What engines have a Bank 2, and which do not?

Bank 2 exists on engines with two cylinder banks: V6, V8, V10, V12 and boxer or flat layouts. Inline-4 and inline-5 engines have only one bank, so codes P0431 through P0434 should never appear on those. Most inline-sixes are single-bank too, but some, such as BMW's N52 and N54, split the exhaust into two halves with their own catalysts and oxygen sensors and report cylinders 4 to 6 as Bank 2, so they can set Bank 2 catalyst codes. If you see P0431 on an inline-4, the scan tool reading is almost certainly wrong.

Why does Bank 2 wear faster than Bank 1 on some engines?

On many V6 and V8 layouts, the Bank 2 exhaust runs closer to the firewall or has tighter packaging, which traps heat and accelerates substrate aging. On other engines, fuel-trim differences between banks (caused by intake manifold geometry or injector wear) lead to one bank running slightly richer and ageing the catalyst faster.

Can I just replace the Bank 2 pre-cat, or do I need to replace both?

If only P0431 is set and Bank 1 is healthy, you can replace just the Bank 2 unit. However, if both banks are at similar mileage (typical), the Bank 1 pre-cat is often within months of failing too. Replacing both at the same time consolidates the labour cost into one job.

Will P0431 alone cause a periodic technical inspection failure?

A stored catalyst code with the check engine light on is the kind of defect that gets a car rejected, whichever bank is affected, though each country runs and grades its own inspection. P0431 must be resolved and the readiness monitors must be set before retesting.

P0431: Warm Up Catalyst Efficiency Below Threshold (Bank 2)

Quick Answer

Safe to drive (short-term)

P0431 means the warm-up catalyst (the small pre-cat closest to the exhaust manifold) on Bank 2 is not heating up fast enough or converting cold-start emissions efficiently. Bank 2 is the cylinder bank that does not contain cylinder 1. It exists on V6, V8, V10, V12 and flat engines, and on some inline-sixes, such as BMW's N52 and N54, that split their exhaust into two halves and report cylinders 4 to 6 as Bank 2.

Severity: Moderate · Emissions
Estimated repair cost: From €125 to €1720+, varies by vehicle

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What does P0431 mean?

P0431 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0431 and illuminates the check engine light.

Where it is on the car

Engine
Oxygen sensor before the converter (sensor 1)
Catalytic converter
Oxygen sensor after the converter (sensor 2, or 3 on some cars)
Silencer and tailpipe

This code is for bank 2. V6, V8 and flat engines have two cylinder banks, each with its own converter and sensors; bank 2 is the side without cylinder 1.

What are the symptoms of P0431?

If your vehicle has triggered P0431, you may notice one or more of these symptoms:

Check engine light illuminated, often after a cold start
Elevated cold-start emissions, especially visible on a cold morning
Slight loss of fuel economy
Faint sulfur smell from exhaust during warm-up
May fail emissions or periodic technical inspection

What causes P0431?

Here are the most common causes of P0431:

Failing or worn warm-up catalytic converter on Bank 2 (pre-cat)
: high
Faulty downstream O2 sensor (Bank 2, Sensor 2)
: medium
Exhaust leak between Bank 2 manifold and pre-cat
: medium
Previous misfire history on Bank 2 cylinders damaging substrate
: low
Oil consumption from one Bank 2 cylinder fouling the pre-cat
: low

Is it safe to drive with P0431?

Generally yes, for short-term driving. Generally safe to drive short-term. P0431 needs a Bank 2 with its own catalyst: V6, V8, V10, V12 and flat engines, plus some inline-sixes that split their exhaust in two. Continued driving with active misfire codes can damage the downstream main converter on Bank 2.

How do you diagnose P0431?

Follow these steps to pinpoint the root cause of P0431:

Confirm the engine actually has two banks. If you see P0431 on an inline-4, suspect a scan tool error or a wiring fault
Read freeze frame data and verify coolant temperature was below 40°C when the code set, which is typical of cold-start warm-up cat faults
Scan for Bank 2 misfire codes (P0305 to P0308) and address those before condemning the pre-cat
Graph Bank 2 upstream and downstream O2 sensors during cold start. The downstream sensor should stabilise near 0.45 V within 60 to 90 seconds on a healthy pre-cat
Compare Bank 1 and Bank 2 downstream O2 sensor traces. If only Bank 2 is showing oscillation, the Bank 2 pre-cat is the likely culprit

How much does P0431 cost to fix?

Repair costs for P0431 vary depending on the root cause and your vehicle. Here are typical estimates:

Repair	Parts Cost	Labor Cost	Total Estimate	DIY Difficulty
Replace warm-up catalytic converter (Bank 2)	€280–€1400	€120–€320	€400–€1720	Professional
Replace downstream O2 sensor (Bank 2)	€55–€170	€70–€150	€125–€320	Moderate
Fix exhaust leak / replace manifold gasket (Bank 2)	€30–€130	€100–€240	€130–€370	Moderate

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

Full repair guides

Catalytic Converter Replacement Cost

P0431 on specific vehicles

P0431 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

P0431 on BMW

BMW 3 Series

See BMW-specific causes & cost

P0431 on Volkswagen

Volkswagen Touareg

See Volkswagen-specific causes & cost

P0431 on Audi

Audi A6

See Audi-specific causes & cost

P0431 on Mercedes-Benz

Mercedes-Benz E-Class

See Mercedes-Benz-specific causes & cost

These codes are often seen alongside P0431 or indicate related issues:

Common questions about P0431

What engines have a Bank 2, and which do not?: Bank 2 exists on engines with two cylinder banks: V6, V8, V10, V12 and boxer or flat layouts. Inline-4 and inline-5 engines have only one bank, so codes P0431 through P0434 should never appear on those. Most inline-sixes are single-bank too, but some, such as BMW's N52 and N54, split the exhaust into two halves with their own catalysts and oxygen sensors and report cylinders 4 to 6 as Bank 2, so they can set Bank 2 catalyst codes. If you see P0431 on an inline-4, the scan tool reading is almost certainly wrong.
Why does Bank 2 wear faster than Bank 1 on some engines?: On many V6 and V8 layouts, the Bank 2 exhaust runs closer to the firewall or has tighter packaging, which traps heat and accelerates substrate aging. On other engines, fuel-trim differences between banks (caused by intake manifold geometry or injector wear) lead to one bank running slightly richer and ageing the catalyst faster.
Can I just replace the Bank 2 pre-cat, or do I need to replace both?: If only P0431 is set and Bank 1 is healthy, you can replace just the Bank 2 unit. However, if both banks are at similar mileage (typical), the Bank 1 pre-cat is often within months of failing too. Replacing both at the same time consolidates the labour cost into one job.
Will P0431 alone cause a periodic technical inspection failure?: A stored catalyst code with the check engine light on is the kind of defect that gets a car rejected, whichever bank is affected, though each country runs and grades its own inspection. P0431 must be resolved and the readiness monitors must be set before retesting.

Read our detailed guides

Scan your car with Skanyx to diagnose P0431

Your car logged P0431. Skanyx reads it on a standard OBD2 adapter (Bluetooth LE on iPhone), tells you whether it is safe to keep driving, and shows the €125-€1720 repair range so you know the number before a shop quotes you. Free on iPhone and Android, and you do not need to be a mechanic to read it.