A great many oxygen sensors get replaced for no reason, because the numbering is confusing and because people assume both sensors do the same job. They do not.

Sensor 1 runs the engine

The sensor ahead of the catalytic converter is sensor 1, and it is the one that matters for how the engine runs.

The computer works out how much fuel to inject from how much air it measures coming in. Then it reads sensor 1 to see whether the mixture actually came out right, and corrects. That correction is the fuel trim. Without sensor 1 the computer has no feedback at all and falls back to a preset table, which costs fuel and emissions immediately.

A healthy sensor 1 signal swings up and down constantly and quickly, several times a second. That oscillation is not a fault. It is the computer deliberately hunting either side of the ideal mixture, because the catalytic converter works best when the exhaust alternates slightly rich and slightly lean rather than sitting exactly in the middle.

Sensor 2 grades the converter

The sensor behind the converter is sensor 2, and it has almost nothing to do with running the engine.

A working converter stores and releases oxygen as it processes the exhaust, and that buffering smooths out the swings coming from the front. So on a healthy system, sensor 1 oscillates rapidly and sensor 2 sits comparatively flat.

The computer compares the two. When sensor 2 starts mirroring sensor 1, the buffering has stopped and it sets P0420, or P0430 for bank 2.

That is the entire job of the rear sensor. It is a test instrument for the converter, not a fuelling control.

Reading the numbering

The names are positional, and once you know the rule they are unambiguous.

Bank is which side of the engine. Bank 1 always contains cylinder number one. On an inline four there is only one bank. On a V6 or V8, which physical side is bank 1 depends entirely on the make, and it is worth confirming rather than guessing.

Sensor is the position in the exhaust. Sensor 1 is before the converter. Sensor 2 is after. Some vehicles have a sensor 3 further downstream.

So bank 1 sensor 2 is the rear sensor on the side of the engine with cylinder one on it.

Getting this wrong is expensive on a V engine, because it means removing a sensor that was never faulty from the harder side to reach.

Heater codes are not sensor codes

An oxygen sensor produces no usable signal until it is around six hundred degrees. Waiting for exhaust heat alone takes minutes, and during those minutes the computer cannot use it. So every modern sensor has a small electric heater built in, and the computer monitors that heater circuit separately.

P0135 and P0141 are heater circuit codes for bank 1 sensor 1 and sensor 2 respectively. They say the heater is not drawing the current a working element draws. They say nothing about whether the sensor is measuring oxygen correctly.

This is why a car with P0141 drives completely normally. The fuelling is controlled by the front sensor, which is fine. What you lose is the fast warm up and, importantly, the ability to complete the catalyst monitor, which needs a working rear sensor to run at all.

Before replacing a sensor for a heater code, check whether more than one heater code is stored. Many vehicles run all the sensor heaters from a single fuse, and one blown fuse looks exactly like several dead sensors.

Before you replace a rear sensor for P0420

A converter efficiency code is a comparison between two sensors, so there are three ways to get it: a dead converter, a lazy rear sensor, or an exhaust leak drawing fresh air across the sensor.

The rear sensor is the cheapest of the three to replace, which makes it tempting, but replacing it blindly on a converter that has genuinely failed just delays the answer.

Watch both traces at a steady 2000 rpm with the engine warm. If the front swings and the rear mirrors it, the converter has stopped buffering. If the rear barely moves at all, or moves very slowly, the sensor itself is suspect. And on a V engine, the other bank gives you a known good reference for what the trace should look like on the same car, the same fuel and the same journeys.