# P0431: Warm Up Catalyst Efficiency Below Threshold (Bank 2) **Verdict:** Drive to a shop this week Emissions - Generic OBD-II code, applies to all gasoline cars and light trucks sold in the US since 1996. - **Typical cost:** $150-$2,800 - **Urgency:** This week - **Most likely:** Worn bank 2 warm-up converter - **DIY?:** Steps 1-2 ## Quick answer P0431 means the engine computer has decided the warm-up catalytic converter on bank 2 is not working efficiently. Bank 2 is the side of a V6, V8, or flat engine that does not contain cylinder 1, so this code only appears on engines with two banks. A worn converter is the usual cause, but leaks, a slow oxygen sensor, or bank 2 misfires can set it too. ## What does P0431 mean? V6, V8, and flat engines split their cylinders into two banks, each with its own exhaust manifold, oxygen sensors, and usually its own converters. Bank 2 is the side opposite cylinder 1. The warm-up converter on each bank sits close to the engine so it starts working within moments of a cold start, when an engine produces most of its pollution. Which side is bank 2 varies by engine, so a shop confirms it from service information before replacing parts. To grade the converter, the engine computer compares the bank 2 oxygen sensor ahead of it with the bank 2 sensor behind it. A converter that still stores oxygen keeps the rear signal fairly calm. If the rear sensor mirrors the front one during the test, which usually runs at warm, steady cruise, a pending code sets. Some makers use P0431 for a small pre-converter, others for that bank's only converter. Because each bank is graded separately, P0431 on its own points to something specific to that side. ## What are the symptoms of P0431? - Check engine light on with no change in how the car drives - Failed emissions inspection - Rotten-egg smell from the exhaust on some vehicles - Rattle from one side of the engine if the converter core has broken - Weak acceleration if the bank 2 converter is partly blocked ## What causes P0431? Ranked from most to least likely. Check them in this order: the cheap, common causes first. 1. **Worn bank 2 warm-up converter** (most common). Converters wear gradually as heat and age reduce the coating's ability to store oxygen. On a V engine the two banks do not always age at the same rate, so one converter can fall below the threshold first. If bank 1 later sets P0421 too, both converters are probably wearing out together, which is worth knowing when you compare quotes. 2. **Exhaust leak on the bank 2 side** (common). A cracked manifold, failed gasket, or leaking joint on the bank 2 side lets outside air reach the sensors and skews the test. Rear-bank manifolds on some transverse engines are hard to see and can crack after many heat cycles. A ticking noise from one side of the engine on a cold start is a common clue. 3. **Bank 2 downstream oxygen sensor** (common). The sensor behind the bank 2 warm-up converter can slow with age or contamination and make the converter look worse than it is. Watch for mixed-up wiring after earlier repairs, because bank 1 and bank 2 sensor connectors can sometimes be swapped. Also check for a heater code on the same sensor, such as P0161. 4. **Misfires or fuel trim problems on bank 2** (less common). A weak coil, worn plug, or leaking injector on a bank 2 cylinder sends raw fuel into just that converter and overheats it. A lean or rich fuel trim on one bank only, shown by codes such as P0174 or P0175, points to a problem on that side. Fix these first, or a new converter can be damaged quickly. 5. **Oil or coolant reaching bank 2 exhaust** (rare). A leaking head gasket, cracked cylinder head, or worn valve seals on bank 2 can let coolant or oil into that bank's exhaust, where it coats the converter. White smoke, coolant loss, or rising oil use alongside P0431 is a warning sign. That needs engine repair before any replacement converter will last. ## How do you diagnose P0431? 1. Scan for all codes and fix misfire, fuel trim, and oxygen sensor heater codes on bank 2 first. 2. Confirm which side is bank 2 for your engine from service information before testing or ordering parts. 