P0143 indicates the Bank 1, Sensor 3 oxygen sensor (the post-catalytic-converter / downstream O2 sensor) is reporting a voltage that stays below the calibrated threshold for too long. The powertrain control module expects the downstream sensor to hover near a steady bias voltage (roughly 0.4–0.5 V) and only swing slowly, so a persistently low reading sets this code.
The sensor element itself is the most common failure; the internal reference or heater can fail and force the signal line to read near 0 V.
Wiring near the exhaust is exposed to heat, vibration, and road debris, and the signal wire can short to ground, opening the circuit to a low reading.
Engine oil consumption, a leaking head gasket, or silicone sealant fumes can foul the sensor tip and pull its output low.
Although leaks more often set other O2 codes, a severe false-air leak at the post-cat area can skew the reading and contribute to a low-voltage condition.
A corroded ground splice or damaged signal-return wire in the engine harness can pull the sensor's reported voltage below normal.
Rare; the control module's sensor input circuitry can fail, but this is only considered after the sensor, wiring, and grounds are ruled out.
Usually not. P0143 points to the downstream O2 sensor circuit, not converter efficiency (that is P0420). A weak downstream sensor can, however, cause a false P0420 because the monitor relies on its readings.
Yes, in most cases. The downstream sensor does not directly control fueling, so the car will usually run normally, but emissions readiness may be lost and a smog check can fail.
No. Despite what some sources suggest, replacing the PCM for P0143 is almost never the correct fix. The downstream sensor, its wiring, and its connector fail far more often; the PCM should only be suspected after those are thoroughly tested.
Unplug the sensor and check for battery voltage on the heater feed, measure heater resistance, and inspect the signal wire for shorts to ground with the harness wiggled. A clean signal/ground at the connector with the sensor unplugged, combined with low voltage while plugged in, points to a bad sensor.