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2004 Cadillac DeVille P0420 Code: Misfire Causes, Sensor Issues & O2 Sensor Replacement History

Model: 2004 Cadillac 420 Fault Code: P2177 Posted: 2011-11-20 16:19

I own a 2004 Cadillac DeVille with 164,000 miles. The vehicle consistently throws a P0420 diagnostic trouble code, which triggers the engine warning light. Despite replacing all relevant sensors three times and the catalytic converter twice, the issue persists. The engine exhibits intermittent misfires—sometimes running smoothly, then suddenly stumbling or misfiring. It occasionally runs rough during startup before stabilizing. I've already performed a vacuum leak inspection, including replacement of a rubber boot between the intake manifold and throttle body. My current theory is that the misfire is causing inaccurate sensor readings, but my mechanic has found no underlying issues. I'm seeking reliable insights—this isn't just about replacing parts blindly. Any technical explanations or diagnostic suggestions would be greatly appreciated.

Related fault codes
P2177
Comments (3)
Anonymous 2011-11-21 06:00

I recommend checking the live data from the OBD2 scanner, especially fuel trim values and cylinder misfire counts during both stable and rough running conditions.

Anonymous 2011-11-22 12:50

That's a solid point—since I'm relying on my own eyes to interpret the data, it's clear that an independent technician or diagnostic tool might reveal more accurate insights.

Anonymous 2011-11-22 19:45

Great follow-up question: What are the fuel trim values when the engine is running smoothly versus when it misfires? Which cylinders show persistent misfire activity? Also, what brands and models of oxygen sensors and catalytic converters have you tried so far?