2008 Chrysler 300 Intermittent Check Engine Light with P0406 Code After Tune-Up
I recently performed a tune-up on my mother's 2008 Chrysler 300 with a 2.7L engine. The car had no Check Engine Light (CEL) prior to the service. To access the spark plugs and PCV valve, I removed the upper intake manifold. The original OEM spark plugs were found to be faulty, so I replaced them along with the PCV valve, air filter, and all intake gaskets. I also added a fuel treatment product. After driving the car for one week without any issues, I noticed a steady drip of fuel while refueling. Upon inspection under the vehicle, no visible leak was found—however, it appeared to be originating from the top of the fuel tank. A technician suggested this might be due to a shutter valve at the top of the tank. Shortly after, the Check Engine Light activated with codes P0406, P0455, and P0456. After removing the intake manifold, I discovered that the EGR tube connection to the EGR valve was missing its gasket. I installed a new gasket, which resolved the P0455 and P0-456 codes (related to evaporative emissions leaks). The remaining code, P0406 (EGR system malfunction or excessive exhaust pressure), was cleared but returned after one week. I have since inspected all wiring and hoses for blockages—no issues were found. After clearing the code, it reappeared again after a week. I'm seeking advice on how to properly diagnose and resolve this intermittent P0406 issue without further complications.
The P0455 and P0456 codes are related to evaporative emissions leaks—your repair of the EGR tube gasket likely resolved those. However, you did not address the P0406 code. The most reliable method is a smoke test to locate any fuel or exhaust system leaks. Smoke tests do not run continuously, so it's normal for codes to reappear after clearing them. Always remember: absence of a Check Engine Light does not mean there are no underlying fault codes—use an OBD2 scanner to verify all stored codes.