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2001 Subaru Legacy Wagon P0420 Code: Front O2 Sensor Reads 3-4V, Cat Efficiency Issue & DIY Diagnostic Tips

Model: Subaru Legacy Fault Code: P0420 Posted: 2009-10-24 11:26

I own a 2001 Subaru Legacy Wagon (manual transmission) with 137,000 miles. The Check Engine Light is on, and I've been receiving the P0420 diagnostic trouble code — indicating poor catalytic converter efficiency for Bank 1. I’ve already ruled out exhaust leaks by tapping all pipes with a hammer and sealing a previous leak behind the catalytic converter. I replaced the rear O2 sensor in July with an OEM Denso unit purchased from RockAuto, and it has been functioning well since then. Today, I had a technician perform a diagnostic using their Snap-on OBD-II scanner. The front oxygen sensor reading showed 3–4 volts — a value that is not typical for a properly operating upstream O2 sensor (which should read between 0.1V and 0.9V under normal conditions). The mechanic confirmed this reading directly on the scanner, but noted it was likely measuring heater circuit voltage rather than signal voltage. The technician did not charge me for the diagnostic — which I appreciate — though they acknowledged it was inconclusive. They suggested possible causes including a failing engine control module (ECM), damaged wiring harnesses, or degraded catalytic converter performance. The vehicle had been involved in an accident in September 2004, prior to this issue emerging. They also pointed out that the heat shields around the catalytic converter may be loose, and noted that Subaru often recommends replacing the converter as part of a repair for such issues. I plan to take it to the official dealership tomorrow night for their $98 diagnostic. However, I wanted to share this post to see if any readers have suggestions or troubleshooting steps I could try in the meantime. I’ve read that re-flashing the ECM at a dealer can sometimes resolve P0420 issues on older Subaru models — especially when the problem stems from software misinterpretation rather than hardware failure. This may extend the life of an aging converter. As a side note, I've seen online products that allow connecting a laptop directly to a car’s OBD-II port to read diagnostic data. While I’m technically inclined, I’d appreciate recommendations for affordable, reliable tools (e.g., OBD2 cable and software) that could help me monitor the system independently. Any insights or experiences with this specific model and code would be greatly appreciated!

Related fault codes
P0420
Comments (3)
Anonymous 2009-10-24 15:20

You took it to PepBoys for diagnostics — their reputation, especially for imported vehicles like Subarus, is generally weak. A reading of 3–4 volts from the front O2 sensor is highly unusual and likely indicates they're measuring heater circuit voltage rather than actual signal voltage. This suggests a misinterpretation of data. It's reasonable to suspect that your catalytic converter may be degraded — many users report that cheaper or lower-quality converters fail to resolve P0420 codes. If the dealer recommends an ECM reflash, consider it; while it won’t fix a failing converter, it can help prolong the life of the existing one.

Anonymous 2009-10-24 22:43

Thanks for clarifying — the front O2 sensor reading of 3–4 volts was confirmed by the mechanic using their Snap-on OBD-II scanner. The technician didn’t charge me because the diagnostic was inconclusive, which I understand and respect. However, it's important to note that a reading in this range is not typical for an upstream oxygen sensor. It’s likely they were measuring heater voltage, not signal voltage. For P0420 diagnosis on a 2001 Subaru Legacy, the correct sensor to monitor is actually the downstream O2 sensor after the catalytic converter. I’d strongly recommend visiting your authorized Subaru dealer for both a reflash and possibly replacing the catalytic converter.

Anonymous 2009-10-25 08:18

The only positive thing they did was not charge you — but if they keep using their scan tool for free, they’ll be paying for it eventually. It's highly likely that they're reading the applied heater voltage instead of the actual signal from the O2 sensor. Regardless, when diagnosing P0420 on a 2001 Subaru Legacy, the correct sensor to evaluate is the downstream one after the catalytic converter. I’d strongly advise scheduling a reflash with your dealer and considering a replacement catalytic converter — especially given the vehicle’s age and history of prior damage.