A small exhaust leak developed at the connection between the mid-pipe and header downpipe after reinstalling a stock exhaust system (stock catted mid-pipe and muffler) on a 2020 Club 6MT with a custom Fab9 tune for 93 octane and bolt-ons. The vehicle was driven for approximately 40 miles without issue. However, after restarting the car following a wash, the check engine light illuminated with an “Engine Malfunction” message. The car continued to operate normally without noticeable power loss or abnormal behavior during the 5-mile drive home.
A potential cause of the exhaust leak is a worn-out gasket between the mid-pipe and header. The vehicle has a custom tune and a full tank of 93 octane fuel. The check engine light had never previously illuminated. Why would it take 45-50 miles for the light to activate?
Several factors could contribute to the delayed onset of the check engine light. The OBDII system monitors various engine parameters and requires specific conditions to be met before triggering a code and illuminating the malfunction indicator lamp (MIL). A small exhaust leak might not immediately trigger a code. The system may require multiple drive cycles or a certain threshold of detected emissions to be exceeded.
An exhaust leak can introduce excess oxygen into the exhaust stream, affecting the oxygen sensor readings and potentially causing a lean condition. The engine control unit (ECU) compensates for this by adjusting the fuel mixture. Over time, these adjustments may exceed acceptable parameters, triggering an OBDII code related to the fuel system or oxygen sensor. Additionally, a leaking exhaust can increase noise levels and potentially damage other components due to excessive heat.
Possible OBDII codes related to exhaust leaks include:
- P0420: Catalyst System Efficiency Below Threshold (Bank 1)
- P0430: Catalyst System Efficiency Below Threshold (Bank 2)
- P0133: O2 Sensor Circuit Slow Response (Bank 1, Sensor 1)
- P0134: O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 1)
- P0135: O2 Sensor Heater Circuit Malfunction (Bank 1, Sensor 1)
To diagnose the specific issue, retrieving the OBDII code with a code reader is crucial. Resolving the exhaust leak by replacing the old gasket is recommended. Additionally, ensuring the proper installation of the oxygen sensor and verifying the compatibility of the custom tune with the stock exhaust system are important steps. If the problem persists after addressing the exhaust leak, returning to the factory tune may help isolate the cause of the issue. Cold weather conditions and summer tires are unlikely to directly contribute to the engine malfunction.
