P1138: Lack of HO2S Switches, Sensor Indicates Rich (Bank 1, Sensor 2)
2 vehicles · 1 makes
Most Reported On
P1138 is an OBD-II diagnostic trouble code meaning “Lack of HO2S Switches, Sensor Indicates Rich (Bank 1, Sensor 2).” Manufacturer-specific - the definition above is Ford's. The PCM saw the downstream heated oxygen sensor on bank 1 stay above the rich threshold (roughly 0.45 V) instead of switching, indicating either a genuine rich condition or a lazy/failed sensor. Symptoms are usually a check engine light, reduced fuel economy, and possibly a rough idle or fuel smell. Important: other makes define P1138 completely differently - Audi and Volkswagen use it for "Long Term Fuel Trim Add. Fuel, Bank 2 - System Too Lean," which points at a vacuum leak rather than an oxygen sensor. Confirm the definition for your specific make before replacing parts. This code is most commonly reported on 2014-2025 Audi SQ5, and 2009-2025 Audi Q5.
Common Causes
Diagnostic Tools
P1138 does not use the standard P/B/C/U prefix. It may be a manufacturer-specific code, so verify that a scanner supports your exact year, model and module before buying.
FAQ
What does P1138 mean?
P1138 stands for "Lack of HO2S Switches, Sensor Indicates Rich (Bank 1, Sensor 2)." Manufacturer-specific - the definition above is Ford's. The PCM saw the downstream heated oxygen sensor on bank 1 stay above the rich threshold (roughly 0.45 V) instead of switching, indicating either a genuine rich condition or a lazy/failed sensor. Symptoms are usually a check engine light, reduced fuel economy, and possibly a rough idle or fuel smell. Important: other makes define P1138 completely differently - Audi and Volkswagen use it for "Long Term Fuel Trim Add. Fuel, Bank 2 - System Too Lean," which points at a vacuum leak rather than an oxygen sensor. Confirm the definition for your specific make before replacing parts.
What are the most common causes of P1138?
The most common causes of P1138 are: Failed or contaminated downstream heated oxygen sensor (bank 1, sensor 2), Exhaust leak upstream of the sensor skewing readings, Genuinely rich fuel mixture: leaking injector, high fuel pressure, or faulty fuel pressure regulator, Damaged O2 sensor wiring, connector, or heater circuit, Contaminated MAF sensor or a failing engine coolant temperature sensor driving mixture rich, On VW/Audi, an unmetered-air (vacuum) leak causing a lean bank 2 fuel trim - verify the make-specific definition first. The specific cause varies by vehicle.
How much does it cost to fix P1138?
Repair costs vary widely depending on the root cause and vehicle.
Which vehicles are affected by P1138?
Au7o has documented P1138 across 2 vehicle models from 1 manufacturers: Audi.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.