Denso 3121 Spark Plug

Denso 3121 U-Groove Conventional Spark Plug

Denso 3121 is a U-Groove Conventional spark plug built for direct ignition service across a wide range of import and domestic applications. Its U-groove ground electrode design is engineered to improve ignitability, helping support more efficient combustion in engines that call for a conventional plug with a 14 mm thread and flat seat.

With a copper center electrode core, nickel center tip, nickel ground electrode tip, and 5,000 ohms of resistance, this plug is built for stable secondary ignition performance. The 1.1 mm pre-gap, 19.00 mm reach, and 16.0 mm hex size make Denso 3121 a practical OE-style service part for vehicles such as Subaru Legacy, Volvo S60, and Saab 9-5 applications.

Key Features

• U-Groove ground electrode design for improved ignitability
• Conventional spark plug construction for broad vehicle coverage
• Copper core center electrode for consistent spark delivery
• Nickel ground and center electrode tip materials
• 5,000 ohm resistance for secondary ignition control
• 1.1 mm pre-gap, 19.00 mm reach, 14.00 mm thread diameter
• Flat seat design with a 16.0 mm hex size

Product Line Details

Denso’s U-Groove Conventional line focuses on dependable ignition performance in everyday driving conditions. The U-shaped ground electrode helps expose the spark kernel more effectively than a standard flat design, which supports cleaner combustion in engines that specify this style of plug. Denso 3121 is part of that conventional service lineup for vehicles that require an OE-type replacement spark plug.

Detailed Description

This spark plug cross-references DENSO Auto Parts K20PR-U11 for the same application. Denso 3121 is listed across many fitments, including 2003-2007 Volvo S60, 2001-2009 Saab 9-5, and 1998-2004 Subaru Forester. It also appears in applications such as 2004-2008 Chevrolet Aveo, 1999-2008 Suzuki Grand Vitara, and 2007-2009 Porsche 911, making it a useful service part for drivers maintaining factory-style ignition performance.

Compatible vehicle coverage also includes 2008-2009 Mercedes-Benz SLK55 AMG, 2005-2006 Mercedes-Benz C55 AMG, 2003-2006 Subaru Baja, and 2004-2006 Volvo S80. The part number 3121 is commonly searched for replacement work on Subaru Legacy, Volvo XC70, and Saab 9-3 ignition service.

Common Issues This Plug Can Address

• Hard starting caused by worn spark plug electrodes
• Rough idle from weakened ignition output
• Misfire conditions related to aged secondary ignition components
• Reduced fuel efficiency from incomplete combustion
• Loss of throttle response due to inconsistent spark delivery

Installation Notes and Tips

Verify thread diameter, reach, seat type, and heat range before installation. Denso 3121 uses a 14.00 mm thread, 19.00 mm reach, flat seat, and manufacturer heat range 20, so it should be matched to the engine specification listed for the vehicle. Check the gap against the engine requirement before installation, and confirm the application by year, make, and model for vehicles such as Subaru Legacy, Volvo V70, and Saab 9-5.

Quality Assurance

Denso manufactures this U-Groove Conventional spark plug to OE-style dimensional and material specifications for secondary ignition service. The design uses nickel and copper electrode materials selected for dependable spark transfer in the listed applications.