2003 Mercury Grand Marquis P1406 and P0355 Codes: Surging and Rough Idle Diagnosis
My 2003 Mercury Grand Marquis with a 4.6L engine is experiencing surging during acceleration and rough idle. The vehicle has triggered two diagnostic trouble codes: P1406 (EGR system malfunction) and P0355 (misfire detected in cylinder 5). I'm looking for guidance on how to properly diagnose these issues. Specifically, the P0355 code points to a potential faulty ignition coil or wiring problem in cylinder 5, while P140-6 may indicate a downstream EGR valve issue—such as a clogged hose, disconnected sensor, or electrical circuit fault. I'd like to know the recommended steps for troubleshooting these codes, including how to check hoses and verify electrical circuits before replacing components.
Hi Chris — thanks for posting. You're right to suspect a bad ignition coil or wiring issue, especially since P0355 refers specifically to cylinder 5. Use a multimeter and test light to verify the circuit integrity. The P0355 code indicates a failure in ignition coil number 5. For P1406 related to the DPFE sensor, check if the downstream hose is clogged or disconnected. If so, inspect for physical obstructions or sensor detachment. Always perform electrical circuit testing after verifying mechanical restrictions — avoid replacing parts without first diagnosing. While DPFE sensors are common failure points, I recommend testing circuits before swapping components.