Could a faulty O2 sensor make 98 Chevy S10 43 liter engine light come on with 420 code catalytic converter operating under threshold and if so where is it?

Yes, a faulty O2 sensor can absolutely cause a P0420 code (catalytic converter below threshold) on a 1998 Chevy S10 with a 4.3L engine. The O2 sensor readings are crucial for the PCM (Powertrain Control Module) to determine the efficiency of the catalytic converter. If the O2 sensor is providing inaccurate readings (either too lean or too rich), the PCM might incorrectly diagnose the cat as malfunctioning.

Here's why:

* O2 Sensor Function: The O2 sensor measures the amount of oxygen in the exhaust. The PCM uses this information to adjust the air/fuel mixture. A malfunctioning sensor can lead to an incorrect air/fuel mixture, causing the cat to work harder and possibly triggering a P0420 code.

* Misdiagnosis: A bad O2 sensor often gets blamed (and sometimes rightfully so) for other issues, P0420 being a prime example. Replacing the sensor is frequently a cheaper first step than replacing a catalytic converter.

Location of O2 Sensors on a 1998 Chevy S10 4.3L:

The 1998 S10 4.3L typically has two O2 sensors:

* Upstream O2 Sensor: Located in the exhaust manifold, before the catalytic converter. This sensor is more critical for the air/fuel mixture and is more likely to cause a P0420 code if faulty.

* Downstream O2 Sensor: Located after the catalytic converter. This sensor monitors the converter's efficiency. A faulty downstream sensor *can* cause a P0420 code, but it's less common than a faulty upstream sensor causing the issue.

To find them: You'll need to look under your truck, near the exhaust manifold. The upstream sensor is closer to the engine. They're threaded into the exhaust system and will usually have a wiring harness connected to them. Refer to a repair manual or online diagrams specific to your year and model for precise locations. A Haynes or Chilton manual would be very helpful.

Important Note: While a bad O2 sensor is a common cause of a P0420 code, it's not the only one. Other potential causes include:

* Actually failing catalytic converter: If the cat is truly failing, replacing the O2 sensor won't fix the P0420 code.

* Exhaust leaks: Leaks before the O2 sensor can affect readings.

* Faulty PCM: Less likely, but possible.

Recommendation: Before replacing the catalytic converter, replace the upstream O2 sensor first. It's a cheaper and easier repair. If the code remains after replacing the sensor, then you'll need to investigate other potential causes. Consider getting the code read with a professional OBD-II scanner, as this can provide more details and potentially point you to a different problem.