A 1999 Toyota Tacoma (California emissions) with 150,000 miles presented a P0420 code. After a tune-up (plugs, wires, PCV valve, fuel filter), the code persisted. The rear downstream oxygen sensor (S2) and the upstream air/fuel sensor (S1) were replaced with Denso sensors. However, unusual voltage readings were observed.
Understanding the Oxygen Sensor Readings
The upstream sensor (S1) voltage hovered around 0.655V at idle and during normal driving, fluctuating between 0.63-0.67V. At wide open throttle (WOT), it briefly reached 0.7V, dropped to 0.3V, then settled back to around 0.655V.
The downstream sensor (S2) exhibited erratic behavior, fluctuating between 0.2-0.8V regardless of engine load or speed.
Research suggests S1 should read around 3.3V, and while S2 can fluctuate between 0-1V, the observed fluctuations were excessive. These readings suggest a potential issue with the catalytic converter(s).
Diagnosing the P0420 Code with an NT301
An NT301 CAN OBDII EOBD code reader can be a valuable tool in diagnosing the P0420 code on a 1998 Toyota Tacoma. This code indicates a potential problem with the catalytic converter’s efficiency. The NT301 allows for real-time monitoring of oxygen sensor data, enabling a more accurate diagnosis. By observing the live data stream from both S1 and S2 with the NT301, the following can be assessed:
- Sensor Response Time: The NT301 can measure how quickly the sensors react to changes in exhaust gas composition. Slow response times can indicate a failing sensor.
- Voltage Fluctuations: The erratic readings from S2 can be further analyzed with the NT301 to determine the frequency and amplitude of the fluctuations. This helps pinpoint whether the issue lies with the sensor or the catalytic converter.
- Oxygen Sensor Heater Circuit: The NT301 can check the functionality of the oxygen sensor heater circuit. A malfunctioning heater can impact sensor performance and trigger a P0420 code.
Potential Causes of the P0420 Code
While failing catalytic converters are a common cause of the P0420 code, other possibilities exist:
- Exhaust Leaks: Leaks before the catalytic converter can introduce fresh air into the exhaust stream, affecting sensor readings.
- Engine Misfires: Misfires can cause unburnt fuel to enter the exhaust, damaging the catalytic converter and affecting sensor readings.
- Faulty Wiring: Damaged or corroded wiring to the oxygen sensors can cause inaccurate readings.
Next Steps
Further diagnosis is recommended using the NT301. Checking for exhaust leaks, confirming proper fuel pressure, and inspecting the wiring harness for damage are crucial steps. If the NT301 confirms erratic S2 behavior and other potential causes are ruled out, catalytic converter replacement may be necessary.
