How to Reset P0420 Code
How to Reset P0420 Code
The right way to reset a P0420 code, what resetting actually does, and why the code keeps coming back if the cause has not been fixed.
Resetting P0420 code using OBD2 scanner
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How to Reset P0420 Code
Resetting a P0420 code is straightforward and takes about two minutes with the right tool. The quickest and most reliable method is to use an OBD2 scanner to clear the code directly from your car’s ECM. You can also disconnect the battery for 15 to 30 minutes, though this method clears all stored codes and resets other ECM data at the same time, which is not always ideal. The important thing to understand before resetting is that clearing the code does not fix the underlying problem. If the cause of P0420 is still present, the code will return within a few days of normal driving once the ECM completes its next monitoring cycle. Resetting is useful for verifying whether a repair worked, but it is not a fix on its own.
🔧 What You Need to Reset and Monitor P0420
- 👉 OBD2 Scanner (Easiest Way to Reset the Code)
- 👉 Fuel System Cleaner (Try Before Resetting)
- 👉 MAF Sensor Cleaner
- 👉 Replacement Downstream O2 Sensor
- 👉 Catalytic Converter Replacement
💡 An OBD2 scanner lets you clear the code, monitor readiness status, and track whether P0420 returns after a repair, all from one device.
3 Ways to Reset the P0420 Code
Method 1: OBD2 Scanner (Best Method)
This is the cleanest, fastest, and most recommended way to reset P0420. An OBD2 scanner connects to the diagnostic port under your dashboard, usually located just below and to the left of the steering column. Once connected, navigate to the code clearing function, select P0420 or clear all codes, and confirm. The check engine light turns off immediately and the code is removed from the ECM’s memory. The scanner also lets you monitor readiness status so you know when the ECM has completed its self-checks after the reset.
✅ Recommended: Fast, clean, no data loss
Method 2: Disconnect the Battery
Disconnecting the negative battery terminal for 15 to 30 minutes forces the ECM to lose power and clears all stored codes including P0420. Reconnect the battery and the check engine light will be off. The downside is that this method also resets all ECM readiness monitors to incomplete, clears your radio presets on some vehicles, and may require you to re-enter a security code on some older vehicles. It works but is a blunter approach than using a scanner.
Method 3: Drive Until the Code Times Out (Not Recommended)
On some vehicles, if the fault condition does not repeat over a set number of drive cycles, the ECM will eventually remove a pending code from memory on its own. This is passive and unreliable. A confirmed P0420 caused by a real mechanical fault will not time out on its own. This method is not worth relying on unless you have strong reason to believe the fault was completely temporary.
⚠️ Only works for pending codes triggered by temporary conditions
How to Reset P0420 with an OBD2 Scanner: Step by Step
The OBD2 diagnostic port is almost always located under the dashboard on the driver’s side, within reach of the steering column. It is a 16-pin trapezoidal connector. Every car sold in the US after 1996 has one.
Connect your OBD2 scanner to the port. Turn the ignition to the ON position without starting the engine. The scanner will power on and begin communicating with the ECM.
Before clearing anything, navigate to the read codes function and note every code present. This gives you a record of what was stored in case the codes return and you need to compare. Pay attention to whether P0420 is listed as pending or confirmed.
Navigate to the clear codes option on your scanner and confirm the action. The ECM will erase all stored and pending codes. The check engine light will turn off within a few seconds.
Start the engine and let it idle for a minute. Plug the scanner back in and check for any codes that returned immediately. An immediate return of P0420 after clearing indicates a persistent and active fault condition.
Drive your normal routes for a full week including some highway driving. Check the scanner periodically to see whether P0420 returns. If it does not return after a week, the repair was successful or the fault was temporary. If it returns within a few days, the underlying cause is still present and needs to be addressed.
Why Does P0420 Keep Coming Back After Resetting?
If P0420 returns within a few days of clearing it, the root cause is still active. Here are the most common reasons it keeps coming back and what to do about each one.
The converter fails the efficiency test every time the ECM runs it. Needs replacement. Start with a fuel system cleaner first on borderline cases.
Sending inaccurate readings on every drive. Replace the downstream O2 sensor and retest.
Outside air keeps corrupting sensor readings. The leak needs to be physically repaired before the code will stay gone.
Oil burning or misfires keep stressing and contaminating the converter. Fix the engine issue before anything else.
The Bottom Line
Resetting P0420 takes two minutes with an OBD2 scanner and is a perfectly valid step after completing a repair. What it is not is a substitute for actually fixing the problem. If the code keeps returning after every reset, the cause is still there waiting to be found. Work through the possibilities cheapest to most expensive: run a fuel system cleaner, check for exhaust leaks, replace the downstream O2 sensor, and only then consider a catalytic converter replacement as a last resort. Reset and monitor after each step and you will find the fix that actually makes the code stay away.