ranks here
TroubleCodes.net lists defective oxygen sensors, since the code is judged from their readings. One Wrangler owner reports replacing all four without curing the code, so it does not always resolve it.
What would change this
Rises if the sensor signals are slow or stuck. Falls if the sensors cycle correctly and the converter still reads poorly.
How to test the oxygen (o2) sensor
Cause 3: Exhaust leak near the sensors or converter
Possible — needs testing
Why it ranks here
TroubleCodes.net lists an engine exhaust leak, which can feed air to a sensor and distort the reading.
What would change this
Rises if there is a ticking or hissing from the exhaust at cold start. Falls if the exhaust is tight.
Cause 4: Damaged sensor wiring, or a faulty airflow or manifold pressure sensor
Possible — needs testing
Why it ranks here
TroubleCodes.net lists burnt, chafed or disconnected wiring and defective mass airflow or manifold pressure sensors.
What would change this
Rises if wiring near hot exhaust parts is burnt, or other airflow codes are stored. Falls if wiring and airflow data look normal.
How to test the mass airflow (maf) sensor

What to test first

Start at step 1. Each result tells you what it means and where to go next.

Step 1: Read all stored codes and freeze-frame data with a scan tool, and note any oxygen sensor, misfire or airflow codes stored with P0431.
Low riskTool: OBD-II scan tool
- IfOxygen sensor, misfire or airflow codes are stored too
  It meansFix those first, since they can cause or mimic P0431.
  Points to: Damaged sensor wiring, or a faulty airflow or manifold pressure sensor
- IfP0431 is the only code
  It meansInspect the exhaust and wiring.
  Go to step 2
Step 2: With the engine cold and off, look at the oxygen sensor wiring near the exhaust for burns, chafing or loose connectors, and look for soot marks that show an exhaust leak.
Use caution
Warning:Exhaust parts stay hot for a long time after the engine stops. Let the exhaust cool fully, and support the car safely if it is raised.
- IfWiring is burnt or a connector is loose
  It meansRepair the wiring.
  Points to: Damaged sensor wiring, or a faulty airflow or manifold pressure sensor
- IfSoot or a hissing leak is found at a joint or the converter
  It meansRepair the leak and recheck.
  Points to: Exhaust leak near the sensors or converter
- IfEverything looks sound
  It meansCompare the oxygen sensor signals.
  Go to step 3
Step 3: With the engine warm, watch the upstream and downstream oxygen sensor readings on a scan tool at idle. TroubleCodes.net says the upstream sensor should cycle between about 0.1 and 0.9 V while the downstream sensor sits near mid-range.
Low riskTool: scan tool with live data
- IfThe downstream sensor cycles like the upstream sensor
  It meansThe converter is not working well.
  Points to: Catalytic converter worn out or damaged
- IfA sensor is slow, stuck or reads wrongly
  It meansSuspect that oxygen sensor.
  Points to: Failed or slow oxygen sensors
- IfBoth look normal
  It meansAsk about bulletins and recheck after a drive cycle.
  Go to step 4
Step 4: Ask a dealer whether your VIN has a bulletin or extended converter warranty for P0431. Mitsubishi has one for 2006-2008 Eclipse and Galant models.
Low risk
- IfA bulletin or warranty applies
  It meansUse it before paying for a converter.
  Points to: Catalytic converter worn out or damaged
- IfNothing applies
  It meansHave a shop test the converter and sensors.
  Go to step 3

Symptoms

Check engine light on
Poorer fuel economy
Weak engine performance Gets more serious: stop and have it checked.
Failed emissions test

How to fix

Step 1: With the exhaust fully cool, look over the oxygen sensor wiring near the exhaust for burnt or loose parts.
Step 2: Ask a dealer whether your VIN has a converter warranty extension or bulletin for P0431.

