How to Pass OBD2 Emissions with Catless Downpipes: A DIY Guide

Navigating OBD2 emissions testing can be a headache for car enthusiasts, especially when modifications like catless downpipes are involved. Many performance upgrades that enhance your vehicle’s power also remove essential emissions control components, leading to inevitable failures during standard emissions tests. This guide provides a practical, DIY approach to help you pass an OBD2 emissions test even with catless downpipes installed, focusing on a cost-effective and reversible method.

Understanding the OBD2 Emissions Test and Catless Downpipes

The On-Board Diagnostics II (OBD2) system is designed to monitor various aspects of your vehicle’s performance, including emissions. A crucial part of this system is the catalytic converter, which reduces harmful pollutants in your exhaust gases. OBD2 systems use oxygen sensors (O2 sensors) both before (upstream) and after (downstream) the catalytic converter to measure its efficiency.

Catless downpipes, as the name suggests, eliminate the catalytic converters. While this modification can significantly improve exhaust flow and boost engine performance, it also removes the component responsible for cleaning up exhaust gases. Consequently, the downstream O2 sensors will detect readings outside of the expected parameters, typically triggering a Check Engine Light (CEL) and causing failure in emissions tests due to “catalyst inefficiency” codes.

Furthermore, tuning software, like BM3 (Bootmod3), often disables the O2 sensor monitors to prevent CELs when catless downpipes are installed. While this avoids the immediate warning light, it also means your vehicle will fail an emissions test because the readiness monitors for the oxygen sensors and catalyst will show as “not ready.” Emissions testing requires these monitors to be in a “ready” or “passed” state.

The Defouler Solution: A Step-by-Step Guide

This guide focuses on using spark plug defoulers as a method to pass OBD2 emissions with catless downpipes. Defoulers are inexpensive and relatively easy to install, offering a temporary solution for testing purposes. This method works by physically spacing the downstream O2 sensor out of the direct exhaust stream, tricking it into reading cleaner gases, similar to what it would expect with a functioning catalytic converter.

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Here’s a step-by-step guide to utilizing this method:

Step 1: Gather Your Materials

You will need the following items:

• Spark Plug Defoulers: Purchase spark plug defoulers from an auto parts store. Specifically, O’Reilly’s part number 42009 or similar is a suitable option. You’ll need two defoulers per downstream O2 sensor. For most vehicles with two downstream sensors, you’ll need four defoulers in total (two packs).
• OBD2 Scanner (Optional but Recommended): A Bluetooth OBD2 scanner and a smartphone app (like Car Scanner ELM OBD2) will allow you to monitor your readiness monitors in real-time. This is highly recommended for efficiency.
• Basic Hand Tools: Wrenches to remove and reinstall your O2 sensors.
• Vehicle Lift or Ramps (Optional but Recommended): Lifting your vehicle will provide easier access to the O2 sensors.

Step 2: Install Spark Plug Defoulers

1. Locate Downstream O2 Sensors: Identify the downstream O2 sensors. These are located after the catalytic converter (or where the catalytic converter would be in the case of catless downpipes). Typically, there are two downstream sensors.
2. Remove Downstream O2 Sensors: Carefully disconnect and remove the downstream O2 sensors using the appropriate wrench. Be cautious not to damage the sensors or their wiring.
3. Assemble Defoulers: Take two spark plug defoulers and screw them together, end-to-end. This stacked configuration provides sufficient spacing for the O2 sensor.
4. Install Defoulers and O2 Sensors: Screw the stacked defouler assembly into the O2 sensor bung on your exhaust pipe. Then, carefully screw the downstream O2 sensor into the end of the defouler assembly. Ensure they are snug but not over-tightened. Repeat this process for the other downstream O2 sensor.

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Step 3: Flash Your ECU to Stock Tune

If you are running a performance tune that disables O2 sensor monitors, you need to flash your Engine Control Unit (ECU) back to the stock tune. Using your tuning software (like BM3), revert to the factory software settings. This step is crucial because the stock tune will enable the readiness monitors, which are necessary for passing the emissions test. You do not need to relock your DME (Digital Motor Electronics) if it’s unlocked; simply flashing back to the stock map is sufficient.

Step 4: Complete the OBD2 Drive Cycle

After installing the defoulers and flashing to the stock tune, you need to perform an OBD2 drive cycle. This is a specific driving pattern that helps the OBD2 system run all its diagnostic checks and set the readiness monitors to “ready.” Drive cycles vary by vehicle manufacturer, but a general drive cycle includes:

• Cold Start: Start the engine when it’s cold (engine coolant temperature below 122°F or 50°C and within 6°F or 3°C of ambient air temperature). Do not idle for long periods upon startup.
• Idling: Idle the vehicle in drive for two and a half minutes with the air conditioning and all accessories off.
• Acceleration: Accelerate smoothly to 55 mph (88 km/h) at part throttle.
• Cruising: Maintain a speed of 55 mph (88 km/h) for three minutes.
• Deceleration: Decelerate smoothly, without braking, and coast down to 20 mph.
• Acceleration: Accelerate again to 55 to 60 mph (88-96 km/h).
• Cruising: Maintain a speed of 55 to 60 mph (88-96 km/h) for five minutes.
• Deceleration: Decelerate smoothly and come to a stop.
• Idling: Idle for five minutes in neutral or park with the air conditioning and all accessories off.

You can monitor the readiness monitor status using an OBD2 scanner app during the drive cycle. The “Emissions Test” or “Readiness Monitors” feature in your app will show the status of each monitor.

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Step 5: Verify Readiness Monitors and Take the Emissions Test

Once your OBD2 scanner app indicates that all relevant readiness monitors (specifically Catalyst Monitor and Oxygen Sensor Monitor) are set to “ready” or “passed,” you can proceed to your emissions testing station. With the defoulers installed and the stock tune active, your vehicle should now pass the OBD2 emissions test.

Step 6: Revert to Performance Tune (Optional)

After successfully passing the emissions test, you can revert your ECU back to your performance tune and, if desired, remove the spark plug defoulers. While some choose to leave defoulers installed, it’s generally recommended to remove them for optimal sensor accuracy during regular driving conditions, especially with performance tunes.

Important Considerations and Disclaimer

• Legality: Modifying your vehicle’s emissions system, even temporarily, might be against local regulations. Check your local laws regarding emissions control and testing.
• Environmental Impact: Catless downpipes remove catalytic converters, which are essential for reducing harmful emissions. While this DIY method may help you pass an emissions test, it does not change the fact that your vehicle will be producing higher levels of pollutants when catless downpipes are installed and catalytic converters are bypassed.
• Effectiveness: While defoulers are often effective, they are not a guaranteed solution for all vehicles. Results may vary depending on the car model, engine management system, and specific modifications.
• Temporary Solution: This method is intended as a temporary workaround to pass emissions testing. For regular driving, especially with performance tunes, consider the implications of bypassing emissions controls.

Conclusion

Passing OBD2 emissions with catless downpipes can be achieved using spark plug defoulers and flashing back to a stock tune. This DIY guide provides a cost-effective and reversible method to temporarily meet emissions requirements. By following these steps carefully and understanding the considerations, you can navigate emissions testing while enjoying the performance benefits of your modified vehicle. Remember to always prioritize legal compliance and environmental responsibility in your vehicle modifications.