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2003 Saturn L200 P0420 Error After Resolving P0300 and P0141/P0172 - O2 Sensor or Catalytic Converter Issue?

Model: 2003 Saturn L200 Fault Code: P0141 Posted: 2012-11-20 10:01

My 2003 Saturn L200 (2.2L 4-cylinder, 130,000 miles) has developed a P0420 error two days after successfully resolving previous issues of P0300 and P0141/P0172. I replaced several components to fix earlier faults: - Ignition coil (OEM) - Ignition controller (OEM) - Spark plugs (OEM) - Downstream O2 sensor (B1S2, OEM) The P0300 misfire was resolved after these replacements. The P0141 and P0172 codes disappeared following the downstream O2 sensor replacement and a vacuum leak check — no leaks were found. However, now I'm seeing a persistent P0420 code (catalytic converter efficiency below threshold). When I replaced the downstream O2 sensor, the probe was warped at approximately 30 degrees, which raised concerns about potential damage to the catalytic converter. I performed two basic checks: 1. After driving for some time, I inspected the exhaust pipe and catalytic converter for glowing — no signs of overheating or glow were visible. 2. I tapped the converter with a tool to listen for rattling — no unusual sound was detected. Given that the upstream O2 sensor (original part) is now likely nearing end-of-life, I suspect it may be sending incorrect signals to the engine control module (ECM), leading to poor fuel trim and reduced catalytic converter efficiency. This could explain the P0420 code even if the converter itself is functional. I’ve already invested around $400 in repairs over the past week, and I’m currently out of budget until next month. While a new O2 sensor (upstream) would cost less than a catalytic converter ($150–$300), I'm unsure whether to replace just the upstream sensor or invest in a full converter. My questions: - Is it safe to continue driving the vehicle for at least one month with short 20-minute commutes (back and forth to work) without damaging previously replaced parts? - Could this P0420 be solely due to a failing upstream O2 sensor, or is there a high likelihood that the catalytic converter itself is faulty? I’m considering replacing the upstream O2 sensor since it’s approaching its normal lifespan — but I’d like to confirm whether this would resolve the issue before spending more money.

Related fault codes
P0141P0172P0300P0420
Comments (4)
Anonymous 2012-11-20 11:16

You mentioned the replacement of B1S2 downstream O2 sensor — did you install an OEM part or a generic, low-cost one? A scanner is essential to check actual sensor readings. The persistent P0172 (rich mixture) code makes me wonder about current fuel trim settings. Check spark plug condition: wet or dry? Also inspect the fuel pressure regulator vacuum hose for any signs of damage — these are simple but critical checks.

Anonymous 2012-11-20 11:54

Thanks, Kev. The check engine light is no longer flashing — it's been off since the O2 sensor replacement. My car is old and all parts were original OEM as recommended by everyone I've spoken to. While I didn't do a detailed inspection of the spark plugs initially, they appeared worn and sooty. After research, I now suspect oil leaks could be contributing — oily spark plugs are a clear sign. The plug wasn’t shiny at all, but I was just looking for general wear. Lesson learned: don’t skip inspections. All hoses and connections look fine. As long as the car runs normally, driving it until next payday is acceptable. I’m tempted to replace the upstream O2 sensor since it’s likely past its lifespan — but budget constraints make that a tough call.

Anonymous 2012-11-20 12:10

The most reliable way to diagnose this is using an OBD2 scanner to monitor real-time O2 sensor readings and fuel trim values. Compare upstream vs downstream data — this helps determine whether the issue stems from faulty sensor input or actual converter failure. While you're right that the upstream O2 sensor may be due, replacing it could resolve the P0420 without needing a full catalytic converter replacement. This is a more cost-effective and safer option than guessing.

Anonymous 2012-11-20 12:26

Just bought a basic $60 OBD2 scanner — will use it to monitor sensor data soon!