P0420 Code: Catalyst System Efficiency Below Threshold (Bank 1)

A common emissions code seen in the field, a P0420 trouble code indicates that the catalytic converter isn’t functioning efficiently. To work at peak efficiency, the emissions or feed gases entering the catalytic converter need to be perfectly balanced and not excessively elevated.

The car’s combustion process leaves behind nitrogen oxides (NOx), carbon monoxide (CO), and unburned fuel, which are bad for the environment. The catalytic converter turns them “safer” by reducing NOx to nitrogen and oxidizing the rest into CO2 and water.

What Does P0420 Code Mean?

The P0420 code signals a low catalyst system efficiency. This code suggests that the oxygen levels are below the desired threshold (Bank 1), which most often results from problems with your car’s exhaust or fuel systems. It can sometimes be tricky to diagnose.

The catalytic converter is a component of the vehicle’s exhaust system. It works by oxidizing the remaining unburned fuel and carbon monoxide (CO) from the combustion process into carbon dioxide (CO2) and water (H2O). It also reduces nitrogen oxides (NOx) into nitrogen.

The converter uses two oxygen sensors – one on the upstream and the other on the downstream.

If the upstream oxygen sensor is working properly, its readings should fluctuate from when the car is at operating temperature and when running in a closed loop. If the downstream oxygen sensor is working properly, and there isn’t an issue with the catalytic converter, its readings should remain steady.

When the oxygen sensors have similar readings to each other, it indicates that the catalytic converter is not working as intended. If the voltage of the downstream oxygen sensor decreases and begins to fluctuate like the upstream oxygen sensor, it means the oxygen levels are too high and the Powertrain Control Module (PCM) will trigger the P0420 code, causing the check engine light to come on, alerting you that you have a catalyst problem.

P0420 Code

Key Summary:

  • P0420 stands for “Catalyst System Efficiency Below Threshold (Bank 1).”
  • This trouble code is triggered when the on-board computer detects that bank 1 of the catalytic converter is underperforming.
  • A faulty catalytic converter (usually due to overheating and contamination) is the most common reason why this code is logged.
  • Once a P0420 code is detected, your vehicle can exhibit symptoms like an illuminated check engine light and a failed emissions test.

Common Causes of The P0420 Code

A code P0420 may mean that one or more of the following has happened:

  • Leaded fuel was used where unleaded was called for (unlikely).
  • A damaged or failed oxygen/O2 sensor.
  • Downstream oxygen sensor (HO2S) wiring is damaged or connected improperly.
  • The engine coolant temperature sensor is not working properly.
  • Damaged or leaking exhaust manifold/catalytic converter/muffler/exhaust pipe.
  • Failed or underperforming catalytic converter (likely).
  • Retarded spark timing.
  • The oxygen sensors in front and behind the converter are reporting too similar of readings.
  • Leaking fuel injector or high fuel pressure.
  • Cylinder misfire.
  • Oil contamination.

4 Common Symptoms of A P0420 Code

A bad catalytic converter generally doesn’t reflect any indicators. However, you can look out for these common symptoms if you suspect your car has problems with its exhaust system. They are:

1. Lack of Power

If your vehicle fails to gain power or operate smoothly after warming up, it could point to a faulty catalytic converter. This could be due to uneven fuel pressure caused by the worn-out converter. You may also notice jerky movements while driving or complete stall-outs.

2. Inability to Speed Up

A bad catalytic converter will cause exhaust buildup in the exhaust pipe, reducing performance. So, you may notice that your car doesn’t speed up beyond 30-40 MPH or has diminished fuel efficiency.

3. Foul Smell from the Exhaust Pipe

This smell occurs from the incorrect amount of oxygen in the catalytic converter, which leads to excess sulfur in the fuel tank, creating the odor from the exhaust system.

4. Check Engine Light (CEL) Turns On

The most prominent sign of any engine code diagnosis is the check engine light turning up on your dashboard. While the check engine light doesn’t necessarily indicate DTC P0420, it could be one of the fault codes keeping your check engine light on.

  • You may also see a failed emissions test due to a bad catalytic converter.
  • If your check engine light is lit, be sure to have the problem diagnosed!

Can I Drive with P0420?

Yes, it is usually alright to continue driving in the short term with P0420. It does not pose a risk to the driver.

Driving long term with this code can damage your engine and exhaust system. You don’t need to call a tow truck if you see this code, but you should try to diagnose and fix the problem as soon as you can.

