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2003 Sunfire P0420 Code: MIL Light Off, Cat Replacement Cost & Diagnostics

Model: 03 Sunfire Fault Code: P0420 Posted: 2015-08-19 13:12

My 2003 Sunfire (2.2L) has been showing a persistent P0420 diagnostic trouble code, but the Check Engine Light (MIL) has turned off and remained off for several days. I used a graphing scan tool to monitor both upstream and downstream oxygen sensors — the downstream O2 sensor is tracking closely with the upstream one, which led me to believe the catalytic converter (CAT) may be failed. The vehicle is at 124,000 miles. Despite no visible MIL illumination, the P0420 code remains stored in the ECU. I'm concerned: could the MIL light have simply burned out? Or is there a deeper issue with sensor readings or system behavior? I've also noticed that when the engine idles, the downstream O2 sensor flatlines at approximately 0.7 volts. At 2500 RPM, it tracks the upstream O2 sensor (which normally switches between 0.2 and 0.9 volts, about twice per second). The downstream sensor appears to lag by roughly a quarter cycle, and after throttle snap shut, it takes around 10 seconds to return to flatline. The STFT values are within normal range (+/-5%), while LTFT is at -3.5%. No vacuum leaks were detected, and I haven't tested further due to the apparent functionality of the sensors. I'm wondering if this behavior indicates a failing catalytic converter — especially given that newer units are sometimes referred to as 'LOC' (Low Oxygen Catalyst), where the post-cat sensor should track the pre-cat sensor. However, I've only seen one article on this topic and remain skeptical. The dealer is quoting $1600 for a replacement CAT, which seems extremely high. Are there more affordable or reliable options? What are common diagnostic steps before replacing the converter? Thanks for any insights — I'm not an expert in OBD diagnostics but would appreciate practical advice from experienced users.

Related fault codes

P0420

Comments (5)

Anonymous 2015-08-19 15:21

It's not uncommon for the MIL light bulb to fail. If a code is present, the light should typically illuminate — especially during startup. The bulb performs a self-test at every engine start. Check if the light comes on when starting the vehicle; if not, it may be a faulty bulb or wiring issue.

Anonymous 2015-08-21 22:24

Before spending $1200+ on an OEM catalytic converter, I'd like to confirm what my scan tool is showing. The engine enters closed-loop within about 20 seconds after startup. Both O2 sensors are 4-wire (heated), and the heater function appears normal. At idle, the downstream O2 sensor flatlines at ~0.7V. At 2500 RPM, it tracks the upstream O2S1, which normally switches between 0.2V and 0.9V about twice per second. The downstream sensor lags by roughly a quarter cycle. After throttle snap shut, it takes around 10 seconds to return to flatline. The STFT is stable within ±5%, while LTFT sits at -3.5%. I've seen references suggesting that newer 'LOC' (Low Oxygen Catalyst) converters require the post-cat sensor to track the pre-cat sensor — but this seems like a rare or outdated concept. Given my observations, does it still point to a failing catalytic converter? The engine runs smoothly with no performance issues. No vacuum leaks detected. I haven't tested sensors further because they appear functional. I understand diagnosing remotely is challenging, so I'm asking: does this behavior suggest a bad CAT? Any recommendations for affordable or reliable alternatives?

Anonymous 2015-08-22 12:29

P0420 should trigger the MIL light — could you be viewing a 'Pending' or 'History' code instead of an active one? The downstream O2 sensor reading appears high (flatlined at 0.7V), which may indicate sluggish performance. A slow response time suggests a non-responsive sensor, not just a failing CAT. Are there any other related codes present? Did you check the Engine Coolant Temperature (ECT) sensor?

Anonymous 2015-08-23 13:41

The ECT appears to be functioning correctly — it sends accurate temperature readings to the dashboard and is visible in real-time on my scan tool. The engine temperature climbs normally and stabilizes. I believe the ECT is working properly. Thanks for your input. Any additional diagnostic steps or troubleshooting tips would be greatly appreciated.

Anonymous 2015-08-23 15:25

If the scanner shows an ECT reading of around 200°F, that's a good sign — it indicates proper engine temperature. By the way, is this a 2.2L engine? I don't think spending $1600 on a catalytic converter is justified — some parameters used in CAT evaluation are arbitrary and expensive. Let’s try low-cost, quick fixes first: add one bottle of SeaFoam or Techron fuel cleaner, inspect spark plugs (AC Delco, tan/grey color), ensure the air filter isn't oily. Check for PVC valve issues to prevent oil from entering combustion chambers. A clean engine and proper operating temperature are essential — the CAT needs heat to function effectively. Try these steps before replacing the converter.