P0140
O2 Sensor Circuit No Activity / Slow Response (Bank 1 Sensor 2)
The ECU detects no switching or excessively slow response from the downstream oxygen sensor on bank 1. Typical for an aged or poisoned sensor; downstream sensors should show a relatively flat signal but must respond to fueling changes during diagnostic monitor runs.
- Check engine light: Yes
- Emissions related: Yes
- Repair difficulty: easy
Symptoms
- Check engine light on
- Increased fuel consumption
- Possible rough idle or hesitation
- Failed emissions test
Common causes
Ordered by how often each turns out to be the cause.
- Sensor aged or poisonedCommon
- Sensor heater circuit weakPossible
- Catalytic converter so degraded that downstream signal mirrors upstreamLess likely
Parts involved
- Oxygen Sensor B1s2
- Oxygen Sensor Heater Circuit
- Catalytic Converter Bank1
P0140 is a generic OBD-II code defined by SAE J2012. Exact wording and diagnostic procedure vary by manufacturer and model. Code definitions are drawn from the public-domain OBDex dataset; the workshop notes are our own.