P1138: How to Diagnose & Fix — Lack of HO2S Switches, Sensor Indicates Rich (Bank 1, Sensor 2)
2 vehicles · 1 makes
How to diagnose and fix P1138: most-reported vehicles
Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.
Audi SQ5 · 2014–2025
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.
Common Symptoms
- Rough running after a cold start
- Excessive engine cranking time
- Hesitation while driving
- Rough idle
- Reduced engine performance
- Poor fuel economy
- MIL with misfire, lean-system or air/fuel-ratio imbalance faults
How to Fix
Confirm fuel quality, symptoms and stored DTCs before replacing or cleaning components. If contaminated gasoline is suspected, advise changing the fuel source; if low detergent quality is suspected, Audi recommends Top Tier detergent gasoline. Continue workshop diagnosis if symptoms persist. On 2018-2025 EA839 SQ5 models, do not recommend unapproved cleaners or additives; follow the owner manual and use only Audi-approved petrol additives. Do not prescribe routine walnut blasting, a catch can or chemical cleaning from this record.
Sources for this issue (2)
Audi Q5 · 2009–2025
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.
Common Symptoms
- Rough running after a cold start
- Excessive engine cranking time
- Hesitation while driving
- Rough idle
- Reduced engine performance
- Poor fuel economy
- MIL with misfire, lean-system or air/fuel-ratio-imbalance faults
How to Fix
Confirm the exact gasoline-powered vehicle, fuel quality, symptoms and stored DTCs before cleaning or replacing components. If contaminated gasoline is suspected, change fuel source; if detergent quality is suspect, Audi recommends Top Tier detergent gasoline. Continue guided diagnosis if the condition persists and identify the actual deposit location before selecting any cleaning procedure. Do not prescribe routine walnut blasting, a catch can, chemical cleaner or high-RPM driving from this record.
Sources for this issue (1)
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P1138 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
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.