P0143: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 3)

P0143 means the third oxygen sensor on bank 1, fitted after a second (usually under-floor) catalytic converter, is reporting a voltage that stays too low for too long. Only a minority of cars have a Sensor 3, typically petrol engines built to the strictest emissions standards with two catalysts in series; most cars have two sensors per bank and cannot set this code.

P0143 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0143 and illuminates the check engine light.

If your vehicle has triggered P0143, you may notice one or more of these symptoms:

• Check engine light illuminated
• Slight increase in fuel consumption
• Possible failed emissions test
• Usually no noticeable change in driveability
• May appear alongside catalyst codes such as P0420

Here are the most common causes of P0143:

Generally yes, for short-term driving. Safe to drive short term. Sensor 3 is a secondary post-catalyst sensor used for fine-trim emissions monitoring. The vehicle will run normally, but a lit check engine light can get the car rejected at an emissions test and may mask a developing catalyst problem.

Follow these steps to pinpoint the root cause of P0143:

1. Confirm the vehicle's exhaust layout. Sensor 3 is only present on cars with a third oxygen sensor after a second catalytic converter
2. Read freeze frame data and look for companion codes (P0420, P0136, P0137) that indicate a broader catalyst or sensor issue
3. Inspect the exhaust system from the upstream catalyst all the way to Sensor 3 for leaks, cracked welds, and loose clamps
4. Inspect the Sensor 3 connector and wiring for corrosion, heat damage, or chafing against the chassis
5. Use a scan tool to view Sensor 3 voltage. A healthy post-cat sensor reads around 0.6-0.8V steady. A reading stuck below 0.2V points to sensor failure or an air leak

Repair costs for P0143 vary depending on the root cause and your vehicle. Here are typical estimates:

These codes are often seen alongside P0143 or indicate related issues: