P0420 Code Subaru WRX - What's Wrong with the Car?

Are you having a hard time figuring out what the P0420 trouble code means and what the issue with your Subaru WRX could be? Well, you're not alone! Getting the right diagnosis can be a nightmare for many car owners and even technicians. This article will cover everything you need to know about the trouble code to help you zero in on the right diagnosis. Let's first look at what the code means.

P0420 code highlights

• Level of urgency:Medium
• Possible culprits:catalytic converter, exhaust pipe
• Price for repair:$500 - $1,500
• If neglected:Engine stalling, power loss, failing emission tests
• DIY repairs:Impossible
• Can you drive?Yes

What Does the P0420 Code in a Subaru WRX Mean?

The P0420 Code in a Subaru WRX usually means there's a problem with the catalyst efficiency. This means that the efficiency is below the threshold in bank 1. It's also common in other Subaru models such as Forester, Impreza, Outback, and Legacy. The check engine light comes on when the Powertrain Control Module triggers the P0420 code indicating a problem in the emissions system. 

What Does Catalyst System Efficiency Mean?

Harmful gases such as carbon monoxide(CO), hydrocarbons(HC), and Nitrogen Oxides are converted to less harmful gases by the catalytic converter in the car's exhaust system. For instance, carbon monoxide and hydrocarbons are oxidized to carbon(iv)oxide, and water and Nitrogen oxides are converted to inert Nitrogen gas.

Catalyst system efficiency means the percentage of harmful gases removed from the exhaust system. If the percentage is high then the catalyst system is highly efficient. Oxygen sensors measure the amount of gases removed from the exhaust.

Catalyst efficiency below the threshold means that not enough pollutants are being removed from the exhaust gas system triggering the P0420 code. In this case, the upstream sensors(located in front of the catalytic converter) and the downstream sensor (located at the back of the catalytic converter) show similar readings.

If the exhaust system works efficiently, the downstream sensor should show a lower reading than the upstream sensor to indicate efficiency in removing harmful gases by the catalytic converter. 

Bank 1 of the Subaru WRX

Different banks and sensor locations can be difficult to trace and mostly depend on the model of your car. Bank 1 refers to the engine part with cylinder one, while bank 2 refers to the part with cylinder 2.

To confirm the proper cylinder bank, your best bet would be to check the car repair manual or simply use google search with your car's model. The sensor numbers are also used to denote where the oxygen sensors or the exhaust temperature sensors are located. 

The first sensor is installed closest to the engine, while you can find the last at the back of the exhaust system. For oxygen sensors, sensor one is installed in front/upstream of the catalytic converter, while sensor two is at the back/downstream of the catalytic converter. Bank 1 is associated with the P0420 code, while bank 2 is associated with the P0430 code.

Can You Drive with the P0420 Code in Place?

Generally, it's safe to drive your car with the P0420 code save for the emission of toxic gases to the environment. This is because the catalyst system on bank 1 is not performing its work efficiently. Also, if the issues associated with the code arise from the fuel system, ignition system, or engine misfires, this can lead to catalytic converter damage if not quickly addressed.

Replacing a catalytic converter is a costly affair and can blow a hole in your budget. This means that as soon as the check engine light illuminates and you get the diagnostic trouble code, try to make the right diagnosis and fix the issue as soon as possible before the issue worsens.

What are the Symptoms of the P0420 Code?

Common tell-tale signs of the P0420 code are:

• Check engine light coming on.

This is usually the first sign of the diagnostic trouble code P0420 and requires a diagnostic tool to read the code since there are many reasons the check engine light may come on. 

• A decline in the engine power output and performance.

A decline in engine performance reduces the ability of the car to accelerate. This becomes an issue when the vehicle cannot exceed the speed limit on highways.

• A rotten egg smell emanating from the exhaust of the vehicle.

The rotten egg smell comes from sulfur or hydrogen sulfide accumulation in the fuel tank. Whenever gasoline enters the catalytic converter, hydrogen sulfide or sulfur traces are converted to odorless sulfur dioxide.

When this doesn't occur due to clogging of the catalytic converter or breakage in the fuel and exhaust system, the hydrogen sulfide accumulates to produce the rotten egg smell.

