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P0420 Error in 2004 Ford Taurus SE: Catalytic Converter or Dirty O2 Sensors?

Model: Taurus Fault Code: P0420 Posted: 2010-11-08 10:29

I own a 2004 Ford Taurus SE sedan and am currently experiencing a check-engine light. The diagnostic trouble code displayed is P0420. Attached is the full OBD-II data for reference: [OBD-II Data for 2004 Ford Taurus SE](http://www.robvangogh.com/misc/obd_ford_taurus_2004.pdf). The issue is whether this P0420 code indicates a failing catalytic converter or simply dirty oxygen (O2) sensors. Could the problem be caused by one of these components? What are the most likely causes and how should I proceed with diagnosis or repair?

Related fault codes

P0420

Comments (4)

Anonymous 2010-11-09 06:52

I recommend replacing the catalytic converter on Bank 1. The O2 sensors appear to be functioning properly. However, the vehicle seems to be running slightly lean. Please inspect all vacuum hoses, including those in the PCV system, and ensure the Mass Air Flow (MAF) sensor is clean and calibrated. Good luck—please update us on your findings.

Anonymous 2010-11-09 07:39

Thanks for the input, Mohawkmtrs. What specific value should I be looking at to confirm that my vehicle is running lean?

Anonymous 2010-11-10 07:43

Check the Short Term Fuel Trim values. A healthy system should show variation of no more than ±5% to ±10%. In your case, the trim value is +23%, indicating fuel is being added to compensate for a lean condition. The positive sign (+) confirms that the engine is running lean—though not severely enough to trigger a P0420 code directly.

Anonymous 2010-11-17 21:50

P0420 in a 2004 Ford Taurus SE almost always points to a failing catalytic converter. Regardless of other factors, you should replace it with an OBD-II compliant catalytic converter. If you choose a low-quality unit, the code will likely return. Invest in a high-quality replacement to ensure long-term reliability.