Denso 4125 Spark Plug

Denso 4125 U-Groove Spark Plug Overview

The Denso 4125 is a conventional spark plug built with a U-Groove ground electrode design to improve ignitability and support more efficient combustion. It uses a copper core center electrode with nickel tip materials for dependable secondary ignition performance in the right application.

For drivers searching for a Denso spark plug for Honda Civic or Toyota Corolla service, this part number 4125 is a direct-fit style option where specified by the vehicle application. The Denso U-Groove Conventional line focuses on stable spark delivery and consistent engine operation.

Key Features

• U-Groove ground electrode tip design for improved spark exposure
• Conventional spark plug construction for standard ignition systems
• Copper core center electrode for efficient heat transfer
• Nickel center and ground electrode tip materials
• 5000 ohm resistance for resistor-type ignition circuits
• Pre-gapped to 0.9 mm
• Flat seat design, 12.00 mm thread diameter, 19.00 mm reach, and 18.0 mm hex size
• Manufacturer heat range 20

Product Details

Denso 4125 is part of the U-Groove Conventional spark plug line, a product family engineered around a shaped ground electrode that helps expose the spark kernel more effectively. That design can improve ignitability in engines that call for a conventional nickel/copper spark plug with a flat seat and standard ground configuration. If you are replacing worn plugs in a Honda Civic or Toyota Corolla, matching the specified heat range, reach, thread diameter, and resistance is important for proper ignition system performance.

This part cross-references DENSO Auto Parts (e.g., X20EPR-U9) for the same application.

Common Issues This Part Can Address

• Hard starting caused by worn or fouled spark plugs
• Rough idle from inconsistent spark delivery
• Misfire conditions related to degraded plug electrodes
• Reduced fuel efficiency from incomplete combustion
• Loss of throttle response from weak ignition performance

Installation Notes and Tips

Verify the vehicle’s required thread diameter, reach, seat type, heat range, and pre-gap before installation. Use the correct torque specification for the cylinder head material, and inspect ignition wires or coil-on-plug components at the same time to avoid repeat misfire concerns. For Honda Civic and Toyota Corolla service, confirm the exact engine application before ordering Denso 4125.

Quality Assurance

Denso manufactures this spark plug as part of its U-Groove Conventional line, using a copper core and nickel electrode materials for standard ignition service. Part number 4125 is built to the listed specifications for applications that call for this configuration.