What Does P1143 Mean?
This code indicates the Bank 1, Sensor 3 heated oxygen sensor signal is consistently below 0.45 volts, indicating a lean air/fuel ratio in the exhaust downstream of the second catalytic converter or at a third monitoring point. Sensor 3 is typically a rear catalyst monitor on vehicles with dual catalytic converters or extended exhaust monitoring. The persistent low voltage suggests either a lean exhaust condition at that point or a sensor malfunction.
Common Causes
30%
Faulty HO2S Bank 1 Sensor 3 biased lean or with degraded sensing element
25%
Exhaust leak near the Sensor 3 location allowing ambient air infiltration
20%
Upstream lean condition carrying through both catalytic converters
15%
Wiring issue causing low signal voltage (high resistance or partial open)
10%
Deteriorated rear catalytic converter affecting exhaust gas composition at the sensor
Diagnostic Steps
1
Monitor Bank 1 Sensor 3 voltage at steady cruise; compare against expected behavior for the vehicle application.
2
Check for exhaust leaks near the Sensor 3 mounting location using a visual inspection and listening for exhaust hissing.
3
Review Bank 1 Sensor 1 and Sensor 2 data to determine if a lean condition exists upstream.
4
Inspect the Sensor 3 wiring harness and connector for damage, corrosion, or high resistance.
5
If available, perform a propane enrichment test near the sensor to verify the sensor responds to a known rich condition.
Estimated Repair Cost
$100 - $400
Parts + labor, varies by vehicle and location
This code indicates the Bank 1, Sensor 3 heated oxygen sensor signal is consistently below 0.45 volts, indicating a lean air/fuel ratio in the exhaust downstream of the second catalytic converter or at a third monitoring point. Sensor 3 is typically a rear catalyst monitor on vehicles with dual cata...
The most common cause of P1143 (HO2S Bank 1 Sensor 3 Signal Below 0.45v (A/F Ratio Too Lean)) is: Faulty HO2S Bank 1 Sensor 3 biased lean or with degraded sensing element
Typical repair costs for P1143 range from $100 to $400, depending on the vehicle, location, and whether you do it yourself or go to a shop.
No direct safety hazard. This sensor primarily monitors rear catalyst efficiency for emissions compliance. The vehicle will continue to run normally but may not pass emissions testing.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P1143 to identify the root cause.
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Quick Info
Category
Powertrain
System
Emissions / Catalyst Monitoring
Difficulty
Type
Manufacturer
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