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

P0143 means the third oxygen sensor on Bank 1 (typically placed after a second catalyst stage or in commercial vehicle aftertreatment systems) is reporting a voltage that stays too low for too long. This code applies primarily to specific engine configurations with three sensors per bank, common on diesel commercial vehicles and some V6/V8 multi-converter setups.

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
• Often appears alongside P0420 or other aftertreatment codes

Here are the most common causes of P0143, ranked by how likely they are to be the culprit:

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 you risk failing emissions testing and may mask a developing aftertreatment problem.

Follow these steps to pinpoint the root cause of P0143:

1. Confirm the vehicle's exhaust layout. Sensor 3 is rare and only present on multi-catalyst configurations or diesel commercials with extended aftertreatment
2. Read freeze frame data and look for companion codes (P0420, P0136, P0137) that indicate a broader aftertreatment 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:

P0143 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:

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