OBD-II Code · Sensors
P0143
O2 Sensor Low Voltage (Bank 1 Sensor 3)
Severity
low
Lowest on our scale. Fix it when convenient.
Can you drive it
Only if running normally
A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety.
Indicative cost (USD)
$200–$500
Varies by vehicle, diagnosis, parts, and local labor. This is not a repair quote.
B1S3 sensor stuck low.
How a shop works this
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed.
Note every code that is set, not just this one.
Codes that appear together usually narrow the cause faster than any one of them read alone.
Read the freeze frame — the conditions the fault was recorded under.
Cold or warm, idle or load, road speed and engine speed. A fault that only appears cold is a different fault from one that only appears under load, even with the same code on the scanner.
Confirm the fault is current rather than history.
Clear the code and see whether it returns under those same conditions. A code that does not come back may have been logged during a repair, a flat battery, or a bad connection that has since been disturbed.
Then work the likely-causes list in the order shown.
That order is what a shop checks first for this code — wiring and connections before the component they run to, because a connector costs nothing to inspect and is a common answer.
If the code comes back after a repair, the underlying fault is still there. That is the point to bring in a scanner with live data and freeze-frame, or a shop that has one.
Common symptoms
- Check engine light
- Lean trim
Likely causes
- Failed sensorMost common
- Exhaust leak
- Wiring short to ground
Read the full diagnostic procedureP0143 means the ECM is reading a stuck-low voltage from the bank 1, sensor 3 O2 sensor — specifically, signal voltage below the lower threshold (typi…
P0143 means the ECM is reading a stuck-low voltage from the bank 1, sensor 3 O2 sensor — specifically, signal voltage below the lower threshold (typically under 0.10V for longer than 60-120 seconds with the engine in closed loop). A narrow-band O2 sensor that is functioning normally should swing between roughly 0.1-0.9V on a pre-cat application or hold steady around 0.6-0.8V on a post-cat application; pinned-low means either the sensor element has lost reference, the signal wire is shorted to ground, or the exhaust ahead of the sensor has a leak letting fresh atmospheric oxygen flood the pipe and bias the reading lean.
Cheapest-first ladder
- Smoke-test the exhaust from the manifold back to the sensor 3 location — a pinhole in the mid-pipe ahead of sensor 3 pulls ambient air into the negative pressure pulses and pegs the sensor at 0V.
- Disconnect the sensor and check the signal wire at the connector with KOEO — you should see the ECM's bias voltage (~0.45V) on the signal pin; if you see 0V with the sensor unplugged, the signal wire is shorted to chassis ground somewhere in the harness.
- Compare B1S3 voltage to B1S1 — if both are pegged at 0V, the engine is running genuinely lean (vacuum leak, low fuel pressure under 35 psi, MAF over-reporting) and the sensor is correct; chase the lean condition, not the sensor. The expensive misdiagnosis is replacing the bank 1 catalyst when the actual cause is an exhaust leak between the cats — the new cat will throw P0143 within a week.
Vehicle-specific patterns2007-2014 Chevy Silverado/Tahoe 5.3L and 6.2L L-series V8 with Active Fuel Management throws P0143 on bank 1 sensor 3 from the deactivated-cylinder l…
- 2007-2014Chevy Silverado/Tahoe 5.3L and 6.2L L-series V8 with Active Fuel Management throws P0143 on bank 1 sensor 3 from the deactivated-cylinder lifter dumping unburned air into the exhaust during AFM mode, which pins the rear sensor lean for extended periods.
- 2004-2009Dodge Ram 5.7L Hemi commonly sees P0143 from a cracked exhaust manifold-to-pipe gasket ahead of the rear bank 1 sensor — Mopar issued service-pack updated gaskets for the V-clamp joint.
- 2005-2011Ford F-150 5.4L 3V sees P0143 from a blown bank 1 exhaust manifold stud (a known weakness on the 3V heads) letting atmospheric air into the upstream cat.
- 2008-2013Toyota Tundra/Sequoia 5.7L 3UR-FE throws P0143 on bank 1 sensor 3 from a degraded sensor element after extended highway running with E85 contamination.
Estimated repair$80 to $520.
Look up another code
P0143 — frequently asked
What does a P0143 code mean?
B1S3 sensor stuck low.
What causes a P0143 code?
Common causes: Failed sensor; Exhaust leak; Wiring short to ground.
What are the symptoms of a P0143 code?
Typical symptoms: Check engine light; Lean trim.
Is it safe to drive with a P0143 code?
Only if running normally. A short trip to a shop may be reasonable if the light is steady and the car runs normally. Stop safely for a flashing light, severe shaking, overheating, loss of power, or another urgent warning. The code alone cannot confirm safety. Estimated repair: $200-$500.
How do I diagnose a P0143 code?
Work from evidence to part, not the other way round. A code names a circuit or a symptom; it does not name the part that failed. 1. Note every code that is set, not just this one. 2. Read the freeze frame — the conditions the fault was recorded under. 3. Confirm the fault is current rather than history. 4. Then work the likely-causes list in the order shown.
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