Bosch 12108 Oxygen Sensor - Downstream or Upstream

Bosch Engineered Oxygen Sensor for Toyota and Geo

Bosch 12108 is a specific-fit, narrow-band non-heated oxygen sensor for select Toyota and Geo applications. It provides air-fuel-ratio feedback to the ECU for stable closed-loop fuel control and emissions monitoring.

The Bosch Engineered Oxygen Sensor line builds on Bosch OE development in oxygen sensing, including early thimble-style designs and later planar narrowband technology. This Bosch 12108 sensor uses a threaded metal housing, blade terminals, and an included wiring harness for direct replacement service on the listed upstream and downstream applications.

Key Features

• Part number: 12108
• Specific-fit oxygen sensor for listed Toyota and Geo vehicles
• Narrow-band, non-heated sensor type
• Threaded mounting with 18 mm thread size and 22 mm hex
• Blade terminals, 2-wire configuration, male connector
• Wiring harness included; harness length 13.6 in
• Overall length: 17.3 in; sealing surface to connector end: 16.1 in
• Steel finish with metal housing for standard replacement service

Detailed Description

Bosch 12108 is built for oxygen sensor replacement on select Toyota Celica, Toyota Paseo, Toyota Tercel, Toyota Corolla, Toyota RAV4, and Geo Prizm applications. It is a narrow-band oxygen sensor that reports exhaust oxygen content to the ECU so the engine can maintain proper fuel mixture control during normal operation. Bosch’s OE-focused sensor development emphasizes accurate signal output, fast response characteristics in narrowband designs, and durable construction for heat and vibration exposure.

Fits 1999 Toyota Celica, upstream. Fits 1997-1999 Toyota Paseo, upstream. Fits 1997-1999 Toyota Tercel, upstream. Fits 1996-1998 Toyota Celica, downstream. Fits 1994 Toyota Celica, downstream. Fits 1995-1997 Geo Prizm, upstream. Fits 1995-1997 Toyota Corolla, upstream. Fits 1997 Toyota RAV4, upstream, without California Emission System. Fits 1996 Toyota RAV4, upstream. Fits 1995 Toyota Celica, downstream, without California Emission System. Fits 1995 Toyota Celica, downstream, with California Emission System.

This part cross-references 8946512380 and 8946520420 for the same application.

What It Helps Address

• Check engine light related to oxygen sensor signal faults
• Rich or lean fuel trim conditions caused by a worn sensor
• Slow or unstable closed-loop fuel control
• Emissions test failures tied to oxygen sensor performance
• Driveability issues from inaccurate exhaust oxygen feedback

Installation Notes and Tips

Confirm the sensor position before ordering, since Bosch 12108 is listed for both upstream and downstream locations depending on vehicle and emission package. Verify the connector shape, thread size, and harness routing against the original sensor before installation. Because this is a non-heated narrow-band sensor, it should be matched only to the exact applications shown in the fitment data.

Quality Assurance

Bosch backs this oxygen sensor with more than five decades of OE development in engine management and emissions sensing. The Bosch Engineered Oxygen Sensor line is built to OE-style standards for signal accuracy, thermal durability, and consistent operation in demanding underhood conditions.