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![How to Pass a Failed Emissions Test [Even with a P0420 Code]](https://www.crcindustries.com/media/amasty/blog/P0420-Code-Failed-Emissions.webp)
Table of Contents:
What is an Emissions Test? | Top Reasons Cars Fail | What is the P0420 Code? | What is an Emissions System Cleaner? | How to Use Emissions System Cleaner | How Cleaners Help with P0420 | Additional Tips for Passing | Wrap Up
Failing an emissions test can be a frustrating experience—especially when your check engine light is on and you're dealing with a diagnostic trouble code like P0420. But before you head to the mechanic or consider costly repairs, there's a potential solution that's both affordable and easy to try: an emissions system cleaner.
In this article, we'll explore why cars fail emissions tests, how emissions system cleaners work, how to use them effectively, and how they can help clear a P0420 code and get your vehicle to pass its emissions test.
What is an Emissions Test?
An emissions test—also known as a smog check or vehicle emissions inspection—is a diagnostic procedure used to measure the amount and type of pollutants a vehicle emits from its exhaust system. These tests are required in many regions to ensure that vehicles meet environmental standards and are not excessively contributing to air pollution.
The test identifies pollutants such as:
- Carbon monoxide (CO) – A poisonous gas from incomplete combustion.
- Hydrocarbons (HC) – Unburned fuel that contributes to smog.
- Nitrogen oxides (NOx) – Gases that contribute to acid rain and smog.
- Particulate matter (PM) – Tiny particles that can harm respiratory health.
Top Reasons Cars Fail Emissions Tests
1. Check Engine Light Is On
In many regions, an illuminated check engine light is an automatic failure. It indicates that the vehicle's onboard diagnostics system (OBD-II) has detected a problem—often related to emissions.
2. Faulty Oxygen Sensors
Oxygen sensors monitor the air-fuel mixture and help the engine control module (ECM) adjust fuel delivery. A malfunctioning sensor can cause the engine to run too rich or too lean, increasing emissions.
3. Failing Catalytic Converter
The catalytic converter is responsible for converting harmful gases into less harmful ones. If it's clogged, contaminated or damaged, it won't function properly, leading to high emissions, especially of CO and NOx.
4. Rich Fuel Mixture
Too much fuel and not enough air in the combustion process leads to high levels of CO and HC. This can be caused by:
- Dirty or clogged air filters
- Leaking fuel injectors
- Faulty or dirty mass airflow (MAF) sensors
5. Engine Misfires
Misfires result in unburned fuel entering the exhaust system, which increases hydrocarbon emissions. Common causes include:
- Worn spark plugs
- Faulty ignition coils
- Vacuum leaks
6. Evaporative Emissions Control (EVAP) System Leaks
The EVAP system prevents fuel vapors from escaping into the atmosphere. A loose or cracked gas cap, or leaks in the EVAP system, can trigger emissions-related codes and cause a test failure.
7. Carbon Buildup
Over time, carbon deposits can accumulate in the intake valves, combustion chambers, and exhaust system. This buildup can reduce engine efficiency and increase emissions, especially in high-mileage vehicles.
Key Point: When no mechanical issues are found, and a vehicle still fails its emissions test, chances are that the fuel system is dirty. Deposit buildup can lead to increased emissions, as well as reduced performance and poor fuel economy. Symptoms may include rough idling, hard starts, hesitation or an overall reduction in power.
What is the P0420 Code?
The P0420 code stands for "Catalyst System Efficiency Below Threshold." It means your car's onboard diagnostics system has detected that the catalytic converter isn't performing as efficiently as it should.
Common Causes:
- Carbon buildup in the exhaust system
- Contaminated or aging oxygen sensors
- Incomplete combustion due to dirty injectors or valves
- A failing catalytic converter
What is an Emissions System Cleaner?
An emissions system cleaner is a chemical additive formulated to clean components that directly affect your vehicle's emissions output. These components include:
- Oxygen sensors
- Catalytic converter
- EGR (Exhaust Gas Recirculation) system
- Combustion chambers
Unlike general fuel system cleaners, emissions system cleaners are specifically engineered to reduce harmful exhaust gases and restore the efficiency of emissions-related components. While any of the problems listed earlier could be the culprit for your emissions test failure, there's a good chance using an emissions cleaner can solve the issue and get you through your re-inspection.
How to Use an Emissions System Cleaner to Pass an Emissions Test
Choose the Right Cleaner
Look for a product labeled as an emissions system cleaner or catalytic converter cleaner. One of the best on the market is CRC Guaranteed To Pass® Emission System Cleaner. Safe to use on all gasoline engines, it has an advanced detergent formula that removes harmful carbon deposits from fuel injectors, intake valves and combustion chambers. It reduces carbon monoxide (CO), hydrocarbons (HC), and Nitrogen Oxide (NOx) emissions, which lead to adverse environmental effects.
CRC Guaranteed To Pass® Emission System Cleaner restores the performance of the entire fuel system and promotes a healthy catalytic converter. It increases start and acceleration response time, saves gas by increasing your MPG, and clears check engine light: P0420 / P0430 and other related codes.
Directions for Using Emissions System Cleaner
- Add entire content of bottle to ½ full tank of gas.
- Drive the vehicle normally for about 20-40 miles.
- After driving, fill the tank completely.
- Drive the vehicle normally until the tank is nearly empty (1/8 full).
- Refill gas tank and take for emissions test.
Maintenance Tip: Use cleaner 4 times per year to maintain a healthy fuel/emissions system.
Clear the Check Engine Light
If the issue was due to carbon buildup or sensor contamination, the light may turn off after using the emissions system cleaner. If the check engine light remains on:
- Use an OBD-II scanner to read and clear the code.
- If the light returns, further diagnostics may be needed.
How Emissions System Cleaners Help with P0420 Code
These cleaners can help resolve a P0420 code by:
- Cleaning oxygen sensors and restoring accurate readings to the ECU.
- Improving combustion by reducing unburned fuel that can overload the catalytic converter.
- Removing carbon buildup from the exhaust system and catalytic converter, improving flow and efficiency.
Additional Tips for Passing Your Retest
- Replace your air filter if it's dirty.
- Check your gas cap. A loose or cracked cap can cause emissions-related codes.
- Use premium fuel during the cleaning process. It burns cleaner and leaves fewer deposits.
- Avoid idling for prolonged periods before the test.
- Drive your car before the inspection. A 15–20-minute drive helps bring the engine and catalytic converter to optimal temperature.
Emissions Cleaner Wrap Up
Using an emissions system cleaner is a smart, low-cost way to address minor emissions issues—especially if you've failed a test due to a P0420 code. While it's not a substitute for mechanical repairs, it can be a powerful tool to restore emissions performance and help your vehicle pass its retest.