Engine Troubleshoot

P0420 Catalyst System Efficiency Below Threshold (Bank 1)


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What is engine code P0420?

  • P0420 Technical Definition: Catalyst system efficiency below threshold (Bank 1).
  • Most common cause: Faulty Catalytic Converter
  • Risks for the engine/driver: MEDIUM. It’s safe to drive the car but you should get it checked out as soon as possible
  • Emissions severity: HIGH. The car won’t pass the emissions control.
  • Estimated repair time: 1-2 days.
  • Estimated repair cost: $200-$1000

The P0420 code is stored in memory when the ECU detects a low efficiency in the catalyst system for bank number 1 (the bank of cylinders where cylinder number 1 is located).

The ECU determines the efficiency of the catalytic converter (CAT) using two sensors: the upstream oxygen sensor, located before the CAT, and the downstream oxygen sensor located after the catalytic converter.

bad O2 sensor

The Oxygen sensors are located near the catalytic converter

To calculate the catalyst efficiency, the ECU compares the values of both oxygen sensors in real-time. The expected result is a “cleaner” reading in the downstream O2 sensor. When the values of both sensors are similar the ECU infers a low efficiency in the catalyst system.

6 Common Symptoms When Code P0420 is Present

The most common symptoms of data trouble code P0420 are:

  1. Check Engine Light lit (sometimes even blinking)
  2. Poor engine performance under all driving conditions.
  3. Poor fuel economy
  4. Increased emissions
  5. Possible engine overheat (clogged catalytic converter)
  6. Gas and rotten eggs smell from the exhaust pipe

What Causes Trouble Code P0420

The most common causes of data trouble code P0420 are:

  • Oxygen sensor wiring (open, shorted, burnt) 
  • Oxygen sensor connector (loosely, corroded, disconnected or bent pins)
  • Bad upstream oxygen sensor
  • Bad downstream oxygen sensor
  • Exhaust pipe / catalytic converter leaks
  • Bad catalytic converter

How To Diagnose The Cause Of P0420

For the purpose of this article, it’s assumed that you have a basic knowledge of safety precautions while working on your vehicle.

IMPORTANT: the diagnostic procedures listed below assumes you only have the data trouble code P0420 present. If you also have any other code related to the ignition system, emissions system or fuel system then you should start diagnosing them first.

Depending on the catalytic converter installed on your vehicle, you may need one or more of the following tools:

  • OBD2 Code Reader
  • Professional Digital Infrared Thermometer
  • Professional Automotive Scan Tool

Always refer to the appropriate OEM literature when possible. Original manufacturer diagnostic procedures should always have precedence over a generic workflow.

That said, let’s start the diagnostic process!

1. Preliminary steps

To discard a possible intermittent DTC condition, you’ll need to clear the ECU memory and complete a driving cycle.

  • Read data trouble codes and take note of them.
  • Clear data trouble codes memory.
  • Perform a driving cycle (at least 5-10 minutes).

In case the Check Engine Light stays off then you may have an intermittent problem. If the light lit during your driving cycle then continue with the diagnostic process.

2. Visual Inspection

> Catalytic Converter inspection: perform a meticulous visual inspection of the catalytic converter as well as its inlet and outlet pipe connections. Any sign of exhaust leaks, overheating or excessive corrosion may be an indication of replacement.

> Oxygen Sensor wiring and connector: perform a meticulous visual inspection of the upstream and downstream O2 sensor wiring and connectors. Look for burnt, damaged, corroded or deteriorated wires, also unplug each sensor and look for bent terminal pins, loosely connections, corrosion or any other possible indication of a bad connection.

Fix any wiring problems or replace damaged sensors before continuing.

3. Scan Tool Tests

> Catalytic Converter Test: using a professional scan tool can save you a lot of time regarding CAT diagnostics. Start your engine and allow it to reach its normal temperature. Watch the upstream and downstream oxygen sensor values for bank 1.

The upstream O2 sensor should be constantly changing between a rich/lean condition but the downstream O2 should be more stable with a tendency to a lean mixture. If both readings are too similar you can assert the CAT is faulty. 

> Upstream Oxygen Sensor Test: start the engine to enter KOER mode. If your scan tool has a graphing ability turn it on. Graph engine RPM and O2 sensor output simultaneously. Accelerate the engine to 2000 RPM. You should see the voltage varying during acceleration and then stabilize. Decelerate to idle. You should see the opposite variation in the O2 output. Repeat the procedure several times.

Some manufacturers include special tests for the O2 sensors. Follow on-screen instructions to complete those tests. Please notice that oxygen sensor output is completely different between wideband models and narrowband models, refer to the appropriate OEM literature to determine their expected behavior.

> Downstream Oxygen  Sensor Test: start the engine to enter KOER mode. If your scan tool has a graphing ability turn it on. Graph engine RPM and O2 output simultaneously. Accelerate the engine to 2000 RPM. You should see the voltage fairly stable. Now allow the engine to return to idle. 

You should not see much variation in the O2 output. Repeat the procedure several times. If the downstream O2 sensor output closely resembles the upstream oxygen sensor output then you may have a bad catalytic converter. Some manufacturers include special tests for the O2 sensors. Follow on-screen instructions to complete those tests. 

How To Repair Engine Trouble Code P0420

Depending on the diagnostics results you may need to do the following:

  • Repairing the oxygen sensor(s) wiring and/or connectors.
  • Replacing the catalytic converter
  • Replacing the upstream, downstream (or both) oxygen sensors.

On most late model vehicles, the ECU does an excellent job diagnosing the catalyst system. Meaning that if you don’t have any oxygen sensor code it’s more than likely that your catalytic converter is no longer functioning as it should. This article will assume this is the case and your CAT needs replacement. For detailed instructions on how to replace your oxygen sensors refer to our faulty oxygen sensor write up.

The catalytic converter is located in the exhaust pipe just after the upstream oxygen sensor, and it resembles a standard muffler.

catalytic converter on exhaust system

The catalytic converter is usually located near the engine end of the exhaust

1. Preliminary steps: ensure the ignition switch is off and then disconnect the vehicle battery. Detaching the negative terminal is usually enough.

2. Remove the upstream and downstream O2 sensor connectors: this is optional, but depending on your vehicle it may be a good idea to keep the sensor wiring safe from an accidental burn.

3. Remove old catalytic converter: some catalytic converters are screwed to the exhaust pipe using two bolts on each side, while others are welded. Depending on the case you may need to cut the pipe (welded CAT) or just remove the bolts.

4. Install the new catalytic converter: remove the old CAT and then install the new one. Double check that you’re using the adequate part number. Be sure to use a good torque wrench when reinstalling bolts, especially when reconnecting sensors as they can break very easily if overtightened. 

Here's a video showing you how to replace a catalytic converter:

About the author

Matt Taylor

Matt Taylor

Hey, I'm Matt, founder of themotorguy.com

I've been a car fanatic all of my life. As a youngster, I loved nothing more than taking things apart and rebuilding them. I would also spend my weekends helping my dad with all sorts of car maintenance jobs.
For the past decade I've been a keen amateur mechanic with a love of classic BMW's. Over the years I've learned lots about car maintenance, car detailing and troubleshooting common engine problems.
It's my goal to share my knowledge with fellow amateur car fanatics around the world, and to help people to diagnose and fix their own cars.
If you have any questions please contact me, I'd love to hear your feedback and suggestions!