Emission controls · Powertrain · generic (SAE-defined)
P0431: Warm Up Catalyst Efficiency Below Threshold (Bank 2)
P0431 means the warm-up catalytic converter on Bank 2, the side of the engine opposite cylinder 1, is operating below the efficiency the engine computer requires.
Standard title per SAE J2012 / ISO 15031-6 · Updated 2026-11-07 · By CarWhere Research
How P0431 sets
Where a small warm-up converter sits close to the Bank 2 exhaust manifold, the powertrain control module (PCM) compares oxygen sensor readings before and after it. If the downstream signal tracks the upstream signal too closely during a qualifying warm drive, the PCM sets P0431. Converter layout and monitoring methods vary by manufacturer.
Common causes of P0431
Roughly most likely first; the actual cause depends on the vehicle and needs testing to confirm.
- Worn or damaged Bank 2 warm-up converter
- Converter damage from misfires, oil consumption, or coolant entering the exhaust
- Exhaust leak on Bank 2
- Faulty Bank 2 downstream oxygen sensor
- Fuel control problems on Bank 2
P0431 symptoms
- Check engine light on
- Usually no drivability change
- Possible sulfur odor
- Loss of power if restricted
How P0431 is diagnosed
- Check for and fix misfire, fuel trim, and oxygen sensor codes first
- Identify Bank 2 and the warm-up converter location in service information
- Inspect for exhaust leaks at the manifold and flanges
- Compare upstream and downstream sensor signals at warm cruise
- Test downstream sensor response
- Check for oil burning or coolant loss
Typical fixes for P0431
- Correct the underlying engine problem
- Repair exhaust leaks
- Replace a failed oxygen sensor
- Replace the warm-up converter or manifold-converter assembly
Replace parts only after testing confirms the cause — a code names the circuit or condition that failed a check, not necessarily the faulty part. We don’t publish per-repair prices; for what households actually spend on upkeep, see car repair costs.
Manufacturer notes
Definitions can vary by manufacturer; check factory service information. Many vehicles report this condition as P0430 instead.
Related codes & warning lights
P0431 FAQ
What does code P0431 mean?
P0431 means the warm-up catalytic converter on Bank 2, the side of the engine opposite cylinder 1, is operating below the efficiency the engine computer requires. The standard definition is "Warm Up Catalyst Efficiency Below Threshold (Bank 2)."
Can I drive with a P0431 code?
In most cases yes, if the engine runs normally. A flashing check engine light, misfiring, or lost power means the converter may be overheating or restricted, so minimize driving until it is diagnosed.
Where is the Bank 2 warm-up converter?
It is typically built into or bolted directly to the Bank 2 exhaust manifold, upstream of the main underbody converter.
Is P0431 common?
It is less common than P0430 because fewer manufacturers monitor the warm-up converter as a separate component.
Keep researching
More emission controls codes
Sources
- SAE J2012 — Diagnostic Trouble Code Definitions (standard code titles)
- ISO 15031-6 — Road vehicles: communication between vehicle and external equipment for emissions-related diagnostics, Part 6: Diagnostic trouble code definitions
- U.S. EPA — Vehicle Emissions Inspection and Maintenance (I/M) programs
- U.S. EPA, 40 CFR 86.1806-17 — Federal onboard diagnostics requirements for light-duty vehicles
- California Air Resources Board — On-Board Diagnostic (OBD) Program
General information written by CarWhere Research, not a diagnosis of your vehicle. Manufacturers can add their own sub-definitions and test conditions; confirm with the factory service information for your year, make and model. Before paying for a repair, check NHTSA for open recalls and your warranty booklet: federal law requires an emissions warranty on major emission-control parts such as the catalytic converter and engine computer.