What Does P0143 Mean?
The ECM has detected that the oxygen sensor on Bank 1 Sensor 3 is producing a voltage signal that is lower than the expected minimum. Sensor 3 is typically a third O2 sensor located downstream of a second catalytic converter in a multi-cat exhaust system. A consistently low voltage indicates the sensor is either stuck lean, has an open sensing element, or the sensor circuit has a wiring fault. This affects the monitoring of the secondary catalytic converter.
Common Causes
35%
Failed O2 sensor (Bank 1 Sensor 3) with degraded or non-responsive sensing element producing low voltage
25%
Open circuit in the O2 sensor signal wire causing a low voltage default
20%
Exhaust leak upstream of Sensor 3 introducing ambient air and biasing the sensor lean
10%
O2 sensor heater failure keeping the sensor below operating temperature (reads lean when cold)
10%
Corroded connector pins creating high resistance in the signal circuit
Diagnostic Steps
1
Monitor Bank 1 Sensor 3 live data. If the voltage is stuck below 0.1V or consistently reads near zero regardless of engine load, confirm the low-voltage condition.
2
Check the O2 sensor heater operation — if the heater is not working, the sensor may not be at operating temperature and will read low. Verify heater current draw is within spec.
3
Inspect the O2 sensor connector for corrosion, water intrusion, or damaged pins. Disconnect, clean, and reconnect.
4
Check for exhaust leaks between the second catalytic converter and the Sensor 3 location. Even minor leaks can introduce enough air to hold the sensor lean.
5
Measure the signal voltage at the ECM connector while disconnecting the O2 sensor. If the ECM reads 0.45V bias with the sensor disconnected, the wiring is good and the sensor is faulty.
Estimated Repair Cost
$100 - $350
Parts + labor, varies by vehicle and location
The ECM has detected that the oxygen sensor on Bank 1 Sensor 3 is producing a voltage signal that is lower than the expected minimum. Sensor 3 is typically a third O2 sensor located downstream of a second catalytic converter in a multi-cat exhaust system. A consistently low voltage indicates the sen...
The most common cause of P0143 (02 Sensor Circuit Low Voltage (Bank 1 Sensor 3)) is: Failed O2 sensor (Bank 1 Sensor 3) with degraded or non-responsive sensing element producing low voltage
Typical repair costs for P0143 range from $100 to $350, depending on the vehicle, location, and whether you do it yourself or go to a shop.
Safe to drive. Bank 1 Sensor 3 is used for secondary catalyst monitoring only and does not influence engine fuel control. The check engine light will be on and emissions testing will fail.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0143 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Exhaust
Difficulty
Type
Generic (SAE)
Recommended Tools
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