P0420 Code in 2002 Ford Cougar with 4-Cylinder: Diagnosis and Solutions
I own a 2002 Ford Cougar with a 4-cylinder engine that is displaying a P0420 diagnostic trouble code. I have inspected the exhaust system thoroughly and found no visible damage. I recently replaced the secondary oxygen (O2) sensor, cleared the code, but it has returned repeatedly. Both the primary and secondary O2 sensors are showing voltage readings between 100 and 999 mV (which typically corresponds to 0.1 to 0-9 volts). The vehicle has approximately 62,000 miles on it. I'm seeking advice or potential causes for this recurring P0420 issue. Any insights into possible underlying problems would be greatly appreciated.
You're likely measuring O2 sensor voltages with a digital voltmeter (DVOM), and you're referring to readings between 0.1 and 0.9 volts — that's correct. What's actually more critical than voltage alone is the O2 sensor cross-counts or fuel trim values from the vehicle's engine control module (ECM). To properly diagnose, ensure there are no vacuum leaks, verify fuel pressure levels, and double-check the functionality of the front oxygen sensor as well.