P1138 on Audi
Lack of HO2S Switches, Sensor Indicates Rich (Bank 1, Sensor 2)
P1138 on Audi vehicles indicates lack of ho2s switches, sensor indicates rich (bank 1, sensor 2). Au7o has documented this code across 2 Audi models — most commonly on Q5, SQ5. 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.
Common Causes of P1138
- •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
P1138 on Audi by Model
Audi Q5(1 issue)
- 2009-2025 Q5 Gasoline-Quality Deposit Diagnosis - TSB 2014753/132009-2025
Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits on intake valves, the intake manifold, fuel injectors and combustion chambers. Possible results include rough running after cold start, extended cranking, hesitation, rough idle, reduced performance, poor fuel economy or an MIL with misfire, lean-system or air/fuel-ratio-imbalance faults. This does not establish a universal Q5 intake-valve defect, a mandatory walnut-blasting interval or a fixed failure mileage.
Audi SQ5(1 issue)
- Fuel-Quality or Deposit-Related Rough Running and Misfire Diagnosis2014-2025
Audi states that contaminated gasoline or gasoline with insufficient deposit-control additives can contribute to deposits in injectors, combustion chambers, the intake manifold and intake valves, with rough running, hesitation, reduced performance, poor fuel economy or related ECM faults. This does not establish a universal SQ5 intake-valve defect, a mandatory walnut-blasting interval or a fixed mileage. Audi separately warns that unapproved fuel additives are not permitted on 2018-2025 EA839 V6 TFSI SQ5 models because a negative effect on engine life cannot be ruled out.
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View P1138 across all makes →Frequently Asked Questions
What does P1138 mean on Audi?▼
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. On Audi specifically, this code is documented across 2 models.
What causes P1138 on Audi vehicles?▼
Common causes on Audi: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P1138 on a Audi?▼
Repair costs vary widely depending on the root cause and specific Audi model.
Which Audi models have P1138 documented?▼
Au7o has documented P1138 on 2 Audi models: Q5, SQ5.