O2 Sensors - All

This procedure covers the replacement of all oxygen sensors (upstream and downstream) on the 2016-2021 Honda Civic 1.5L turbocharged engine.

Warnings

Tools required

Parts

• Upstream O2 sensor (Air/Fuel sensor) × 1 — 36531-5AA-A01 or equivalent
• Downstream O2 sensor × 1 — 36532-5AA-A01 or equivalent

Preparation

1. Ensure the exhaust system is completely cold to the touch
2. Park vehicle on level ground and set parking brake
3. Place wheel chocks behind rear wheels
4. Safely raise and support the front of the vehicle on jack stands
5. If an O2 sensor warning light is present, use an OBD-II scanner to identify which sensor has failed
6. Spray penetrating oil on sensor threads if sensors are original or have not been replaced recently, then wait 15-20 minutes

Procedure

1. 1

   Disconnect battery negative terminal
   Disconnect the negative battery terminal to prevent accidental shorts and clear ECU fault codes. Wait 3 minutes before proceeding to allow capacitors to discharge.
2. 2

   Locate upstream O2 sensor (A/F sensor)
   From underneath the vehicle, locate the upstream Air/Fuel sensor. It is positioned on the exhaust manifold or turbo outlet pipe before the catalytic converter, typically on the driver's side. The sensor has a gray 4-pin connector.
3. 3

   Disconnect upstream O2 sensor electrical connector
   Press the release tab on the gray 4-pin connector and carefully pull the connector apart. Do not pull on the wires. If the connector is tight, gently wiggle it while pulling. Trace the wire back and unclip any wire harness retainers.
4. 4

   Remove upstream O2 sensor
   Using a 22mm O2 sensor socket and ratchet with extension, turn the sensor counterclockwise to remove. The sensor socket has a slot to allow the wire to pass through. If the sensor is extremely tight, apply additional penetrating oil and allow more time to soak, or use a breaker bar with controlled force to avoid breaking the sensor body.
   ⚠If the sensor breaks off leaving threads in the bung, you will need a thread chaser or may require professional extraction.
5. 5

   Clean upstream sensor bung threads
   Use a wire brush to clean the threads in the exhaust manifold/turbo outlet bung. Remove any carbon deposits, old anti-seize, or corrosion. Ensure threads are clean and dry before installing the new sensor.
6. 6

   Install new upstream O2 sensor
   Apply a thin coat of anti-seize compound to the sensor threads only, being extremely careful not to get any on the sensor tip or element. Thread the new sensor into the bung by hand, ensuring it turns smoothly and is not cross-threaded. Once hand-tight, use the O2 sensor socket and torque wrench to tighten to specification.
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
7. 7

   Reconnect upstream sensor connector
   Connect the 4-pin gray connector until it clicks into place. Route the sensor wire properly and secure with any harness clips that were removed. Ensure the wire is not in contact with hot exhaust components or moving parts.
8. 8

   Locate downstream O2 sensor
   The downstream O2 sensor is located after the catalytic converter in the mid-pipe section, typically underneath the passenger area of the vehicle. It has a black 2-pin connector and is smaller than the upstream A/F sensor.
9. 9

   Disconnect downstream O2 sensor electrical connector
   Press the release tab on the black 2-pin connector and separate the connector halves. Unclip any wire harness retainers securing the sensor wire to the chassis or exhaust hangers.
10. 10

    Remove downstream O2 sensor
    Using the 22mm O2 sensor socket and ratchet with extension, turn the sensor counterclockwise to remove. Access may be easier from directly underneath the vehicle. Apply the same precautions as with the upstream sensor regarding excessive force.
11. 11

    Clean downstream sensor bung threads
    Use a wire brush to thoroughly clean the threads in the exhaust pipe bung. Remove all carbon buildup and old anti-seize to ensure proper thread engagement and torque accuracy.
12. 12

    Install new downstream O2 sensor
    Apply a thin coat of anti-seize to the sensor threads only, avoiding the sensor element completely. Start the sensor by hand, turning it several full rotations to confirm proper thread engagement. Once hand-tight, torque to specification using the torque wrench.
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)
13. 13

    Reconnect downstream sensor connector
    Connect the 2-pin black connector until it clicks. Secure the sensor wire with all retaining clips, ensuring the wire has adequate slack and will not contact the exhaust or be pinched by suspension components.
14. 14

    Reconnect battery and clear codes
    Reconnect the negative battery terminal. If using an OBD-II scanner, clear any stored diagnostic trouble codes. The ECU will relearn sensor parameters during normal driving.

Reassembly

1. Double-check that both sensor electrical connectors are fully seated and locked
2. Verify all wire harness clips are secured and wires are routed away from heat and moving parts
3. Lower the vehicle from jack stands safely
4. Remove wheel chocks

Verification

• Start the engine and allow it to idle for 2-3 minutes, listening for any exhaust leaks at the sensor locations
• Check that the check engine light does not illuminate or return immediately
• Take the vehicle for a 10-15 minute test drive including highway speeds to allow the ECU to complete sensor diagnostics
• If available, use an OBD-II scanner to verify both sensors are reading properly and no pending codes are present
• Verify no exhaust leaks are present at sensor bungs after the system has cooled

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