QuestionI got a P0421 code error, warm-up catalyst below threshold, Bank 1. I am attempting to rule out the oxygen sensor not properly functioning. How do I measure the voltage of the oxygen sensor and secondly, what should the reading be if it is good.
AnswerThe easiest way to check the oxygen sensor voltages is with a scan tool which will display data numerically as well as graphically. Presuming you do not have access to such a tool, you can also do it with a voltmeter. To do this, you'll need to locate the signal wire on each bank on oxygen sensor.
On the 2.4 Santa Fe, the ground wire will be gray or green on the upstream oxygen sensor and black on the downstream sensor. It will be difficult to determine which wire this is on the upstream sensor because there will be two wires of this color in the connector. On the downstream sensor, make note of the location of the black wire; it will be in the same position as the gray/green wire in the connector of the upstream sensor.
On the 3.5 Santa Fe, the you'll be interested in the bank one (rear bank)_sensors. On this vehicle, it'll be the red wire in each connector.
Once you've located the signal wires described above, attach a voltmeter between the wire and the negative battery terminal and run the engine. At operating temperature, you should experience the results I'll describe. The upstream sensor should fluctuate between 0V and 1V, spending no more than 3 seconds above .5V and no more than 3 seconds below. The rear sensor should be a relatively stable value, typically around .7V. If the upstream sensor fluctuates pretty much normally and the rear sensor fluctuates just like the front sensor, you've pretty much determined that the catalyst in question is not performing its function.
Another thing to check is that the upstream sensor is not plugged into the downstream sensor connector and vice versa. I've seen this cause the code in question, and it'd be difficult for even a trained eye to pick up the problem with the sensor signal without a tool which is capable of displaying both signal waveforms at the same time for direct comparison.
Most likely, you'll need the converter.
