Can a P0420 Code Go Away?

The honest answer on whether P0420 clears on its own, when it might, and what actually makes it go away for good.

Can a P0420 Code Go Away on Its Own?

In rare cases, yes, a P0420 code can go away on its own, but it depends entirely on what caused it. If the code was triggered by a temporary condition such as a one-time engine misfire, a tank of bad fuel, or moisture in the exhaust system, it may clear itself after a few normal drive cycles once the underlying condition resolves. However, if the code is being caused by a worn catalytic converter, a failing oxygen sensor, or an exhaust leak, it will not go away on its own and will keep returning until the root cause is actually fixed. Most people who see P0420 disappear briefly and then return are dealing with a real mechanical issue that is getting progressively worse.

⚠️ Important: Even if P0420 clears temporarily, the underlying issue is still present if the converter or sensor is failing. It will return, and typically sooner each time.

🔧 Help It Go Away the Right Way

👉 OBD2 Scanner to Monitor If the Code Returns
👉 Fuel System Cleaner (Can Clear Borderline Cases)
👉 MAF Sensor Cleaner
👉 Replacement Downstream O2 Sensor
👉 Catalytic Converter Replacement

💡 A bottle of fuel system cleaner is the cheapest first step and has been known to permanently clear P0420 in borderline cases.

When P0420 Might Go Away on Its Own

After a Tank of Bad or Low-Quality Fuel

Low-quality fuel can cause incomplete combustion and temporarily stress the catalytic converter enough to trigger P0420. Once you fill up with quality fuel and drive through a few normal cycles, the code may clear and not return. If this is the case, running a bottle of fuel system cleaner with your next tank helps clean any residue left behind.

After a One-Time Engine Misfire

A single misfiring event, sometimes caused by a momentary fuel delivery hiccup or spark plug issue, can send raw fuel through the exhaust and briefly trigger P0420. If the misfire was truly a one-time event and the underlying cause resolved, the code may not return after a drive cycle or two.

In Cold or Damp Weather Conditions

Moisture in the exhaust system during very cold starts can sometimes produce unusual sensor readings temporarily. In some rare cases this can store P0420 briefly. These instances are uncommon and the code typically does not return once the car is fully warmed up and driven normally.

After a Drive Cycle Naturally Clears Pending Codes

P0420 can sometimes be stored as a pending code before becoming a confirmed code. In its pending state, if the conditions that triggered it do not repeat during the next drive cycle, it may erase itself automatically. This is not the same as the problem being fixed, it just means the ECM did not see the fault again during that monitoring window.

When P0420 Will NOT Go Away on Its Own

Worn Catalytic ConverterA physically degraded converter will never recover on its own. The code will return every time and get more consistent as the converter continues to deteriorate.

Failing O2 SensorA downstream oxygen sensor that is sending bad data will keep sending bad data. It will not fix itself. The code will return reliably until the sensor is replaced.

Exhaust LeakAn exhaust leak does not seal itself. Air will keep entering the exhaust stream and corrupting sensor readings until the leak is physically repaired.

Oil or Coolant BurningIf your engine is burning oil or coolant internally, that contamination continues coating the converter until the engine issue is repaired. The code will return repeatedly.

How to Make P0420 Go Away for Good

Step 1: Do not just clear the code and hope for the best.Clearing P0420 with an OBD2 scanner without fixing the cause is a temporary measure at best. The code will return, often within a few days of normal driving.

Step 2: Try a fuel system cleaner first.Pour a bottle of fuel system cleaner into a full tank and drive normally for the entire tank. Clear the code after finishing the tank and monitor whether it returns. This resolves P0420 permanently in some borderline cases.

Step 3: Inspect for exhaust leaks.Listen near the exhaust manifold with the engine warm for any ticking or hissing. Exhaust leaks are a cheap fix and removing that source of false readings sometimes clears P0420 permanently.

Step 4: Replace the downstream O2 sensor.If the code keeps returning consistently, testing or replacing the downstream O2 sensor is the next logical step. This fix permanently resolves P0420 in a significant number of cases and costs far less than a new converter.

Step 5: Replace the catalytic converter as a last resort.If every cheaper fix has been tried and the code keeps returning, the converter itself is failing. A quality replacement catalytic converter will resolve P0420 permanently once installed correctly and any underlying engine issues are addressed first.

The Bottom Line

P0420 can go away on its own in a small number of situations involving temporary conditions like bad fuel or a one-time misfire. But if you are seeing it return consistently or it has been there for more than a week of normal driving, it is not going to resolve itself. The code is telling you something is wrong with how efficiently your exhaust is being processed, and that condition will only worsen over time without intervention. Start with the cheapest fixes first, use an OBD2 scanner to monitor whether the code returns, and work your way through the causes until it stays gone for good.

📌 Disclaimer: This guide is for informational purposes only. Always consult a licensed mechanic for an accurate diagnosis before purchasing or replacing any vehicle components. Some links on this page are affiliate links. We may earn a small commission at no extra cost to you.

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Can a P0420 Code Go Away

Can a P0420 Code Go Away?