Two on the 2.5-liter four-cylinder engine and four on the 3.5-liter V6, depending on which engine the car has.
Oxygen sensors (O2 sensors) monitor exhaust gas composition to help the engine control unit tune the air-fuel mixture and monitor emissions. In the 2017 Altima lineup, the sensor count varies with the engine option.
Engine-specific sensor counts
Below is a quick breakdown by engine option.
- 2.5L four-cylinder: 2 O2 sensors total — one upstream sensor before the catalytic converter and one downstream sensor after it.
- 3.5L V6: 4 O2 sensors total — two upstream sensors (one for each exhaust bank, before each catalytic converter) and two downstream sensors (one after each catalytic converter).
With this configuration, you’ll generally find two sensors on the 2.5L Altima and four on the 3.5L model.
Location and identification tips
On the 2.5L engine, the upstream sensor sits on the exhaust manifold or close to it, while the downstream sensor is located along the exhaust pipe after the catalytic converter. On the 3.5L V6, there are sensors on both banks, with one upstream and one downstream per bank.
Why it matters
The oxygen sensors provide feedback to the engine computer to adjust fuel delivery and monitor emission controls. More sensors in the V6 model allow more precise monitoring of two catalytic converters and each exhaust bank.
Summary
For a 2017 Nissan Altima, count depends on engine: two O2 sensors for the 2.5L four-cylinder, and four for the 3.5L V6. If you’re diagnosing or replacing sensors, verify the exact sensor counts using the VIN and service manual, since variations or aftermarket configurations can occur.