P0140 — Post-Cat O2 Sensor No Activity (Bank 1 Downstream)

What does P0140 mean?

P0140 is set when the Bank 1 downstream oxygen sensor shows no meaningful activity at all. The voltage sits flat — typically parked at the ECM's bias voltage of roughly 0.4–0.5 V — and never moves, even under conditions that should force it to move. Bank 1 is the bank containing cylinder number 1, and Sensor 2 is the sensor after the catalytic converter. P0140 is different from a sensor reading too low (P0137) or too high (P0138): with no activity the ECM is effectively looking at an open circuit or a completely dead sensor element.

What are the symptoms of P0140?

• Check Engine Light is illuminated
• Catalyst readiness monitor will not complete
• Often no drivability symptoms at all
• Possible slight reduction in fuel economy
• Vehicle will fail emissions / smog testing
• Frequently sets alongside P0141 when the sensor heater has failed

What causes P0140?

Repair costs are typical US ranges and vary by make, model, model year, and labor rate. A diagnostic trouble code is a symptom, not a guaranteed failed part — confirm the root cause before replacing anything.

Is it safe to drive with P0140?

In most cases a vehicle stays drivable for short trips with P0140 active, but you should diagnose and repair it promptly. This is a moderate-severity code — ignoring it can lead to further damage or a failed emissions test. Exact tolerance depends on your specific make and model.

How to diagnose P0140

1. Pull every O2 code before deciding anything

   P0140 next to P0141 tells a very different story from P0140 alone. With P0141 the heater circuit has failed and the sensor never gets hot enough to produce a signal — the heater is the fault and the "no activity" is a consequence. P0140 by itself points at the signal circuit or the sensor element.

   Tools: Scan tool with full code retrieval

2. Check the O2 heater fuse

   Many vehicles run all O2 heaters from one fuse; some split them by bank. Test the correct fuse with a test light rather than eyeballing it. If a fresh fuse blows straight away, there is a short in the harness or inside a sensor, and that short is your actual repair.

   Tools: Test light or multimeter, Replacement fuse

3. Measure heater resistance at the sensor

   Unplug the downstream sensor and measure across the heater terminals — typically somewhere in the 3 to 15 Ω range depending on the sensor. Infinite resistance means the heater element is open, the sensor never warms up, and it will never produce a signal no matter how good the wiring is.

   Tools: Multimeter, Wiring diagram

4. Verify bias voltage reaches the connector

   With the sensor unplugged and the key on, the scan tool should still show the ECM's bias voltage on the signal PID. If it does, the signal wire is intact back to the PCM and the sensor is the suspect. If the reading goes to zero or reads erratically, the fault is in the harness.

   Tools: Scan tool, Multimeter

5. Force the sensor to respond and watch for movement

   With the engine fully warm, briefly create a rich condition — most easily by snapping the throttle — and watch the downstream PID. Even a lazy but living sensor will show some movement. A trace that never leaves its resting value under any condition confirms the element is dead.

   Tools: Scan tool with graphing PIDs

How do I fix P0140?

• Replace the Bank 1 downstream O2 sensor with an OEM part
• Repair the open O2 signal or ground circuit
• Replace the blown O2 heater fuse
• Repair harness chafed or melted against the exhaust
• Clean or replace the corroded downstream sensor connector

P0140 by make and model

Select your exact year, make, and model below to get a P0140 diagnosis tuned to your vehicle's known failure patterns, recalls, and TSBs.

Related diagnostic codes

• P0136 — Post-Cat O2 Sensor Circuit Fault (Bank 1)
• P0137 — Post-Cat O2 Sensor Low Voltage (Bank 1 Downstream)
• P0138 — Post-Cat O2 Sensor High Voltage (Bank 1 Downstream)
• P0141 — Post-Cat O2 Heater Circuit (Bank 1 Downstream)
• P0156 — Post-Cat O2 Sensor Circuit Fault (Bank 2)
• P0420 — Catalyst Efficiency Below Threshold

Common symptoms of P0140

P0140 often shows up as one of these symptoms. Start from what your car is actually doing:

• Poor fuel economy
• Failed emissions / smog test