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Trouble code P0143 on a Toyota means:

P0143 – O2 Sensor Circuit Low Voltage (Bank 1, Sensor 3)
Let’s break that down and see how to fix it
Meaning
Bank 1 → The side of the engine that has Cylinder 1.
Sensor 3 → The third oxygen sensor, usually after the catalytic converter (downstream).
The Engine Control Module (ECM) is detecting a low voltage from this sensor (usually below 0.1V for too long).
This sensor monitors the catalytic converter’s efficiency and helps control emissions.

Possible Causes
Faulty O2 sensor (Bank 1 Sensor 3) – most common.
Damaged or corroded sensor wiring/connectors.
Exhaust leak before or near the sensor.
Short to ground or open circuit in the O2 sensor wiring.
Faulty Engine Control Module (rare).
How to Diagnose and Fix

Step 1: Visual inspection

Locate the Bank 1 Sensor 3 (rear O2 sensor after the catalytic converter).
Inspect the wiring and connector — look for:
Burned wires near the exhaust pipe
Loose or corroded plug connections
Oil or moisture contamination
Step 2: Check the O2 Sensor voltage

Use a scan tool or multimeter:
With the engine warm, the sensor voltage should fluctuate between 0.1V–0.9V.
If it stays below 0.1V, the sensor or circuit is bad.
Step 3: Check wiring continuity

Test the signal wire between the O2 sensor and ECM for open or short circuits.
Repair or replace damaged wiring.

Step 4: Replace the O2 sensor (if needed)
If the wiring is fine but voltage remains low, replace Bank 1 Sensor 3 with an OEM or quality aftermarket sensor.
Clear the code and test-drive the vehicle.
Step 5: Check for exhaust leaks
If the code returns, inspect for small exhaust leaks near the catalytic converter that could let extra oxygen in.