A failed downstream heated oxygen sensor (bank 1, sensor 3) that is stuck lean or has an internal short.; A wiring problem in the sensor circuit, such as an open ground, chafed wire, or corroded connector.; An exhaust leak between the sensor and the outside air, allowing oxygen to skew the reading lean.; A vacuum leak or unmetered air entering the engine that affects the entire bank.; Low fuel pressure or a clogged fuel injector causing a genuinely lean mixture on bank 1.; A failing catalytic converter that no longer stores oxygen properly, though this usually sets a catalyst efficiency code first.; PCM calibration issues or a faulty PCM driver for the sensor heater circuit, which is rare but possible.

DTC codes / GM codes / P1143

P1143 · GMHO2S Bank 1 Sensor 3 Lean Or Low Voltage

Q: What does code P1143 mean?

HO2S Bank 1 Sensor 3 Lean Or Low Voltage. P1143 on a GM vehicle means the powertrain control module (PCM) has detected that the downstream heated oxygen sensor (HO2S) on bank 1, sensor 3 is reporting a lean condition or a voltage that stays lower than expected. The PCM monitors this sensor after the catalytic converter to judge catalyst efficiency and fuel control, and it sets the code when the signal remains below the calibrated lean threshold for too long.

P1143 on a GM vehicle means the powertrain control module (PCM) has detected that the downstream heated oxygen sensor (HO2S) on bank 1, sensor 3 is reporting a lean condition or a voltage that stays lower than expected. The PCM monitors this sensor after the catalytic converter to judge catalyst efficiency and fuel control, and it sets the code when the signal remains below the calibrated lean threshold for too long.

SystemPowertrain

Code typeGM specific

SeverityMedium

Fix difficultyIntermediate

AreaFuel and air metering

Symptoms you may notice

Illumination of the check engine light, often with no other noticeable symptoms.
A slight decrease in fuel economy if the lean signal is causing the PCM to add fuel.
Occasional rough idle or hesitation during acceleration when the mixture is affected.
A faint sulfur or rotten-egg smell from the exhaust if the converter is being overworked.
Failed emissions inspection due to the stored code and incomplete readiness monitors.

Common causes, most likely first

A failed downstream heated oxygen sensor (bank 1, sensor 3) that is stuck lean or has an internal short.
A wiring problem in the sensor circuit, such as an open ground, chafed wire, or corroded connector.
An exhaust leak between the sensor and the outside air, allowing oxygen to skew the reading lean.
A vacuum leak or unmetered air entering the engine that affects the entire bank.
Low fuel pressure or a clogged fuel injector causing a genuinely lean mixture on bank 1.
A failing catalytic converter that no longer stores oxygen properly, though this usually sets a catalyst efficiency code first.
PCM calibration issues or a faulty PCM driver for the sensor heater circuit, which is rare but possible.

How to diagnose and fix it

Connect a scan tool and record the freeze-frame data and live HO2S bank 1 sensor 3 voltage to confirm the lean condition.
Visually inspect the sensor, its connector, and the exhaust system for leaks, corrosion, or physical damage.
Check the sensor ground and signal circuit with a multimeter for excessive resistance or an open circuit.
Perform a propane enrichment test at the intake to see if the sensor reacts, which helps rule out a stuck sensor.
Measure fuel pressure and inspect for vacuum leaks if the sensor responds normally but the mixture is lean.
Replace the downstream HO2S bank 1 sensor 3 if it fails the response test, then clear the code and verify the repair.
If the code returns, evaluate the catalytic converter and PCM driver circuit as a last step before parts replacement.

What P1143 really tells you about sensor 3

Sensor 3 on bank 1 is almost always the downstream sensor after the catalytic converter, so a lean reading there often points to an exhaust leak rather than a failed sensor.
A dirty or contaminated sensor from oil or coolant burning in the exhaust can skew the reading lean without setting any other code.
Technicians sometimes replace the sensor only to find a pinched wire in the harness near the exhaust, so always wiggle-test the harness while watching live data.
A lean code on only one bank can be caused by a failing intake manifold gasket on that side, which is easy to overlook when focusing on the sensor.
The PCM may set P1143 as a companion to a catalyst efficiency code, so check for other stored codes before condemning the sensor.

Can I keep driving with P1143?

You can usually drive the vehicle with P1143 set, but avoid heavy acceleration and long trips until it is diagnosed because a persistent lean condition can overheat the catalytic converter. If the check engine light starts flashing or you notice misfiring, stop driving and have it inspected immediately.

Tools that help

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Guidance is general and written for GM vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.