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P0420 Fault on 2002 Saturn L200: Causes, Diagnosis, and Fix After Using Lucas Additive

Model: saturn L200 Fault Code: P0420 Posted: 2014-03-25 13:31

I recently purchased a 2002 Saturn L200 with 107,000 miles and it has been well maintained with regular servicing every 3,000 miles. After adding Lucas exhaust system cleaner, the check engine light came on and now displays only fault code P0420 — indicating a catalytic converter efficiency below threshold. I have performed several checks: no visible exhaust leaks, consistent exhaust flow under acceleration, no power loss during uphill driving or speed gains, no coolant leaks or abnormal fluid levels. The engine idles smoothly without vibrations or misfires and behaves normally under load. I’ve used Lucas additives in my previous vehicles (PowerStroke, V10 Excursion, Neon, Grand Caravan) for years with no issues, so I’m concerned this might be a sensor-related issue caused by the additive. The car has no other diagnostic codes besides P0420. Should I simply drive it until the fuel tank is empty and refill to see if the code clears? Or is there a better course of action? I’d like to avoid damaging my engine, so any advice on proper diagnosis or next steps would be greatly appreciated.

Related fault codes
P0420
Comments (6)
Anonymous 2014-03-25 22:13

Use an OBD2 scan tool to monitor the air-fuel (AF) ratio and oxygen (O2) sensor voltage readings. The Lucas additive may have interfered with sensor sensitivity or calibration, causing a false P0420 reading.

Anonymous 2014-03-26 05:28

Okay, I'll use an OBD2 scanner to check the AF and O2 sensor voltages. Would it be safe to also flush the engine with Sea Foam as a preventive measure?

Anonymous 2014-03-26 10:07

I'm not sure how much difference Sea Foam would make compared to Lucas. Focus should remain on diagnostic data from your scan tool — sensor readings are more reliable than additive-based fixes.

Anonymous 2014-03-26 12:08

The issue likely didn't exist before using the Lucas product — it may have overwhelmed or temporarily disabled the oxygen sensors. Try a long drive to heat up the exhaust system, then clear the code and monitor if it returns.

Anonymous 2014-04-01 06:06

I cleared the P0420 code and drove for a short distance — the check engine light came back on immediately. After driving for several days, I took it to a friend who scanned it. The P0420 was gone but a new P0410 appeared (related to the air pollution control pump). I then used Sea Foam to decarbonize the engine by disconnecting the vacuum line at the pump and slowly pouring in half a can while the engine was running, with my wife shutting it off once it started spilling. I let it sit for 30 minutes, checked all hoses, and disconnected the battery during that time. After restarting, there was heavy smoke during initial runs (0-50 mph at WOT), followed by normal operation after a full hour of driving. It's now day 4 with no check engine light. I drove about 1.5 hours away and back on Sunday and around town Monday — everything is functioning normally. I'm sharing this in case others face similar issues with their Saturn L200s. Thanks to everyone for the advice! — Bryan

Anonymous 2014-04-01 09:02

I can't dispute the success story here — Sea Foam may have helped clear carbon buildup and restored engine performance, especially in older vehicles like this one.