What does no activity detected mean?

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.

Can I drive with P0140?

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.

How do I test an O2 sensor with a multimeter?

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 fairly steadily and moves when the mixture changes. A signal that stays fixed, for example flat near 0V, or parked at a mid value of roughly 0.45V that many ECUs apply when the circuit is open, points to a failed sensor or a broken wire.

Can a blown fuse cause P0140?

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.

P0140: O2 Sensor Circuit No Activity Detected (Bank 1, Sensor 2)

Quick Answer

Safe to drive (short-term)

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.

Severity: Moderate · Emissions
Estimated repair cost: From €1 to €310+, varies by vehicle

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What does P0140 mean?

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.

Where it is on the car

Engine
Oxygen sensor before the converter (sensor 1)
Catalytic converter
Oxygen sensor after the converter (sensor 2, or 3 on some cars)
Silencer and tailpipe

What are the symptoms of P0140?

If your vehicle has triggered P0140, you may notice one or more of these symptoms:

Check engine light illuminated
Slight reduction in fuel economy in some cases
Possible failed emissions test
Usually no noticeable change in how the car drives
Live data shows downstream sensor stuck at one value or out of range

What causes P0140?

Here are the most common causes of P0140:

Failed downstream O2 sensor (Bank 1, Sensor 2) with internal open circuit
: high
Open or broken O2 sensor signal wire
: high
Disconnected or heavily corroded sensor connector
: medium
Failed sensor heater preventing sensor from reaching operating temperature
: medium
Faulty ECU input circuit (rare)
: low

Is it safe to drive with P0140?

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.

How do you diagnose P0140?

Follow these steps to pinpoint the root cause of P0140:

Read freeze frame data and check for a heater code for the same sensor (P0141), which points to a power or heater problem at the sensor. Several heater codes at once (for example P0135 and P0141) point to a shared fuse or supply
Inspect the Bank 1 Sensor 2 connector visually for disconnection, corrosion, water intrusion, or melted plastic
Use a multimeter at the sensor connector to verify ground, heater power (battery voltage with key on), and signal continuity back to the ECU
If wiring tests good, unplug the sensor and measure heater resistance directly. Healthy: 2-15 ohms. Open: burned-out heater. The sensor needs replacement
If a new sensor still shows no activity, scope the signal wire for opens or shorts between the sensor and ECU

How much does P0140 cost to fix?

Repair costs for P0140 vary depending on the root cause and your vehicle. Here are typical estimates:

Repair	Parts Cost	Labor Cost	Total Estimate	DIY Difficulty
Replace downstream O2 sensor (Bank 1, Sensor 2)	€40–€180	€50–€130	€90–€310	Moderate
Repair O2 sensor wiring or connector	€5–€25	€40–€120	€45–€145	Moderate
Replace heater fuse and diagnose underlying short	€1–€10	€0–€80	€1–€90	Easy

Prices estimated as of March 2026. Costs vary by region, vehicle, and shop.

Full repair guides

Oxygen Sensor Replacement Cost

P0140 on specific vehicles

P0140 behaves differently depending on your vehicle. Select your car below for model-specific causes and known issues:

P0140 on BMW

BMW 3 Series

See BMW-specific causes & cost

P0140 on Volkswagen

Volkswagen Golf

See Volkswagen-specific causes & cost

P0140 on Audi

Audi A4

See Audi-specific causes & cost

P0140 on Mercedes-Benz

Mercedes C-Class

See Mercedes-Benz-specific causes & cost

These codes are often seen alongside P0140 or indicate related issues:

Common questions about P0140

What does no activity detected mean?: 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.
Can I drive with P0140?: 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.
How do I test an O2 sensor with a multimeter?: 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 fairly steadily and moves when the mixture changes. A signal that stays fixed, for example flat near 0V, or parked at a mid value of roughly 0.45V that many ECUs apply when the circuit is open, points to a failed sensor or a broken wire.
Can a blown fuse cause P0140?: 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.

Read our detailed guides

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