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1996 Ford Explorer XLT 4.0L EFI - P0420 Code After Smog Pass and Coolant Leak Concern

Model: 1996 Ford Explorer XLT Fault Code: P0420 Posted: 2008-10-27 20:18

Hello everyone, I recently encountered a check engine light on my 1996 Ford Explorer XLT with the 4.0L EFI engine, triggering Diagnostic Trouble Code (DTC) P0420. The issue appeared just before my smog inspection. I tested the oxygen (O2) sensors and found both were clean—so I reinstalled them. After clearing the code, the vehicle passed the smog test with excellent results; emissions levels were well below the maximum thresholds. However, about two weeks later, the check engine light came back on. I’m hesitant to replace the catalytic converter (CAT) since the car only has 80,000 miles on it and performance remains normal—no loss of power or drivability. Another issue: a new water pump was recently installed. A few weeks later, I noticed the coolant reservoir was empty. After refilling, I observed that the top of the radiator begins to leak when the engine is warm and running—though the car has never overheated. This makes me suspect a faulty thermostat or a hose with a pressure-related leak. Could these two issues be related? I’ve read that P0420 can sometimes be triggered by a coolant temperature sensor malfunction. Is it more likely to be an O2 sensor issue or a failing catalytic converter? Any insights or troubleshooting steps would be greatly appreciated! Thanks in advance.

Related fault codes
P0420
Comments (6)
Anonymous 2008-10-28 20:18

I’ve never seen a thermostat or coolant temperature sensor directly cause a P0420 code. The most common causes are either a failing catalytic converter or a PCM reprogramming—though the latter is often just a temporary fix to avoid warranty issues.

Anonymous 2008-10-28 22:59

The radiator top is leaking? Could you specify which component is actually leaking?

Anonymous 2008-10-29 20:20

It happens while driving—like at a stoplight—and I can hear fluid flowing out. When the car stops, the reservoir is empty. This is actually my father-in-law’s vehicle. Now that I think about it, I suspect it's a hose. Likely under pressure, it's leaking from the hose connection near the top of the radiator. Back to the catalytic converter: Would you recommend replacing the O2 sensors first before investing in a new CAT? Thanks for your input!

Anonymous 2008-10-29 21:06

P0420 is one of those rare codes that should be taken seriously. If I were troubleshooting, I’d prioritize replacing the catalytic converter—based on my experience, defective O2 sensors are extremely uncommon as a root cause.

Anonymous 2008-10-30 22:10

It’s strange that the CAT failed after only 80K miles and yet produced excellent smog results. If the converter were truly failing, wouldn’t it show poor emissions or fail the smog test? I’ll be testing the vehicle this weekend and will update with my findings—thanks for the input!

Anonymous 2008-11-01 18:00

The 1996 Ford Explorer XLT has two catalytic converters—one per engine bank—so that explains why smog results were good. But the vehicle’s OBD-II system still reports a failure. What a frustrating aspect of modern diagnostics!