You might notice a slight decrease in your car’s performance. This is because your exhaust system is not working at peak efficiency. P0420 can become more serious if there are other codes present that show there is too much fuel in your air/fuel mixture.

Check to see if there are other codes like P0171, P0172, P0174, or P0175. These codes can mean that the engine is getting the wrong fuel/air ratio.

How to Diagnose the P0420 Code?

P0420 is a generic powertrain code that various vehicles can log. The process to diagnose might vary depending on make-model specs.

Here are some of the step’s mechanics take to diagnose it:

  • Determine a baseline: They take note of the freeze frame data upon retrieving the code. The information serves as their baseline for tests.
  • Resolve problems that ruin a Catalyst: Misfires, ignition issues, and fuel and/or intake problems can trigger the P0420 code, so mechanics fix them first to rule them out.
  • Go on a test drive near or at the freeze frame conditions: Mechanics verify whether the rear oxygen sensor is mirroring the front one and/or generating voltage below the required threshold. The catalytic converter is defective if they verify these conditions.

Mechanics might conduct other tests depending on what they find.

If you want to diagnose the code yourself, following these steps is a great start. Research what tools you need to diagnose your car’s catalytic converter, and make sure you have them on hand.

However, if you’re neither familiar with diagnosing trouble nor confident in DIY-ing the process, it’s better to bring your ride to an auto repair shop near you.

Common Diagnosis Mistakes

One of the most common mistakes to avoid when fixing the P0420 code is not fully diagnosing the problem before replacing any parts. People often instinctively change the O2 sensor or oxygen filter when remedying the OBD II code P0420.

However, the problem could be something totally different, like a bad spark plug. So, make sure you properly diagnose all potential causes before deciding on a fix. Better not to assume it’s simply an upstream oxygen sensor or downstream oxygen sensor issue.

How Serious Is Engine Fault Code P0420?

Engine fault P0420 isn’t frequently a threat to driving safety. The defect is unlikely to cause serious drivability or reliability issues. However, your engine may be emitting more dangerous pollutants because the catalyst system on bank 1 is underperforming. As a result of this coupled with the check engine light, you are unlikely to pass emissions inspections.

Like with any engine problem, the safest option is to diagnose and repair the P0420 problem as quickly as feasible. So we’re not suggesting that you overlook these difficulties for weeks or months. Rather, it’s probably safe to keep driving with engine code P0420. Use your best judgment. Play it safe if the automobile feels risky or if there are other engine codes, major symptoms, etc.

What Repairs Can Fix the P0420 Code?

Since the P0420 code can be caused by several issues within the exhaust system, it doesn’t have a single solution. Before trying to fix P0420 make sure you correctly diagnose the cause of the code.

  • Replace muffler or repair leaks in the muffler.
  • Replace exhaust manifold or repair leaks in the exhaust manifold.
  • Replace exhaust pipe or repair exhaust pipe leaks.
  • Replace catalytic converter (most common)
  • Replace the engine coolant temperature sensor.
  • Replace front or rear oxygen sensor.
  • Repair or replace damaged wiring to oxygen sensor(s)
  • Repair or replace oxygen sensor connector(s)
  • Replace or repair fuel injector(s) that are leaking.
  • Diagnose any repair any misfiring problems.
  • Diagnose and repair any other related trouble codes that have been stored by the Power Control Module (PCM)

How Much Does P0420 Code Repair Cost?

The cost of repairing DTC P0420 varies greatly depending on the diagnosis. If it’s just a minor exhaust leak, you might be able to weld the holes yourself for next to nothing. Catalytic converters, on the other hand, can be costly. Essentially, P0420 repairs might cost anywhere from $10 to $2,000 or more.

Below are the cost ranges of having a mechanic fix common repair:

  • Air fuel sensor $200-$300
  • Oxygen sensor replacement $275-$500
  • Catalytic converter $400-$2400
  • A leak in exhaust $100-$200 (if welded to repair)

It will be much cheaper if you do it yourself, but you risk causing more damage if you are not experienced. Before trying to fix P0420 make sure you correctly diagnose the cause of the code.  If you clear the code and it doesn’t come back after driving, you don’t have to spend any money on it.

If there is a small leak in the exhaust system that you can weld, you won’t have to spend much money on the repair. On the other hand, if you need to replace the catalytic converter, it is going to be an expensive repair.

We strongly recommend that if you need to replace the catalytic converter that you replace it with an original manufacturer brand unit (i.e., get it from a dealership). One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts.

So, if you have a newer car but it’s out of its bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five-year, unlimited mileage warranty on these items. It’s worth checking into.