Will P0420 Code Clear Itself? (What You Need to Know)

Edited and Reviewed By:Michael

So, you’re cruising down the road, life is good, then suddenly that dreaded Check Engine Light pops up. You plug in your trusty OBD-II scanner and see the infamous P0420: Catalyst System Efficiency Below Threshold (Bank 1).

If your first thought was, “Ugh, maybe it’ll just clear itself if I drive a little,” you’re not alone. I’ve been there too. When my own car threw a P0420 code, I hoped it was just a fluke. Spoiler alert: sometimes it goes away… but most of the time, it sticks around like that one friend who never leaves your house after a party.

In this article, we’ll dig into what P0420 really means, when it might clear itself, and when you’ll actually need to roll up your sleeves (or open your wallet) to fix it.

What Does P0420 Mean?

Before we start wishing it away, let’s understand what it is.

  • P0420 = Catalyst System Efficiency Below Threshold (Bank 1).
  • Translation: Your car’s computer thinks your catalytic converter isn’t doing its job of cleaning up exhaust gases.
  • How does it know? It compares readings from two oxygen sensors—one before the catalytic converter and one after.

If both sensors show similar readings, it means the cat isn’t storing oxygen properly → boom, P0420 code.

Symptoms of a P0420 Code

Here’s the thing: sometimes you’ll have a P0420 with no noticeable symptoms. Other times, your car might act up a bit.

Common signs include:

  • Check Engine Light (the obvious one)
  • Reduced fuel economy
  • Rotten egg smell (sulfur) from exhaust
  • Sluggish acceleration or loss of power
  • Rattling noise under the car (if the converter is breaking apart)

But FYI: Many people drive for weeks with P0420 and don’t notice much beyond the light. That’s why it’s so tempting to ignore it.

Will P0420 Code Clear Itself?

Alright, the big question: Can P0420 go away on its own?

Short answer: Yes… but usually only temporarily.

Here’s why:

  • If the issue was a one-time glitch (like bad fuel, temporary sensor misread, or even moisture in the exhaust), the code might clear after a few driving cycles.
  • But if the catalytic converter or O2 sensors are genuinely failing, the light will come right back once the computer runs its next system check.

👉 Pro Tip: If you clear the code manually with a scanner and it doesn’t come back after 100–200 miles, it may have been a fluke. If it reappears consistently, you’ve got a real problem.

Common Causes of P0420

Let’s talk about what actually triggers it. Some are minor, others not so much.

1. Bad Catalytic Converter

  • Most common cause.
  • Cats wear out over time or get clogged/damaged.
  • Expensive, but often the real culprit.

2. Faulty Oxygen Sensors

  • O2 sensors can misread and make the ECM think the cat isn’t working.
  • Much cheaper to replace than a catalytic converter.

3. Exhaust Leaks

  • Small leaks before the O2 sensors can throw off readings.
  • Fixing a gasket or weld could clear the code.

4. Poor Fuel Quality or Misfires

  • Running rich or lean for too long damages the catalytic converter.
  • Misfires dump raw fuel into the exhaust—cat killer!

5. Software/ECM Glitches

  • Rare, but sometimes a PCM update from the dealer resolves it.

Step-by-Step Diagnosis

Here’s how to figure out if it’s a ghost code or a real issue.

Step 1: Scan and Note Freeze Frame Data

  • See what conditions (speed, RPM, temp) triggered the code.
  • Helps you replicate the issue later.

Step 2: Inspect for Exhaust Leaks

  • Listen for hissing or popping near the exhaust manifold or pipes.
  • Even a small leak can cause false P0420.

Step 3: Check O2 Sensors

  • Monitor live data on a scan tool.
  • Upstream sensor should fluctuate rapidly.
  • Downstream sensor should stay steady.
  • If both mimic each other → bad cat. If downstream is lazy → bad sensor.

Step 4: Test the Catalytic Converter

  • Infrared thermometer test (check temp difference before and after the cat).
  • Big difference = working cat. Little/no difference = failing cat.

Step 5: Rule Out Engine Issues

  • Check spark plugs, ignition coils, and fuel trims.
  • Misfires or fueling issues can trick the ECM into thinking the cat is bad.

Can You Just Clear the Code and Keep Driving?

Honestly, many people do. But let’s weigh the risks:

Pros:

  • Light goes away (for now).
  • Car might drive fine.
  • Passes the “wife test” (aka no annoying dash light 😉).

Cons:

  • Code almost always comes back if the cat or sensors are failing.
  • Won’t pass emissions inspections.
  • Long-term damage to catalytic converter (if not already toast).
  • Could lead to worse performance and fuel economy.

So yeah, you can clear it—but unless it was a fluke, you’re just delaying the inevitable.

Fixes for P0420

Let’s talk solutions, from cheapest to most expensive.

1. Check/Replace Gas Cap

  • Loose cap can sometimes contribute to EVAP codes that trigger P0420 indirectly.
  • Cost: $15–$30

2. Fix Exhaust Leaks

  • Replace gaskets, patch pipes, weld cracks.
  • Cost: $20–$200

3. Replace Oxygen Sensors

  • Upstream and downstream sensors wear out.
  • Cost: $50–$150 each
  • Often a good first step before replacing a cat.

4. Replace Catalytic Converter

  • The big one.
  • OEM cats: $800–$2,000+
  • Aftermarket cats: $200–$600 (but quality varies).

5. ECU Update or Reflash

  • Some vehicles have TSBs (technical service bulletins) that reprogram the ECM for false P0420 codes.
  • Dealer-only fix.

Cost Breakdown

Here’s a quick reference chart:

FixDIY CostShop Cost
Gas Cap$15–$30$60–$100
Exhaust Leak Repair$20–$200$150–$400
Oxygen Sensor$50–$150$200–$350
Catalytic Converter$200–$2,000+$600–$2,500
ECM Reflash$0–$150$150–$300

👉 Notice how replacing O2 sensors is way cheaper than a cat. That’s why many people start there.

How to Prevent P0420 in the Future

You can’t stop cats from aging, but you can extend their life:

  • Keep up with regular maintenance (oil changes, plugs, coils).
  • Fix misfires immediately.
  • Use quality fuel to prevent carbon buildup.
  • Don’t ignore other codes—rich/lean conditions kill catalytic converters fast.

Conclusion

So, will the P0420 code clear itself?

  • Sometimes yes, but usually no. If it was a glitch, it may disappear after a few drive cycles.
  • If it keeps coming back, you’ve likely got a failing cat, bad O2 sensors, or an exhaust leak.

When mine first appeared, I crossed my fingers and cleared it. It stayed gone for a week, then popped back up. Replacing the downstream O2 sensor solved it—for a while. A year later, I had to replace the cat. Lesson learned: P0420 isn’t a code you can ignore forever.

Bottom line? Clear it once and see what happens. If it comes back, time to dig deeper. At least now you know what to look for and how to fix it.

And hey, the next time a buddy says, “Will P0420 clear itself?” you can grin and reply: “Sure, if you’re lucky. But don’t bet your gas money on it.” 😎

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