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How to Find an Exhaust System Leak

Consequences of Exhaust System Leaks

A leak in the exhaust system can cause big problems for the customer; decreased gas mileage, problems with the catalytic converter and damage to other components. If a leak is suspected, it’s time to do a thorough inspection. Check out the following scenario you may see in the shop and learn what steps you can take to resolve the issue.

How to Inspect for Exhaust Leaks

Proper air/fuel ratio or “fuel control” is required for modern converters to operate efficiently. The engine fuel management system relies on accurate MAF, oxygen and air fuel sensor readings. Very small leaks in the intake or the exhaust system can dramatically impact the accuracy of these readings. During your inspection, do the following:

  • Look for small pinhole leaks in welds around the O2 sensor ports.

  • Review all connections and exhaust components, upstream of the converter, for small leaks.

  • Examine all connections within 3" downstream of the converter outlet for small leaks.

  • Inspect intake boot connections between the MAF sensor and the intake manifold.

  • Check the intake manifold for proper sealing.

Most service providers rely on a smoke machine to help locate these small leaks, but there are a number of other methods that work well. Keep in mind that the exhaust system should be inspected for leaks when it is cold and again after it has been warmed to normal operating temperature.   

Next Steps

Once all intake and exhaust system leaks are resolved, proper MAF, O2 & AFR sensor operation should be verified. It should be noted that both pre- and post-O2 sensor operations are important for converter efficiency. Most service providers find that graphing readings from these sensors are helpful in identifying issues. Since converters are designed to store oxygen, post converter O2 sensor readings of 450mv or higher will typically indicate high converter efficiency.  
When working to resolve converter efficiency concerns, service providers need to be aware that a perfectly balanced air/fuel mixture (technically referred to as Lambda of 1) is one of the main requirements for high converter efficiency. Using a recently calibrated 4- or 5-gas analyzer is one of the fastest and most effective ways to calculate Lambda.  
If a converter has failed, the service provider should identify and resolve the root cause of the failure prior to replacing the converter. Converters don’t fail on their own; the main causes of failure are:

  • Overheated, melted or broken substrate (typically due to engine misfires or AFR cylinder imbalance).

  • Coated/oil fouled substrate (carbon buildup, excessive oil consumption, internal coolant leaks, improper fuels or additives, or use of non-converter safe gasket sealants).

  • Structural damage (corrosion, thermal shock, metal fatigue/stress fractures, stripped O2 threads, or flex pipe failure).


The content contained in this article is for informational purposes only and should not be used in lieu of seeking professional advice from a certified technician or mechanic. We encourage you to consult with a certified technician or mechanic if you have specific questions or concerns relating to any of the topics covered herein. Under no circumstances will we be liable for any loss or damage caused by your reliance on any content.

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