Parts and cost context

Catalytic converter:Converters are one of the costlier emissions parts, so confirm the diagnosis and ask about warranty cover before buying one. Some states have a longer emissions warranty. Labour varies; get a quote.
Oxygen (O2) sensor:Do not replace oxygen sensors without comparing the upstream and downstream readings first.

Labor varies by vehicle and shop; get a quote before you approve a repair.

Compare used part prices on PartPriceGuide

On your vehicle

Mitsubishi

2006–2008 · Galant
Supported by vehicle data
Mitsubishi TSB-13-15-001 describes converter layer separation that sets P0421 or P0431 and extends the converter warranty in some states. Whether it is still current is not verified.
2006–2008 · Eclipse
Supported by vehicle data
The same Mitsubishi bulletin covers the Eclipse and Eclipse Spyder. Whether it is still current is not verified.

TSB-13-15-001Status unverified
Catalyst layer separation, MIL with P0421 or P0431
2006–2008eclipse, galant · issued April 24, 2020 · NHTSA 10173901
Extends the converter warranty to 10 years or 120,000 miles in certain states on affected vehicles whose manifold-mounted converters may have catalyst layer separation.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.
115Status unverified
Tech Talk on DTC P0421 / P0431
2004–2005galant · issued August 10, 2005 · NHTSA 10015715
Technical information on the two codes. The summary does not give the content.
We couldn't confirm whether this bulletin is still current, so it doesn't count as confirmation.

Jeep

2006 · Wrangler
Possible — needs testing
One 2006 Wrangler owner reports P0431 returning after all four oxygen sensors were replaced. This is an owner report, not a verified repair.

All Catalytic converter (catalyst efficiency) codes

P0431 questions

Is P0431 the same as P0421?

They share a title, but P0421 is bank 1 and P0431 is bank 2, the side without cylinder 1.

Do I replace the converter right away?

Compare the oxygen sensor signals first and ask about a warranty. Mitsubishi extended the converter warranty on some 2006-2008 models.

Sources and review

TroubleCodes.net DTC pages (P0342, P0385, P0394, P0400, P0431, P0447; P0413, P0414, P0416, P0417 read but not used) — TroubleCodes.net
Mitsubishi TSB-13-15-001: Catalyst layer separation, MIL with P0421 or P0431 (2006-2008 Eclipse and Galant) — Mitsubishi / NHTSA , April 24, 2020
NHTSA complaint ODI 10358798: 2006 Jeep Wrangler — NHTSA Office of Defects Investigation (consumer complaint)
Dodge 10182004: Added diagnostics for P0421 and P0431 (2003 Ram 1500, 2500, 3500) — Dodge / NHTSA , April 27, 2005
On-Board Diagnostic Trouble Codes (generic code title list) — Delaware Division of Motor Vehicles

Last reviewed: October 16, 2026
Written by: Saurabh K.
Editorial review: Pending
Technician review: Technician review pending

This page explains what a code usually means; it is not a substitute for a professional inspection. How we source and review · Report an error

Severity: Monitor

Most likely causes

Cause 1: Catalytic converter worn out or damaged

Cause 2: Failed or slow oxygen sensors

Cause 3: Exhaust leak near the sensors or converter

Cause 4: Damaged sensor wiring, or a faulty airflow or manifold pressure sensor

What to test first

Step 1: Read all stored codes and freeze-frame data with a scan tool, and note any oxygen sensor, misfire or airflow codes stored with P0431.

Step 2: With the engine cold and off, look at the oxygen sensor wiring near the exhaust for burns, chafing or loose connectors, and look for soot marks that show an exhaust leak.

Step 3: With the engine warm, watch the upstream and downstream oxygen sensor readings on a scan tool at idle. TroubleCodes.net says the upstream sensor should cycle between about 0.1 and 0.9 V while the downstream sensor sits near mid-range.

Step 4: Ask a dealer whether your VIN has a bulletin or extended converter warranty for P0431. Mitsubishi has one for 2006-2008 Eclipse and Galant models.