OBD codes / P0143
P0143
ModerateO2 Sensor Circuit Low Voltage (Bank 1, Sensor 3)
No caution needed for daily driving — P0143 concerns a monitoring sensor at the very end of the emissions system and has zero effect on how the engine runs. Sort it out before an emissions inspection or any catalytic-converter diagnosis.
What this code means
P0143 applies only to vehicles with a third oxygen sensor on Bank 1 — found after the second of two in-line catalytic converters, where it verifies the final exhaust cleanup. The code sets when that sensor's voltage stays low (a constant lean reading) longer than the computer allows. Real-world causes are a failing sensor, a wiring short to ground, a connector corroded by road spray this far back on the underbody, or an exhaust leak near the sensor pulling in outside air, which reads as lean. Because everything aft of the first sensor is monitoring rather than fuel control, this fault does not affect drivability; its significance is emissions compliance and the accuracy of the rear catalyst monitor.
Symptoms you might notice
- Check-engine light and nothing more, typically
- Failed emissions test
- No drivability or economy change
What it costs to fix
Typical range: $100–$450 · about ~0.3–1 hrs of labor
| If the cause is… | Part | Labor |
|---|---|---|
| Oxygen (O2) sensor (post-catalyst, sensor 3) | $30–$250 | ~0.3–1 hrs |
| Wiring / connector repair | $10–$120 | ~0.5–1.5 hrs |
| Exhaust leak repair (gasket or pipe section) | $20–$200 | ~0.5–2 hrs |
Under-floor access usually keeps this a quick job. The exceptions are a rust-seized sensor and — worth checking first — wiring damaged by debris or corrosion, which is cheap to fix if caught by inspection.
The price swings on which cause it turns out to be — so confirm the cause before paying. Diagnose P0143 for my exact vehicle →
Frequently asked
What does Sensor 3 do on my car?
It sits after a second catalytic converter and confirms the exhaust is fully cleaned up. Only vehicles with two converters in series on a bank have one — it is a monitor, not part of fuel control.
Is P0143 safe to ignore for a while?
Mechanically, yes — nothing about engine operation depends on this sensor. Legally and practically, it fails emissions testing, and a wiring fault left alone can worsen, so 'a while' should not become 'forever'.
Why would this sensor read constantly low?
Either it is being fed a lean signal — say, from an exhaust leak admitting outside air nearby — or the signal itself is broken: a worn-out sensor or a signal wire shorted to ground. A meter check of the circuit separates the two quickly.
How much is the fix for P0143?
Usually a modest repair: the sensor is typically accessible under the floor, and post-catalyst sensors are often on the cheaper end. Wiring repairs, if that is the cause, can be cheaper still.
Seeing P0143 on your car? Get a diagnosis specific to your exact year, make and model.
Diagnose P0143 for my vehicleRelated trouble codes
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Generic OBD-II reference. Manufacturer-specific behavior varies — confirm with a scan tool and, for safety-related codes, a professional inspection.