Denso 4221 Spark Plug

Denso 4221 Spark Plug Overview

Denso 4221 is a U-Groove conventional spark plug built for reliable ignition performance in secondary ignition systems. The U-groove electrode design helps improve ignitability, while the copper core and nickel electrode materials support consistent spark delivery for everyday driving and maintenance service.

For drivers searching for a Denso spark plug for Honda Civic, Toyota Corolla, or similar import applications, this part number 4221 offers a straightforward OE-style replacement with a 0.8 mm pre-gap, 14.0 mm thread diameter, 19.00 mm reach, and flat seat design.

Key Features

• Denso U-Groove Conventional spark plug for secondary ignition service
• U-groove electrode profile for improved ignitability and engine efficiency
• Copper center electrode core with nickel tip material
• Nickel ground electrode core material with standard ground configuration
• 5000 ohm resistance for ignition system noise control
• 0.8 mm pre-gap, 16.0 mm hex size, flat seat, 19.00 mm reach
• Manufacturer heat range 20 for specified engine applications

Product Details

The Denso U-Groove Conventional line is built around a practical spark plug design that focuses on stable combustion and dependable ignition performance. Part number 4221 uses a standard single-ground-electrode layout with nickel and copper construction, which supports consistent spark transfer and serviceable durability in the correct engine application. This Denso 4221 spark plug is a direct-fit style component for vehicles that call for the listed thread diameter, reach, seat type, and heat range.

This part cross-references DENSO Auto Parts (e.g., U20ESR-N) for the same application.

Common Issues This Part Can Address

• Hard starting caused by worn or inefficient spark plugs
• Rough idle from weak or inconsistent ignition output
• Misfire concerns related to aged secondary ignition components
• Reduced fuel efficiency from incomplete combustion
• Loss of throttle response from degraded spark plug performance

Installation Notes and Tips

Verify the engine calls for a 14.0 mm thread diameter, 19.00 mm reach, flat seat, and 0.8 mm pre-gap before installation. Check the ignition system condition, inspect plug wires or coil-on-plug components, and confirm the replacement matches the original heat range and resistance specification. For fitment-based searches, this Denso 4221 spark plug is commonly referenced for Honda Civic and Toyota Corolla service applications where the listed dimensions match the engine requirement.

Quality Assurance

Denso manufactures ignition components to OE-level dimensional and performance standards for specified applications. The U-Groove Conventional design is engineered to support reliable ignition and efficient combustion when installed in the correct vehicle application.