1997 Honda Civic DX: P0135, P0141, P0501, P0700, P0720 Codes Appearing Simultaneously - O2 Sensor & Speed Sensor Issues
I own a 1997 Honda Civic DX with a 1.6L engine and automatic transmission. Recently, all the following diagnostic trouble codes appeared at once while driving: P0135, P0141, P0501, P0700, and P0720. The vehicle began exhibiting rough idling, erratic speedometer readings, and abnormal gear shifting. I have already replaced both oxygen sensors within the past 24 months. After replacing the speed sensor, codes P0501, P0700, and P0720 cleared — however, P0135 and P0141 persist. I'm wondering if there's a connection between the speed sensor failure and the oxygen sensor heater circuit errors. Could a grounding issue be causing this? I've visually inspected all wiring and found no obvious faults. Is there a specific fuse that might be involved? I plan to conduct further voltage and resistance testing under daylight conditions to verify sensor readings and wiring integrity. Any insights or troubleshooting steps would be greatly appreciated!
P0135 and P0141 relate to the heated oxygen sensor heater circuits. On a 1997 Honda Civic DX, there are typically two oxygen sensors — one before and one after the catalytic converter. Check the wiring harness for the heater circuits from each oxygen sensor to the ECU. The heater circuit should supply full 12V (battery voltage) at the sensor end. Ensure both terminals on the sensor connector and the ECU side are clean and free of corrosion. Use contact cleaner or a contact spray — contamination can cause intermittent faults even if wiring appears intact.