CodeXOBD

P0140 — O2 Sensor Circuit No Activity (Bank 1 Sensor 2)

P0140 means the downstream O2 sensor signal is flat and not responding. A failed sensor, a disconnected sensor, or a wiring fault are the usual causes.

Low

The engine runs normally. The module cannot monitor converter efficiency.

$20–$200Parts, typical range
0.3–1 hLabour
estimateNot a quote · 2026 basis

What P0140 means

P0140 sets when the downstream oxygen sensor voltage remains at a fixed level and does not respond to changes in exhaust composition. Unlike P0136 (general circuit fault) or P0138 (high voltage), P0140 specifically means no activity — the sensor is alive enough to report a voltage, but that voltage never changes. This is characteristic of a sensor whose sensing element has degraded to the point where it can no longer detect oxygen.

Technical summary
SystemEmission control
How the catalytic converter is gradedExhaust leaves the engine, passes the upstream oxygen sensor, then the catalytic converter, then the downstream oxygen sensor. The module compares the two sensor signals: on a healthy converter the downstream signal is steady while the upstream one oscillates. When the downstream signal starts to mirror the upstream one, the converter is judged to have stopped storing oxygen.Engineexhaust outUpstream O₂sets fuel trimCatalytic converterstores oxygenDownstream O₂grades the converterThe module compares the two sensors — that comparison is the test

Symptoms you may notice

What causes P0140

Downstream O2 sensor contaminated or aged

Very likelylow costeasy

The sensing element has been contaminated by silicone, lead, or simply age, and can no longer detect oxygen changes.

Downstream O2 sensor connector disconnected

Likelylow costeasy

The sensor plug has worked loose or was left off after exhaust work. A disconnected sensor can produce a flat default voltage that registers as no activity.

Wiring fault preventing signal change

Possiblelow costeasy

A partially damaged wire can carry a fixed voltage but not change with the sensor output.

How to diagnose it

1

Monitor downstream sensor with live data

At steady cruise with a warm engine, watch the downstream sensor voltage. It should show some variation. If it is stuck at one value, the sensor is not active.

Tools: OBD-II scanner with live dataExpected: Slow variation around 0.5-0.7VIf it passes: Sensor shows some activity. The fault may be intermittent.If it fails: Voltage is completely flat — replace the sensor.
2

Check the sensor connector

Verify the downstream O2 sensor connector is fully seated and the terminals are clean and tight. A loose connector can produce a flat default signal.

If it passes: Connector is secure. Continue to step 3.If it fails: Reconnect, clear the code, and re-test.
3

Inspect the wiring harness

Trace the sensor wiring from the connector to the module. Look for chafed, pinched, or broken wires that could carry a fixed voltage but not change with the sensor output.

Tools: Inspection light, MultimeterIf it passes: Wiring is intact. The sensor is confirmed faulty.If it fails: Repair the damaged wiring.

Related codes

P0136P0137P0138P0420

Common questions

Is P0140 the same as a dead O2 sensor?

Almost. The sensor is still producing a voltage, so it is not completely dead, but its sensing element can no longer detect oxygen. Functionally, it needs replacement.

Last verified 2026-07-27 · Sources: SAE J2012 — Diagnostic Trouble Code Definitions and Failure Type Byte (SAE International)

This information is for general guidance and is not a substitute for professional diagnosis. Cost figures are estimates, not quotes.