P0420 Catalyst System Efficiency Below Threshold

Technical description of error P0420

Catalyst System Efficiency Below Threshold (Bank 1)

What does code P0420 mean?

This Diagnostic Trouble Code (DTC) is a generic transmission code. It is considered universal as it applies to all makes and models of vehicles (1996 and newer), although the specific repair steps may differ slightly depending on the model. So this article with engine codes applies to Nissan, Toyota, Chevrolet, Ford, Honda, GMC, Subaru, VW, etc.

P0420 is one of the most common trouble codes we see. Other popular codes include P0171, P0300, P0455, P0442, etc. So be sure to bookmark this site for future reference!

The catalytic converter is a part of the exhaust system that looks like a muffler, although its operation is very different from that of a muffler. The job of a catalytic converter is to reduce exhaust emissions.

The catalytic converter has an oxygen sensor at the front and rear. When the vehicle is warmed up and operating in closed loop mode, the signal reading of the upstream oxygen sensor should fluctuate. The downstream O2 sensor reading should be reasonably stable. Normally, the P0420 code will turn on the check engine light if the readings of the two sensors are the same. Oxygen sensors are also called O2 sensors.

This indicates (among other things) that the converter is not performing as efficiently as it should (according to specifications). Catalytic converters are generally not classified as “wear-out,” meaning they do not wear out and do not need to be replaced. If they failed, chances are it was due to something else that caused the crash. This is what P0420 stands for in a simplified way.

Symptoms of error P0420

The primary symptom for the driver is the MIL illuminated. You will most likely not notice any handling problems, although there may be symptoms. For example, if the substance inside the catalytic converter is broken or out of order, it can restrict the release of exhaust gases, resulting in a feeling of reduced vehicle power output.

• No noticeable symptoms or handling problems (most common)
• Make sure the engine light is on
• No power after car warms up
• Vehicle speed cannot exceed 30-40 mph
• Rotten egg smell from the exhaust

Causes of the P0420 code

The P0420 code may mean that one or more of the following events have occurred:

• Leaded fuel used where unleaded fuel was required (unlikely)
• Damaged or failed oxygen / O2 sensor
• Downstream oxygen sensor (HO2S) wiring damaged or incorrectly connected
• Engine coolant temperature sensor not working properly
• Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
• Defective or insufficiently efficient catalytic converter (probably)
• Ignition delay
• Oxygen sensors in front of and behind the transmitter are giving too similar readings.
• Leaking fuel injector or high fuel pressure
• Misfire cylinder
• Oil contamination

Possible solutions

Some recommended steps for troubleshooting and fixing the P0420 code include:

• Check for exhaust leaks in the manifold, pipes, catalytic converter. Repair if necessary.
• Use an oscilloscope to diagnose the oxygen sensor (Hint: the oxygen sensor in front of the catalytic converter usually has an oscillating waveform. The waveform of the sensor behind the converter should be more stable).
• Inspect the lower heated oxygen sensor and replace if necessary.
• Replace catalytic converter.

Diagnostic advice

Generally speaking, you can view the exhaust temperature just before and immediately after the converter with an infrared thermometer. When the engine is fully warmed up, the outlet temperature should be about 100 degrees Fahrenheit higher.

Overall, probably the biggest mistake vehicle owners make when they have a P0420 code is to simply replace the oxygen sensor (sensor 02). It is important to conduct a correct diagnosis so as not to waste money on unnecessary replacement parts.

We strongly recommend that if you need to replace the catalytic converter, replace it with an original manufacturer's brand device (i.e. get it from the dealership). The second option is a quality replacement part, such as a legal 50-state cat. There are many stories on our forums of people replacing a cat with a cheaper aftermarket one only to have the code returned soon after.

It should be noted that many car manufacturers offer longer warranties on emissions-related parts. Therefore, if you have a newer car but is not covered by the bumper-to-bumper warranty, there may still be a warranty for this type of problem. Many manufacturers provide these products with a five-year unlimited mileage warranty. It's worth checking out.

HOW DOES A MECHANIC DIAGNOSTIC CODE P0420?

• Use an OBD-II scanner to retrieve stored trouble codes from the PCM.
• Displays live data of the downstream (rear) oxygen sensor. The downstream oxygen sensor voltage reading should be constant. Determine if the downstream (rear) oxygen sensor is working properly.
• Diagnose any other codes that may be causing DTC P0420.
• Repair misfiring, misfiring and/or fuel system problems as needed.
• Inspects the rear oxygen sensor for damage and/or excessive wear.
• Test driving the vehicle looks at freeze frame data to determine if the downstream (rear) oxygen sensor is working properly.
• Check for available PCM updates if the catalytic converter is faulty. After replacing the catalytic converter, PCM updates will be required.

COMMON ERRORS WHEN DIAGNOSING CODE P0420

The most common mistake is replacing oxygen sensors before the diagnostic process is complete. If another component is causing the P0420 trouble code, replacing the oxygen sensors will not fix the problem.

HOW SERIOUS CODE P0420 IS?

It is normal for a driver to have no handling problems when a P0420 DTC is present. Other than the Check Engine light being on, the symptoms of this DTC may go unnoticed. However, if the vehicle is left by mistake without resolving the problem, it can cause serious damage to other components.

Since there are no symptoms of handling problems associated with DTC P0420, this is not considered serious or dangerous to the driver. However, if the code is not corrected in a timely manner, the catalytic converter may be seriously damaged. Because catalytic converter repairs are expensive, it is imperative that DTC P0420 be diagnosed and repaired as soon as possible.

WHAT REPAIRS CAN FIX CODE P0420?

• Replace muffler or repair muffler leaks
• Replace exhaust manifold or repair exhaust manifold leaks.
• Replace drain hose or repair drain hose leaks.
• Replace catalytic converter (most common)
• Replace Engine Coolant Temperature Sensor
• Replacing the front or rear oxygen sensor
• Repair or replace damaged wiring to oxygen sensors.
• Repair or replace oxygen sensor connectors
• Replace or repair leaking fuel injectors
• Diagnosing any misfiring problems
• Diagnose and correct any other related trouble codes that have been stored by the power management module (PCM).

ADDITIONAL COMMENTS TO BE AWARE OF CODE P0420

Problems with the ignition system, fuel system, air intake, and misfires can damage the catalytic converter if not resolved quickly. These components are the most common cause of DTC P0420. When replacing a catalytic converter, it is recommended to replace it with an original unit or a high quality oxygen sensor.

Aftermarket oxygen sensors often fail, and when this happens, the P0420 trouble code can reappear. You should also contact the manufacturer to see if your vehicle is covered by the manufacturer's warranty on emissions related parts.

How to Fix P0420 Engine Code in 3 Minutes [3 Methods / Only $19.99]

Need more help with your p0420 code?

If you still need help with DTC P0420, post a question in the comments below this article.

NOTE. This information is provided for informational purposes only. It is not intended to be used as a repair recommendation and we are not responsible for any action you take on any vehicle. All information on this site is protected by copyright.