P0140 means the ECU cannot detect any activity from the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2). The sensor voltage is either flatlined or out of range. This usually points to a failed sensor, an open circuit in the wiring, or a connector that has come loose or corroded.
P0140 means the ECU cannot detect any activity from the downstream oxygen sensor on Bank 1 (post-catalyst, Sensor 2). The sensor voltage is either flatlined or out of range. This usually points to a failed sensor, an open circuit in the wiring, or a connector that has come loose or corroded.
P0140 is a emissions-related diagnostic trouble code classified as moderate severity. When your vehicle's ECU detects this condition, it stores P0140 and illuminates the check engine light.
If your vehicle has triggered P0140, you may notice one or more of these symptoms:
Here are the most common causes of P0140, ranked by how likely they are to be the culprit:
Generally yes, for short-term driving. Safe to drive short term. The downstream sensor reporting no activity does not affect fuel mixture, but the ECU loses its catalyst monitoring capability. Repair within a few weeks to maintain proper emissions diagnostics.
Follow these steps to pinpoint the root cause of P0140:
Repair costs for P0140 vary depending on the root cause and your vehicle. Here are typical estimates:
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
P0140 behaves differently depending on your vehicle. Select your car below for model-specific causes, known issues, and adjusted cost estimates:
2006–2024
On N52 and N55, the downstream sensor connector clip is fragile and frequently breaks on older cars. A broken clip lets the connector vibrate loose, which presents as P0140 (no activity) even though the sensor itself is fine. Inspect and re-secure the connector before condemning the sensor. BMW OEM Bosch sensors 110–200 euros, aftermarket Bosch around 60–110 euros.
Estimated costs for BMW 3 Series ownersadjusted higher than average due to BMW-specific parts pricing
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
2005–2024
EA888 TSI engines see P0140 most often from heater element burnout combined with oil-contaminated sensor element. Check the heater fuse first, then heater resistance at the sensor before buying parts. On older MK5/MK6 cars, the sensor harness can chafe against the heat shield, breaking the signal wire and producing P0140.
Estimated costs for Volkswagen Golf ownersadjusted higher than average due to Volkswagen-specific parts pricing
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
2006–2024
On the B7 and B8 A4 with 2.0 TFSI, the downstream sensor connector lives in a tight spot near the gearbox bellhousing and is prone to being knocked loose during clutch or gearbox work. Check for incomplete seating before replacing the sensor if P0140 appeared after recent work. Audi-OEM sensors 130–220 euros, aftermarket Bosch around 70–120 euros.
Estimated costs for Audi A4 ownersadjusted higher than average due to Audi-specific parts pricing
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
2007–2024
The W204 M271 1.8L Kompressor commonly triggers P0140 from heater element burnout around 130,000–150,000 km. The W205 OM651 diesel has the documented harness chafing point near the gearbox mount that can sever the signal wire, producing P0140. Mercedes-OEM Bosch sensors 130–220 euros, aftermarket Bosch around 70–120 euros.
Estimated costs for Mercedes C-Class ownersadjusted higher than average due to Mercedes-Benz-specific parts pricing
Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.
These codes are often seen alongside P0140 or indicate related issues:
The ECU expects the downstream O2 sensor signal to move within a defined range over time. If the voltage stays flat or out of range for the full monitor window, the ECU concludes the sensor has no activity and sets P0140. Most often the sensor itself has failed internally, or a wire has broken.
Yes, short term. The downstream sensor does not drive fuel mixture, so the car runs normally. Long term you lose catalytic converter monitoring, which means you may miss a developing converter problem until it is severe.
Unplug the sensor and measure heater resistance (typically 2–15 ohms). Plug it back in and back-probe the signal wire. With the engine warm and idling, a healthy downstream sensor reads around 0.6–0.8V steady. A flat reading at 0V or 5V with engine running points to an internal sensor failure.
Indirectly yes. If the heater fuse is blown, the sensor never reaches operating temperature and stays inactive, triggering P0140 alongside heater codes like P0141. Check the heater fuse first if you see both code types.
Connect any Bluetooth OBD2 adapter, scan for codes, and get AI-powered diagnostics with severity ratings and repair cost estimates.
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