🔧 Impreza - Oxygen Sensor Replacement

Your Impreza uses two exhaust sensors: an upstream air/fuel (A/F) sensor (before the catalytic converter) and a downstream oxygen (O2) sensor (after the catalytic converter). Replacing the correct one restores proper fuel control and can fix a check-engine light for O2/A/F sensor codes.

Difficulty Level: Intermediate | Estimated Time: 0.8-2.0 hours

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⚠️ Safety & Precautions

• ⚠️ Let the exhaust fully cool (burn risk).
• ⚠️ Support the car with jack stands; never rely on a jack.
• ⚠️ Avoid twisting/pulling on the sensor wiring.
• ⚠️ If working under the car, chock the rear wheels.
• ⚠️ Battery disconnect is not required, but keep ignition OFF while unplugging sensors.

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🔧 Required Tools

You'll need the following tools for this repair:

• Safety glasses
• Nitrile gloves
• Wheel chocks
• Floor jack (rated 2-ton minimum)
• Jack stands (rated 2-ton minimum)
• 22mm oxygen sensor socket
• 3/8" drive ratchet
• 3/8" drive extension set
• Torque wrench (10–80 Nm range)
• Trim clip remover
• Flathead screwdriver
• Penetrating oil
• OBD2 scan tool

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🔩 Required Parts

HowToo sells all the parts you need for this repair:

• Upstream A/F ratio sensor (front) - Qty: 1
• Downstream oxygen sensor (rear) - Qty: 1

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📋 Before You Begin

• 🧰 Park on level ground and set the parking brake.
• 🧰 Let the engine/exhaust cool completely.
• 🧰 Decide which sensor you’re replacing: upstream (in engine bay) or downstream (under the car).
• 🧰 Spray penetrating oil on the sensor threads and let it soak 10–15 minutes.

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🔨 Step-by-Step Instructions

Follow these steps in order:

Step 1: Identify the sensor (upstream vs downstream)

• Use your OBD2 scan tool to read codes.
• P0137/P0138 often point to the downstream sensor; P0031/P0032/P0130-type codes often point to the upstream A/F sensor.
• If unsure, replace only the coded sensor.

Step 2: Replace the upstream A/F sensor (engine bay method)

• Open the hood and locate the sensor on the exhaust manifold area (front of engine).
• Remove any blocking plastic ducting or covers using a trim clip remover and flathead screwdriver.
• Unplug the sensor connector by releasing the lock tab by hand (do not pull on wires).
• Install a 22mm oxygen sensor socket on a 3/8" drive ratchet with a 3/8" drive extension set and loosen the sensor.
• Remove the sensor by hand once loose.
• Thread the new sensor in by hand first (prevents cross-threading).
• Tighten with a torque wrench (10–80 Nm range): Torque to 44 Nm (32 ft-lbs).
• Reconnect the electrical connector until it clicks.

Step 3: Replace the downstream O2 sensor (under-car method)

• Chock wheels with wheel chocks.
• Lift the front with a floor jack (rated 2-ton minimum) and support with jack stands (rated 2-ton minimum).
• Locate the downstream sensor in the exhaust pipe after the catalytic converter.
• Unplug the connector (usually clipped to the body); use a trim clip remover if the harness clip is stubborn.
• Loosen the sensor using the 22mm oxygen sensor socket with a 3/8" drive ratchet and 3/8" drive extension set.
• Remove the sensor by hand, then install the new one by hand first.
• Tighten with a torque wrench (10–80 Nm range): Torque to 40 Nm (30 ft-lbs).
• Route the harness the same way as original and reconnect the connector until it clicks.

Step 4: What to do about anti-seize

• Most new sensors come with thread coating already applied.
• If your new sensor threads are pre-coated, do not add anything.
• If completely dry (rare), apply a tiny amount of nickel anti-seize only to threads, avoiding the tip.

Step 5: Reassemble and lower the car (if raised)

• Reinstall any covers/ducting using the trim clip remover and flathead screwdriver.
• Lower the car safely using the floor jack (rated 2-ton minimum).

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✅ After Repair

• ✅ Clear codes with your OBD2 scan tool.
• ✅ Start the engine and check for exhaust leaks (ticking sound) around the sensor area.
• ✅ Confirm the check-engine light stays off after a short drive.
• ✅ If the light returns immediately, re-check the connector lock and harness routing.

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💰 DIY vs Shop Cost

Shop Cost: $220-$520 (parts + labor)

DIY Cost: $60-$220 (parts only)

You Save: $160-$300 by doing it yourself!

Shop labor rates vary but typically run $100-$150/hour. This repair takes a shop approximately 0.8-1.5 hours.

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🎯 Ready to get started?

HowToo makes it easy: same-day/2-day shipping on every part, plus all the tools and specialty tools you need! Check out the parts and tools sections below to add everything to your cart.