The Quick Answer above is the generic definition. What follows is what is documented for the Audi models that log P0138, checked against manufacturer data and owner reports.
DTC/P0138·Audi
P0138 on Audi: Causes, Symptoms and Fix Cost
Quick Answer
Moderate SeveritySafe to drive (short-term)Audi
P0138 on a Audi means the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2) is reporting a voltage that is too high for too long, above roughly 1.0V. A healthy downstream sensor reads around 0.6–0.8V steady. Persistent high voltage typically indicates a rich exhaust condition, a short to voltage in the wiring, or a contaminated sensor.
Repair cost on Audi€65 - €650
Connect any standard Bluetooth OBD2 adapter to your Audi and Skanyx tells you what P0138 means on that engine, whether it is safe to keep driving, and what the fix should cost. Free on iPhone and Android.
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Audi A4
2006–2024The B7 and B8 A4 with 2.0 TFSI is notorious for HPFP wear (especially early EA113), which raises fuel pressure and causes genuine rich conditions. Walnut blasting may also be needed on any TFSI over 80,000 km to clear intake carbon affecting combustion. Audi-OEM sensors 130–220 euros, aftermarket Bosch around 70–120 euros.
What causes P0138 on a Audi?
Beyond the generic causes listed on the main P0138 page, these are the documented Audi-specific patterns:
Audi A4Early 2.0 TFSI HPFP wear elevating fuel pressure
Audi A4Intake carbon buildup affecting combustion stability
How to diagnose P0138 on a Audi with OBD2
Follow these steps to pinpoint the root cause of P0138:
- Read freeze frame data and check for companion codes (P0172, P0175, P0420) that point to a real rich condition or converter problem
- Use a scan tool to view downstream sensor voltage. A reading stuck above 0.9V at idle indicates a rich condition or a faulty sensor
- Check long-term fuel trims. If LTFT is significantly negative (under -10%), the engine is genuinely running rich
- Inspect the Bank 1 Sensor 2 signal wire for shorts to voltage, especially near heat shields and the battery positive cable
- Substitute a known-good sensor. If high voltage persists, the issue is fuel mixture or wiring, not the sensor
How much does P0138 cost to fix on a Audi? (EUR)
Estimated repair costs for P0138 in EUR, compiled across makes. Expect Audi parts and labour towards the upper end of each range.
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
Related codes that often appear with P0138 on Audi
These codes commonly cluster with P0138 on Audi vehicles:
Looking for the full P0138 reference (all makes, full diagnosis flow, complete repair cost matrix)?
See the main P0138 guideSkanyx · Audi
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