3. Inspect the bank 2 manifold, gasket, and front pipe for leaks with the engine cold. 4. Compare bank 2 sensor readings with bank 1 at steady cruise. A healthy rear sensor stays steadier than its front partner. 5. Check coolant level and oil consumption if the converter looks contaminated. ## How do you fix P0431, and what does it cost? | Fix | Typical cost | What it involves | | --- | --- | --- | | Repair bank 2 exhaust leak | $150-$900 | Rear-bank manifolds can need more labor to reach. | | Replace bank 2 downstream oxygen sensor | $150-$400 | Often the cheapest fix that clears a borderline result. | | Replace bank 2 warm-up converter | $800-$2,800 | Manifold-integrated parts and CARB-compliant converters cost the most. | Full price breakdowns: [catalytic converter replacement cost](/cost/catalytic-converter-replacement), [oxygen sensor replacement cost](/cost/oxygen-sensor-replacement), [exhaust manifold gasket replacement cost](/cost/exhaust-manifold-gasket-replacement). ## Is it safe to drive with P0431? > **Drive to a shop this week.** It is usually safe to drive for a short time while the cause is found. Get the car checked right away if it loses power, a converter rattles, or the check engine light flashes, because a bank 2 misfire can overheat that converter. ## Will P0431 fail an emissions test? P0431 with the check engine light on fails an OBD-based emissions test in most states, exactly as a bank 1 converter code would. Clearing the code resets the catalyst monitor, and it may take several drives before it shows ready again. In California and states using its rules, the replacement converter must be CARB-approved for your vehicle. ## After the repair: clearing P0431 Clear the code once the repair is done and drive normally, including steady cruising with a fully warm engine. The catalyst monitor checks both banks and often needs several trips. The repair worked when the monitor reads complete, neither P0431 nor P0421 returns as a pending code, and the bank 2 rear sensor stays fairly steady. ## How to prevent P0431 - Replace spark plugs and coils on both banks on schedule, including the harder-to-reach side. - Fix misfire codes for bank 2 cylinders right away to protect that converter. - Watch coolant level and oil use, since contamination often affects one bank first. - Repair manifold ticks and exhaust leaks early, before they skew oxygen sensor readings. - Use only sensor-safe sealants near the engine, since ordinary silicone can contaminate oxygen sensors. ## Codes often seen with P0431 - [P0430](/codes/p0430): Catalyst System Efficiency Below Threshold (Bank 2) - [P0421](/codes/p0421): Warm Up Catalyst Efficiency Below Threshold (Bank 1) - [P0420](/codes/p0420): Catalyst System Efficiency Below Threshold (Bank 1) - [P0300](/codes/p0300): Random/Multiple Cylinder Misfire Detected - [P0174](/codes/p0174): System Too Lean (Bank 2) - [P0161](/codes/p0161): O2 Sensor Heater Circuit (Bank 2 Sensor 2) ## Frequently asked questions ### What does bank 2 mean on P0431? Bank 2 is the side of a V6, V8, or flat engine that does not contain cylinder 1. Each bank has its own exhaust and sensors, so P0431 points to the warm-up converter on that side only. Your service manual or a shop can confirm which side is bank 2 on your engine. ### Can I drive with a P0431 code? Yes, you can usually drive with P0431 for a short time, because it mainly affects emissions. You will fail an emissions inspection, though. Get it checked quickly if the engine loses power, a rattle comes from one side of the engine, or the check engine light flashes from a misfire. ### How much does it cost to fix P0431? Most P0431 repairs cost $150 to $2,800 at an independent shop. A bank 2 exhaust leak runs about $150 to $900 and a downstream oxygen sensor $150 to $400. Replacing the bank 2 warm-up converter costs $800 to $2,800, depending on its design and the emissions rules in your state. ### What is the difference between P0430 and P0431? P0430 refers to the catalyst system on bank 2, while P0431 refers to the warm-up catalyst on bank 2. Makers use these definitions