O2 Sensors - All

Complete replacement of all four oxygen sensors (two upstream, two downstream) on the 3.5L EcoBoost engine to restore proper air-fuel ratio monitoring and emissions control.

Warnings

Tools required

Parts

• Upstream O2 sensor (Bank 1) × 1 — Motorcraft DY-1355 or equivalent
• Upstream O2 sensor (Bank 2) × 1 — Motorcraft DY-1355 or equivalent
• Downstream O2 sensor (Bank 1) × 1 — Motorcraft DY-1354 or equivalent
• Downstream O2 sensor (Bank 2) × 1 — Motorcraft DY-1354 or equivalent
• Anti-seize compound × 1 — Nickel-based, sensor-safe

Preparation

1. Ensure engine and exhaust system are completely cold to the touch
2. Park vehicle on level ground, engage parking brake, and place wheel chocks behind rear wheels
3. Raise front of vehicle and support securely on jack stands rated for vehicle weight
4. Spray all four O2 sensor threads with penetrating oil and allow 15-20 minutes to soak if sensors are original or heavily corroded
5. Locate all four sensors: upstream sensors are in the exhaust manifolds near the turbochargers, downstream sensors are in the Y-pipe after the catalytic converters

Procedure

1. 1

   Disconnect upstream O2 sensor electrical connectors
   From the engine bay, locate the electrical connectors for both upstream O2 sensors. They are positioned near the back of each cylinder head. Press the release tab and disconnect each connector. Bank 1 (passenger side) connector is near the firewall; Bank 2 (driver side) may require reaching around the intake plumbing.
2. 2

   Remove Bank 1 upstream O2 sensor
   Using the O2 sensor socket with ratchet and extension, position the socket over the Bank 1 upstream sensor in the passenger-side exhaust manifold. Turn counterclockwise to remove. If sensor is seized, apply heat with a propane torch to the bung (not the sensor) for 2-3 minutes and retry. Thread the wiring harness through as you remove the sensor.
3. 3

   Remove Bank 2 upstream O2 sensor
   Access the driver-side exhaust manifold and remove the Bank 2 upstream sensor using the same technique. This sensor is more difficult to access due to steering and suspension components. Use a long extension and work from above through the engine bay. Thread the wiring harness through as you remove the sensor.
4. 4

   Disconnect downstream O2 sensor electrical connectors
   From underneath the vehicle, locate the electrical connectors for both downstream O2 sensors in the Y-pipe assembly after the catalytic converters. These connectors are typically clipped to the transmission crossmember or frame rail. Press release tabs and disconnect both connectors.
5. 5

   Remove both downstream O2 sensors
   Using the O2 sensor socket from underneath, remove both downstream sensors from the Y-pipe. Bank 1 sensor is on the passenger side of the Y-pipe, Bank 2 is on the driver side. These sensors typically have better access than upstream sensors. Turn counterclockwise to remove.
6. 6

   Clean sensor bungs and inspect threads
   Use a wire brush to clean the threads in all four sensor bungs in the exhaust system. Remove any carbon buildup, rust, or old anti-seize. Inspect threads for damage. If threads are stripped or damaged, the exhaust manifold or Y-pipe may need replacement or thread repair.
7. 7

   Prepare new sensors for installation
   Apply a thin coat of nickel-based anti-seize compound to the threads of all four new O2 sensors. Apply only to the threads, keeping anti-seize away from the sensor tip and perforated protective sleeve. Excess anti-seize can contaminate the sensor element and cause immediate failure.
8. 8

   Install Bank 1 downstream O2 sensor
   From underneath the vehicle, thread the new Bank 1 downstream sensor into the Y-pipe by hand, ensuring wiring harness routes properly. Hand-tighten until seated, then use the O2 sensor socket and torque wrench to tighten to specification. Do not over-tighten.
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
9. 9

   Install Bank 2 downstream O2 sensor
   Thread the new Bank 2 downstream sensor into the Y-pipe by hand, routing the wiring harness properly. Hand-tighten until seated, then torque to specification using the O2 sensor socket and torque wrench.
   Torque spec

   O2 Sensor45 Nm (33 lb-ft)
10. 10

    Install Bank 1 upstream O2 sensor
    From the engine bay, thread the new Bank 1 upstream sensor into the passenger-side exhaust manifold by hand. Route the wiring harness to avoid contact with hot exhaust components or moving parts. Hand-tighten until seated, then use the O2 sensor socket and torque wrench to tighten to specification.
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)
11. 11

    Install Bank 2 upstream O2 sensor
    Thread the new Bank 2 upstream sensor into the driver-side exhaust manifold by hand. Route the wiring harness carefully to avoid interference with steering components, intake plumbing, and exhaust heat. Hand-tighten until seated, then torque to specification.
    Torque spec

    O2 Sensor45 Nm (33 lb-ft)
12. 12

    Reconnect all O2 sensor electrical connectors
    Connect all four O2 sensor electrical connectors, ensuring each clicks securely into place. Verify correct pairing: upstream sensors connect to harnesses near the cylinder heads, downstream sensors connect to harnesses near the transmission crossmember. Ensure all wiring is secured and routed away from heat and moving parts.
13. 13

    Lower vehicle and clear diagnostic codes
    Carefully lower the vehicle from jack stands. If a scan tool is available, connect it to the OBD-II port and clear any stored O2 sensor-related diagnostic trouble codes. If no scan tool is available, disconnect the negative battery terminal for 30 seconds to clear codes, though this will also reset radio presets and other settings.

Reassembly

1. Verify all O2 sensor electrical connectors are fully seated and locked
2. Ensure all wiring harnesses are secured and properly routed away from exhaust heat and moving components
3. Double-check that all four sensors are torqued to specification and no anti-seize compound contacted the sensor tips

Verification

• Start the engine and verify it runs smoothly without misfires or rough idle
• Allow engine to reach operating temperature and verify no exhaust leaks are present at any of the four sensor locations
• Using a scan tool, monitor O2 sensor data in live data mode: upstream sensors should show voltage fluctuations between 0.1-0.9V during closed-loop operation, downstream sensors should show relatively steady voltage around 0.5-0.7V
• Test drive vehicle for 10-15 minutes through varied driving conditions to allow PCM to complete O2 sensor readiness monitors
• After test drive, use scan tool to verify no new O2 sensor codes have set and all readiness monitors are complete or in process
• If check engine light illuminates after installation, retrieve codes immediately to diagnose potential installation issues or defective sensors

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