What are the Causes of the P0420 Code in a Subaru WRX?

There are a variety of reasons that may cause the check engine light to appear with the P0420 code. Some of the main reasons are:

1) Faulty Oxygen Sensors

Faulty oxygen sensors are one of the main causes of the P0420 code. This can be due to an escape of the exhaust gases before being measured by the sensors. Another reason would be that the sensors are not sending the right information to the Engine Control Module(ECM). 

2) A Failing or Clogged Catalytic Converter 

A failing catalytic converter is another major reason for the P0420 code. The decline or loss of performance of the catalytic converter leads to toxic gases not being removed. The catalyst converter is made of ceramic and is supported by a Nickel support shell. It also contains noble metals such as Palladium, Platinum, and Rhodium, which make the converter expensive.

Another rare metal, Cerium, stores excess oxygen and acts like a sponge and soaks oxygen when the mixture becomes lean and releases the oxygen after the mixture becomes rich. The ability to soak up oxygen makes the catalytic converter able to convert the hydrocarbons to water and carbon(iv)oxide.

An issue arises if the Cerium becomes contaminated and cannot store oxygen or the catalytic converter overheats due to carbon build-up.

3) Other causes of the P0420 code that causes failure of the Catalytic Converter 

• Lean or rich air-fuel mixture.
• Engine misfire.
• Leaded gasoline.
• An issue with the PCM.
• Wiring of the oxygen sensor.
• Leakage in the exhaust system.
• Leaking fuel injectors.
• Bad catalytic converter oil.
• Excess fuel pressure.

How to Diagnose the P0240 Code in Your Subaru WRX

A proper diagnosis helps determine the correct issue. For Do-It-Yourselfers, a correct diagnosis can be tough to make, especially if you're a beginner. It also requires advanced tools to make a diagnosis. Tools required to make a diagnosis are a digital multimeter or an OBD 2 scanner. 

Steps for Diagnosis

The first step is using an OBD 2 Scanner tool to read and verify the Diagnostic Trouble Code(DTC) displayed in the Power Control Module. Other codes may also pair along the P0420 code. These codes can help demystify the type of problem present. For instance, the P0420 code paired with misfire codes such as P0308 indicates a misfire diagnosis.

This prevents getting a new catalytic converter which may get damaged again because of not solving the misfire issue. Pairing with a code like P0174 indicates a lean or rich fuel mixture that also causes a catalytic converter failure.

The second step is to examine the exhaust system for any signs of leakages and damage. Leakages may be present in the gasket, exhaust pipes of the precatalytic converter, and exhaust manifold.

Examine the voltage of the downstream oxygen sensor. This is where you can use a digital multimeter, or the diagnostic tool live data feature to measure the voltage reading of the downstream oxygen sensor. If the catalytic converter is in good condition, the tool should produce a steady voltage reading of about 0.45V. But if the tool shows a fluctuation of voltage movement between 0.1V to 0.9V, the catalytic converter is in a deplorable state. 

If the problem is still unclear, examine if there are ignition problems, issues with the fuel system, or misfires, and also check if there are signs of wear and tear on your rear oxygen sensors.

Inspect also the oxygen sensor wiring for any signs of damaged wires.

How to Fix the Code P0240

After making the correct diagnosis, it's now possible to fix the issue and remove the error code. A simple welding procedure will do the trick if you've found an issue with the exhaust leak due to damage to the exhaust system. However, you'll have to replace the exhaust manifold if it's beyond repair.

Other fixes include:

• Replace the front or rear oxygen sensors and their connectors.
• Repair or replace any leaking fuel injectors.
• Fix any bad oxygen sensor wiring or replace the faulty wires.
• Repair present misfiring problems after diagnosis.
• Replace the engine coolant temperature sensor.

After the repair, you can delete the P0420 trouble code using the OBD 2 Scanner tool for the check engine light to disappear.

Bottom Line

There you go - everything you need to know about the P0420 code in your Subaru WRX. If you cannot make the diagnosis or fix the issue on your own, your next option would be to find an ideal auto shop or consult a professional mechanic.