Experiencing a P0420 code on your 2007 Honda Accord, especially alongside unusual OBD2 O2 sensor readings, can be concerning. This fault code, indicating “catalyst efficiency below threshold (Bank 1),” often appears when the downstream oxygen sensor detects that the catalytic converter isn’t working as efficiently as it should. However, if you’re also seeing a flat line voltage reading on your upstream O2 sensor, as with your 2007 Accord, the issue might be more complex than just a failing catalytic converter.
A flat line reading from the upstream O2 sensor is definitely not normal. The upstream sensor, positioned before the catalytic converter, is supposed to be actively monitoring the air-fuel mixture and sending fluctuating voltage signals to the engine control unit (ECU). These fluctuations are crucial for the ECU to make real-time adjustments to maintain optimal engine performance and emissions. A flat line suggests the sensor isn’t detecting any change in oxygen levels, which points to a few potential problems.
One possibility, as you mentioned, is a faulty O2 sensor, even if it’s a new replacement. Manufacturing defects can occur, or the sensor might have been damaged during installation. It’s less likely, but still possible. Another significant area to investigate is the sensor wiring and pigtail. Given that you found it difficult to remove the old sensor, there’s a chance the wiring or connector could have been damaged in the process. Look for any signs of frayed wires, corrosion, or loose connections at the sensor plug and along the wiring harness leading to the ECU.
While a faulty O2 sensor or wiring issue is often the culprit for abnormal readings, we can’t completely rule out catalytic converter problems or even ECU related issues without further diagnosis. However, before jumping to conclusions about the catalytic converter, which is a more expensive repair, focusing on the O2 sensor circuit is a logical first step.
To effectively troubleshoot, you could try a few steps. First, double-check the installation of the new upstream O2 sensor. Ensure it’s properly seated and the electrical connector is firmly attached. Next, a thorough visual inspection of the wiring is essential. If you have access to a multimeter, you can test the O2 sensor circuit for voltage and continuity, comparing readings to the specifications for your 2007 Accord V6 (or 2.4L, depending on your engine). This will help pinpoint whether the issue lies with the sensor itself, the wiring, or potentially the ECU’s sensor input circuit.
If wiring and sensor checks don’t reveal any obvious problems, and the flat line reading persists, further diagnostics might be needed to assess the catalytic converter’s function and rule out less common ECU faults. However, systematically addressing the O2 sensor and its circuit is the most efficient starting point for resolving the P0420 code and the flat line O2 sensor reading on your 2007 Honda Accord.
