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Figuring out why your Check Engine Light is on means deciphering OBD-II (or “OBD2”) codes, and that can be kind of annoying. That’s why Car Bibles did it for you! You’re welcome, now let’s get into solving your issue.

OBD2 P0420: What It Means

All cars have an emissions system, which is designed to break down some of the harmful gasses and junk created by its engine. One key part of the emissions system is the catalytic converter. Code P0420 or “Catalyst System Efficiency Below Threshold (Bank 1)” is triggered when the catalytic converter is not operating as efficiently as it should be.

“Bank 1” refers to the odd-numbered cylinders if this is an engine with more than one bank of cylinders. (Such as a Flat or “V” engine). If this is an inline motor, there is only one bank.

Likely Symptoms 

Here’s what you may experience due to the P0420 code:

  • Illuminated check engine light. A stored P0420 is an automatic CEL trigger.
  • Sluggish performance. The vehicle may be less responsive when fully warmed up, or in the higher RPM range.
  • Vehicle unable to go faster than 40 or 50 mph. A clogged or inoperative catalytic converter may not let the car drive fast.
  • Sulfur or “Rotten Egg” smell coming from car’s tailpipe. There may be a lack of oxygen in the catalytic converter, leading to a build-up of sulfur.
  • In some cases, there may be no symptoms at all.

Probable Causes

Here’s what could be causing the issue:

  • Clogged or broken catalytic converter(s). Catalysts often clog and break due to oil or fuel contamination. Commonly a misfire or oil leak will send unburnt fuel down the exhaust, ruining the catalytic converter.
  • Leaking or damaged exhaust. If the exhaust gasses are leaking out before they reach the catalytic converter, the vehicle’s computer may be getting an inaccurate read on how well the catalyst is working.
  • Broken, damaged, or disconnected oxygen sensor(s). If the sensors are not functioning correctly, it may give a faulty picture of how well the catalytic converter is working.

What Part Is Potentially Affected?

Commonly, the catalytic converter is the culprit for P0420. The Converter is a box filled with expensive heavy metals, like gold and platinum. Those metals filter out a lot of the harmful stuff from the exhaust gases before they’re expelled.

The vehicle’s ECU, or computer, monitors the efficiency of the converter, using the oxygen sensors, one placed before the catalyst, and one placed after. If those two sensors are functioning correctly, it often means the catalytic converter itself is faulty.

Possible Fixes

Here are the most common fixes to remedy the P0420 code:

  • Replace catalytic converter. Converters can be expensive, due to the rare metals inside them. This is why you might have heard about these things getting stolen.
  • Clean catalytic converter. There are some products that claim to be able to clean off the catalytic converter, which may or may not work. Typically when converters fail, unburnt fuel makes its way into the converter, which burns, and melts the catalyst, destroying it.
  • Replace oxygen sensors. If the oxygen sensors are not giving an accurate reading, they may need to be replaced.
  • Repair exhaust leak(s).
  • Diagnose and repair misfire. It’s important to note that catalytic converters do not often fail or clog on their own; they’re typically caused by unburnt fuel or oil contaminating them. This typically comes from a misfire condition, which sends unburnt fuel onto the catalyst.

Finding The Parts You Need

Now that you’ve figured out what’s wrong with your hooptie, let’s talk about where you’re gonna find that part’s replacement.

There are plenty of places you can buy auto parts from, but Car Bibles gets paid if you click this Advance Auto link so that’s the one we’re serving up. Advance Auto Parts also has delivery, curbside pickup, and a host of helpful diagrams to aid your repair!

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