NTK 25540 Oxygen Sensor - Upstream

Direct Fit Oxygen Sensor for BMW

NTK 25540 is a direct fit heated oxygen sensor built for specific BMW applications. It uses a threaded mounting design and a 4-wire configuration to match the original sensor layout for accurate air-fuel feedback.

Key Features

• Direct fit design for specific applications
• Heated oxygen sensor for faster operating response
• 4-wire configuration
• Threaded mounting type with 18 mm thread diameter
• 20.5 in. harness length

Detailed Description

NTK 25540 is part of the Direct Fit Oxygen Sensor line, which focuses on application-specific replacement sensors with OE-style fit, form, and function. NTK draws on its background as a major oxygen sensor manufacturer and uses controlled ceramic technology from raw material through finished production. This sensor is engineered for upstream use on BMW 325i, BMW 325is, and BMW 325iX applications listed for 1987-1988, including Bank 1 Sensor 1 coverage noted in the fitment data. The part number 25540 is also referenced across multiple aftermarket listings for the same application.

What It Helps Address

• Faulty oxygen sensor readings
• Check engine light concerns related to mixture control
• Slow sensor response during closed-loop operation
• Incorrect air-fuel ratio feedback
• Emissions performance issues tied to sensor wear

Installation Notes and Tips

Confirm the upstream location, thread size, and harness length before installation. This sensor uses a threaded mount and a specific 4-wire layout, so connector and routing verification is important before replacement. For BMW 325i, BMW 325is, and BMW 325iX applications, match the fitment notes carefully, especially the 9/87 cutoff listed for 1988 models.

Quality Assurance

NTK states that these oxygen sensors are extensively tested during manufacturing to meet or exceed OE specifications. The brand is part of the NGK family and is recognized for original equipment oxygen sensor production and application coverage.