P0143: O2 Sensor Circuit Low Voltage (Bank 1, Sensor 3)
Quick Answer
Safe to drive (short-term)
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.
- Severity
- Moderate · Emissions
- Estimated repair cost
- From €55 to €380+, varies by vehicle
€55 or €380, depending on your car.
- Repair sensor wiring or connector€55–€160
- Replace Bank 1 Sensor 3€110–€380
A scan shows whether P0143 is really stored on your car, which codes sit next to it and how urgent each one is.
A scan needs a Bluetooth OBD2 adapter (Bluetooth LE on iPhone), about €25-60; the built-in demo runs without one. See which adapter to buy
What does P0143 mean?
P0143 is a emissions-related diagnostic trouble code classified as moderate severity.
Where it is on the car
- Engine
- Oxygen sensor before the converter (sensor 1)
- Catalytic converter
- Oxygen sensor after the converter (sensor 2, or 3 on some cars)
- Silencer and tailpipe
What are the symptoms of P0143?
- 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
What causes P0143?
Failed Bank 1 Sensor 3 stuck at low voltage
: high
Exhaust leak upstream of Sensor 3 allowing fresh air to reach it
: high
Damaged or corroded O2 sensor wiring or connector
: medium
Failing secondary catalyst reducing oxygen storage
: medium
Actual lean condition reaching Sensor 3 (vacuum leak, low fuel pressure)
: low
Is it safe to drive with 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.
How do you diagnose P0143?
- Confirm the vehicle's exhaust layout. Sensor 3 is only present on cars with a third oxygen sensor after a second catalytic converter
- Read freeze frame data and look for companion codes (P0420, P0136, P0137) that indicate a broader catalyst or sensor issue
- Inspect the exhaust system from the upstream catalyst all the way to Sensor 3 for leaks, cracked welds, and loose clamps
- Inspect the Sensor 3 connector and wiring for corrosion, heat damage, or chafing against the chassis
- 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
How much does P0143 cost to fix?
| Repair | Parts Cost | Labor Cost | Total Estimate | DIY Difficulty |
|---|---|---|---|---|
| Replace Bank 1 Sensor 3 | €50–€220 | €60–€160 | €110–€380 | Moderate |
| Fix exhaust leak upstream of Sensor 3 | €20–€120 | €70–€220 | €90–€340 | Moderate |
| Repair sensor wiring or connector | €5–€30 | €50–€130 | €55–€160 | Moderate |
Prices estimated as of October 2026. Costs vary by region, vehicle, and shop.
Related diagnostic codes
Common questions about P0143
- Why does my car have three O2 sensors per bank?
- Some petrol cars built to the strictest emissions standards have a second catalytic converter further down the exhaust, with a third oxygen sensor after it to monitor it. Most passenger cars only have two sensors per bank.
- Can I drive with P0143?
- Yes, for short distances. Sensor 3 is a monitoring sensor and does not drive fuel mixture decisions. However, a lit check engine light can get the car rejected at an emissions test, and you may miss a developing catalyst problem if you ignore it long term.
- Is Sensor 3 the same as Sensor 2?
- No. Sensor 3 sits further downstream, after the second catalytic converter, and monitors that catalyst. Its part number, cable length and plug often differ from Sensor 2, so order by position.
- Why is P0143 less common than P0137?
- Most cars only have two sensors per bank, so they cannot set Sensor 3 codes. P0143 only appears on the relatively few cars with a third oxygen sensor after a second catalytic converter.