2004 Cadillac DeVille P0300 Code: Multiple Cylinder Misfire, Spark Plug & Coil Replacement Failed
I own a 2004 Cadillac DeVille that is currently showing a persistent P0300 code with a flashing check engine light. The vehicle experiences noticeable misfiring and sputtering, especially under load or at idle. I previously diagnosed the issue as a spark plug or coil problem and replaced both spark plugs and two ignition coils—this provided temporary relief (lasting only about two miles). The P0300 code has returned, along with full engine misfire. I have ruled out vacuum leaks, loose electrical connections, and EGR valve blockage. Exhaust emissions are rich but not foul-smelling like rotten eggs. A new fuel pump and filters were installed approximately 6,000 miles ago and do not appear to be the root cause. Using an OBD2 scan tool, cylinder #7 is consistently flagged as misfiring and appears completely dead. I swapped the fuel injector on cylinder #7 with that of cylinder #6—afterward, cylinder #7 still misfires. A new ignition coil was also installed on cylinder #7, but no improvement has been observed. Compression testing showed normal readings, and spark checks confirmed proper spark delivery to all cylinders. Despite these efforts, the engine continues to misfire with the P0300 code active. Possible causes I'm considering include: ignition module failure, crankshaft or camshaft position sensor issues, internal wiring harness faults (especially in the coil-to-ECU path), or a head gasket leak allowing coolant into combustion chambers. Any insights or diagnostic steps would be greatly appreciated.
P0300 indicates a random/multiple cylinder misfire detected. If the issue persists, perform a spark plug visual inspection. Remove all spark plugs and examine them in their original positions—look for sooty black or wet plugs, which indicate a failing cylinder. In your 2004 Cadillac DeVille (which uses individual coil-per-cylinder ignition), consult your shop manual to trace the wiring from the ECU to each coil and injector. Use a digital multimeter to check continuity; if any wire path shows significantly higher resistance, it may indicate an internal break in the harness. If you wiggle the connector and the resistance changes, that confirms a loose or damaged wire. For cleaning: apply electric contact cleaner directly on the female connectors (on the harness side), then dry thoroughly with an alcohol-based blower. For terminals, use a cotton swab dipped in contact cleaner to gently clean surfaces. Sensors are less likely at fault here since no other diagnostic codes are present—unless additional faults have been detected. When does the misfire occur? Does it happen only at idle and improve when revved? Or does it start smoothly at idle and worsen as RPM increases? Or is there a consistent misfire across all engine speeds?