2000 Honda Civic P0135 Code: O2 Sensor Heater Circuit Fault and Rough Idle Diagnosis
Hello everyone, I'm looking for some help with my 2000 Honda Civic. The check engine light came on, and after purchasing an OBD-II scanner during a sale, I pulled up the fault code P0135 — indicating a heater circuit failure in the oxygen sensor for Bank 1 (upstream of the catalytic converter). Before replacing the sensor, my car ran fine but showed poor acceleration, especially when transitioning from moderate speeds (35-45 mph) to stops. After coming to a halt, the engine would idle roughly and sometimes almost stall. The OBD-II reader displayed flashing 'EV' and 'C' fields during code reading — I'm not sure what this means, but it seems significant. I replaced the upstream oxygen sensor (bank 1), and now acceleration is much better. However, the rough idle issue persists. My current concerns are: - Do the flashing EV and C fields indicate a deeper problem? - Could I have a failing PCV valve? - Is it possible that my catalytic converter is damaged or cracked — especially since this is a known issue with older Civic models? I'm not an expert, so any insights on what to check next would be greatly appreciated. I'm feeling stuck and hoping for some clear technical guidance.
The catalytic converter is likely cracked at the location where the oxygen sensor mounts. This is a common issue in older Honda Civics, especially around this model year. A damaged converter can cause significant drivability problems and may be responsible for the rough idle and P0135 code. I've replaced hundreds of these units — if your converter is compromised, it should be inspected or replaced as soon as possible.