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P0420 Fault on 1997 Taurus SHO: Catalytic Converter Issue and O2 Sensor Diagnosis

Model: Sho Fault Code: P0420 Posted: 2007-10-14 10:58

I'm experiencing a recurring P0420 fault on my 1997 Taurus SHO. The check engine light has cleared on its own and remained off for approximately two weeks before reappearing. I've scheduled an emissions test for tomorrow, during which I cleared the code yesterday and completed a 100-mile round trip to work with no warning light activation. The previous owner installed a new oxygen sensor (O2), but this likely did not resolve the underlying issue. My vehicle is equipped with a post-catalytic converter system, featuring three total O2 sensors in a V8 configuration and runs smoothly under normal conditions. I will now verify emission readiness today. I also tested my wife's 2002 Town & Country LXI AWD, which showed no issues during diagnostics—though it reported a missing diagnostic on the EVAP module. Is this a carryover from an older electronic voice module (EVA) system? I believe EVA stands for EVAP, and the van may require driving under specific conditions to complete its EVAP system tests. I'm seeking advice on whether the P0420 is most likely due to a failing catalytic converter or faulty rear O2 sensors. Any insights would be greatly appreciated.

Related fault codes
P0420
Comments (7)
Anonymous 2007-10-14 13:28

It's best to address each issue separately for clarity. Regarding the P0420, this code typically points to a failing catalytic converter. Since the previous owner replaced an O2 sensor (which is not a reliable fix), it's likely the catalyst has been operating inefficiently for some time. I recommend having the catalytic converter professionally diagnosed or replaced. Please open a new post for your Town & Country EVAP module issue.

Anonymous 2007-10-14 14:02

I agree with the catalytic converter being the primary suspect. The EVAP module issue on the Town & Country—what you initially thought was an old 'EVA' module—is actually short for EVAP (Evaporative Emission Control). The van requires a certain amount of driving to activate its EVAP system tests, which may explain why diagnostics show a missing test rather than a fault. Thanks for clarifying!

Anonymous 2007-10-14 16:57

Sorry about that—I accidentally deleted my first post while trying to manage threads related to AllPar and M109riders. Apologies for the oversight! Thank you for your patience and continued contributions to the forum.

Anonymous 2007-10-14 21:20

No worries at all—thank you for your work on maintaining thread organization and community engagement!

Anonymous 2007-11-05 17:40

After inspecting under the vehicle, I noticed that the previous owner only replaced the front O2 sensors—not the rear ones. I recently replaced both rear O2 sensors (Bank 1 and Bank 2) as a preventive maintenance step. To my surprise, this resolved the P0420 issue. The catalytic converter was functioning properly—this was purely a sensor-related fix. I’m grateful for the outcome.

Anonymous 2007-11-06 09:13

I personally question whether replacing only the rear O2 sensors would resolve a P0420. The P0420 code specifically indicates that the rear O2 sensors are switching at a rate similar to the front ones, which suggests the catalytic converter is not effectively converting exhaust gases. In my experience, O2 sensors typically fail by shorting out or exhibiting slow response—never by showing normal switching behavior. A true P0420 requires actual catalyst inefficiency. Please update this thread if you experience the code again.

Anonymous 2007-11-06 18:10

Thanks for the input! I've driven over 1,000 miles since the repair and have not seen the P0420 code return. I passed my Maryland emissions test today with no issues. The code has remained cleared—so far, this solution appears to be effective.