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P0420 and P0141 Diagnostic Issues on 2007 Honda Civic EX with VTEC Engine

Model: 2007 Honda Civic EX Fault Code: P0141 Posted: 2025-08-16 00:01

I own a 2007 Honda Civic EX equipped with an inline-4, 1.8L VTEC engine. A few days ago, the Malfunction Indicator Lamp (MIL) illuminated. Using an OBD2 scanner, I detected two diagnostic trouble codes: P0420 and P0141. - P0420: 'CAT system efficiency below threshold - Bank 1' - P0141: 'O2 sensor heater circuit fault - Bank 1 Sensor 2' Live data readings: - Upstream O2 sensor (O2-S1): reads 0.0 volts — no signal. - Downstream O2 sensor (O2-S2): fluctuates between 0.1 and 0.9 volts, behaving like an upstream sensor. I initially suspected the upstream O2 sensor was faulty, as it had functioned reliably for over 18 years. I replaced it with a new aftermarket unit purchased from Amazon, following the OEM replacement procedure (using proper torque specifications). After starting the engine, the upstream sensor still showed no voltage. The downstream sensor remained erratic. I cleared the DTCs and performed a 20-minute test drive. The MIL turned off, but the O2 readings did not improve — the upstream sensor still reads zero volts, and the downstream sensor continues to behave abnormally. To verify whether my scanner was misreading sensor positions, I replaced the downstream O2 sensor as well. After reinstallation, the MIL lit up again, and P0141 returned. The live data remains unchanged: upstream sensor = 0.0V; downstream sensor = erratic 0.1–0.9V range. I am now at a loss. The issue appears to be persistent despite replacing both sensors. I suspect either the sensors themselves are defective, or there is an underlying wiring or ECU-related problem. Could this be due to faulty sensor heater circuits or improper signal interpretation? Any technical insights or troubleshooting steps would be greatly appreciated.

Related fault codes

P0141P0420

Comments (4)

Anonymous 2025-08-16 00:17

Could the O2 sensors be aftermarket and low-quality? I recommend using OEM parts from an authorized dealership. Did you inspect all wiring harnesses and connectors during installation for signs of damage or corrosion?

Anonymous 2025-08-16 00:37

The sensors I purchased were labeled 'Brand X' (manufactured in China). No visible issues were found with the wiring or connections. I assume OEM parts are significantly more expensive — is that accurate? Could this be a quality issue?

Anonymous 2025-08-16 08:33

Did you retain the original sensors? Sometimes, even if the sensor appears to fail, it might not be the root cause. My son replaced an aftermarket O2 sensor with a cheaper Chinese unit and later found that the OEM Toyota sensor was required for proper performance due to insufficient sensitivity. Yes, OEM parts are more expensive — but they often deliver better accuracy and reliability. Also, ensure you're reading the upstream O2 sensor after the engine has warmed up, not immediately after a cold start. The ECU typically runs in open-loop mode during startup and may ignore O2 readings until warm-up is complete.

Anonymous 2025-08-16 18:03

Webmaster wrote: Sat Aug 16, 2025 8:33 am Did you retain the original sensors? It's possible that the root cause isn't in the sensor itself. My son bought a budget aftermarket O2 sensor to save money and ended up spending twice as much on an OEM Toyota unit because it wasn’t sensitive enough. Yes, OEM parts cost more — but they're worth it for performance and reliability. Are you reading the upstream O2 sensor right after a cold start or after the engine has warmed up? The ECM usually ignores O2 sensors during cold starts and operates in open-loop mode. -------------------- Yes, I still have both original sensors. I found an OEM upstream O2 sensor on eBay for $57.70 — currently awaiting shipping. I plan to return the Chinese aftermarket units to Amazon for a full refund. This is only the second time I've encountered low-quality 'gray market' components from China. I will reinstall the original downstream Honda sensor (which was working fine), clear the codes, and monitor the system behavior after a warm-up cycle. Thanks for the helpful insight!