differently, so on some vehicles P0431 points to the only converter on that side. The testing approach, starting with leaks, sensors, and misfires, is similar for both codes. ### Should I replace both catalytic converters for P0431? Not automatically. If only bank 2 failed and the bank 1 converter tests healthy, replacing one converter is often enough. Some owners replace both on high-mileage engines to avoid a second repair soon after, especially when labor overlaps. Ask the shop to show you the sensor data for each bank. ### Can a bad oxygen sensor cause P0431? Yes, a slow or contaminated downstream sensor on bank 2 can make the converter look bad. Testing it costs much less than replacing a converter. Also confirm the sensor connectors are plugged into the correct bank, because swapped connectors after earlier work can confuse the test and set the wrong code. ### Why would only one catalytic converter go bad? One converter usually fails first because something on that bank stressed it, such as a misfiring cylinder, a leaking injector, or oil or coolant entering that bank's exhaust. Converters can also simply wear at different rates. Finding a bank-specific cause protects the new converter from the same fate. ### Is P0431 covered under the emissions warranty? It may be, if the car is within the federal emissions warranty, which covers catalytic converters for 8 years or 80,000 miles on most US vehicles. Some states, such as California, offer longer coverage on certain models. Damage from neglected engine problems such as ignored misfires may not be covered. ### Can I fix P0431 myself? You can do the first steps, such as reading codes, checking for misfire codes, and listening for a manifold leak. Replacing a bank 2 warm-up converter often means seized studs and tight access, especially on a rear bank, so it is usually a shop job. Confirm the diagnosis before buying parts. ### Will P0431 clear itself? Sometimes the light goes off if the converter passes on later drives, but a worn converter usually sets the code again. The computer keeps a history record even after the light goes out. If P0431 returns more than once, the converter, a sensor, or a bank 2 engine problem needs attention. ## Sources - [SAE J2012: Diagnostic Trouble Code Definitions](https://www.sae.org/standards/content/j2012_201812/) - [EPA: On-Board Diagnostics (OBD)](https://www.epa.gov/state-and-local-transportation/vehicle-emissions-board-diagnostics-obd) ## Other emissions codes - [P0411 Secondary Air Injection System "A" Incorrect Flow Detected](https://diagxcar.com/codes/p0411) - [P0420 Catalyst System Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0420) - [P0421 Warm Up Catalyst Efficiency Below Threshold (Bank 1)](https://diagxcar.com/codes/p0421) - [P0430 Catalyst System Efficiency Below Threshold (Bank 2)](https://diagxcar.com/codes/p0430) - [P0440 Evaporative Emission System](https://diagxcar.com/codes/p0440) - [P0441 Evaporative Emission System Incorrect Purge Flow](https://diagxcar.com/codes/p0441) - [P0442 Evaporative Emission System Leak Detected (small leak)](https://diagxcar.com/codes/p0442) - [P0443 Evaporative Emission System Purge Control Valve "A" Circuit](https://diagxcar.com/codes/p0443) - [P0446 Evaporative Emission System Vent Control Circuit](https://diagxcar.com/codes/p0446) - [P0449 Evaporative Emission System Vent Valve/Solenoid Circuit](https://diagxcar.com/codes/p0449) ## Symptoms - [Why does my car smell like rotten eggs?](https://diagxcar.com/symptoms/car-smells-like-rotten-eggs) - [Why does my car lose power when accelerating?](https://diagxcar.com/symptoms/car-loses-power-when-accelerating) ## Repair costs - [Catalytic converter replacement cost](https://diagxcar.com/cost/catalytic-converter-replacement): $1,000-$3,000 - [Oxygen sensor replacement cost](https://diagxcar.com/cost/oxygen-sensor-replacement): $150-$450 - [Exhaust manifold gasket replacement cost](https://diagxcar.com/cost/exhaust-manifold-gasket-replacement): $300-$1,300 --- Source page: https://diagxcar.com/codes/p0431 Updated: 2026-11-03 General information, not a substitute for a qualified mechanic.