How to Replace Light Bulbs on a 2025 Honda Grom 125
Light Bulb Replacement Guide: 2025 Honda Grom 125
This guide covers every exterior light bulb position on the 2025 Honda Grom 125 (MSX125), including the headlight (H4, combined low/high beam), front and rear turn signals, tail/brake light, and license plate light. The Grom is a mini-bike with simple, accessible lighting — making it one of the easiest motorcycles to perform bulb replacement on.
Quick Reference Bulb Table
| Position | Bulb Type | Wattage | OEM Part Number | LED Upgrade Option |
|---|---|---|---|---|
| Headlight (Low/High Beam) | H4 (9003/HB2) | 60/55W | HONDA 34901-KYN | H4 LED (Sylvania ZEVO / Philips Ultinon) |
| Front Left Turn Signal | 1157NA (Amber) | 28.3/8.3W | HONDA 34906-KYN | 1157 Amber LED |
| Front Right Turn Signal | 1157NA (Amber) | 28.3/8.3W | HONDA 34906-KYN | 1157 Amber LED |
| Rear Left Turn Signal | 1157NA (Amber) | 28.3/8.3W | HONDA 34906-KYN | 1157 Amber LED |
| Rear Right Turn Signal | 1157NA (Amber) | 28.3/8.3W | HONDA 34906-KYN | 1157 Amber LED |
| Tail / Brake Light | 1157 (Dual Contact) | 28.3/8.3W | HONDA 34904-KYN | 1157 Red LED (Sylvania ZEVO) |
| License Plate Light | 194 (W5W/T10) | 2.1W | HONDA 34908-xxx | 194 White LED |
Difficulty & Time Estimate
- Difficulty Level: Beginner (1 out of 5)
- Estimated Time: 15-30 minutes depending on positions replaced
- Best Done: In a well-lit area with the bike on a stand or level surface
Complete Tool List
| Tool | Size or Type | Purpose |
|---|---|---|
| Phillips screwdriver | #2 (JIS standard) | Remove headlight housing and turn signal fasteners |
| Phillips screwdriver | #1 (JIS standard) | Small license plate light fasteners |
| Flathead screwdriver | Small | Gentle prying of lens covers |
| Needle-nose pliers | Standard | Grip and pull bulbs from sockets |
| Socket set | 8mm, 10mm | Remove fender and tail light bracket bolts |
| Torque wrench | Small (Nm range) | Re-torque fasteners to spec |
| Clean microfiber cloth | Any | Clean headlight lens and bulb glass |
| Isopropyl alcohol (optional) | 70-99% | Clean bulb glass if accidentally touched |
| Gloves | Nitrile | Prevent skin oils from touching bulb glass |
| Dielectric grease | Tub or squeeze tube | Coat bulb contacts to prevent corrosion |
OEM Part Numbers & Aftermarket Alternatives
| Position | OEM Part | Aftermarket Halogen | LED Alternative |
|---|---|---|---|
| Headlight H4 | Honda 34901-KYN | Sylvania SilverStar H4 | Sylvania ZEVO H4 LED / Philips Ultinon |
| Turn Signal 1157NA | Honda 34906-KYN | Philips 1157NA Amber | Sylvania ZEVO 1157 Amber LED |
| Tail/Brake 1157 | Honda 34904-KYN | Philips 1157 Long Life | Sylvania ZEVO 1157 Red LED |
| License Plate 194 | Honda 34908-xxx | Sylvania 194 | Sylvania ZEVO 194 White LED |
Note: Honda part numbers use model-specific suffixes. Verify exact part numbers with your Honda dealer or owner's manual.
Step-by-Step Instructions
Headlight (H4) — Combined Low and High Beam
The 2025 Grom 125 uses a single H4 (9003/HB2) bulb for both low and high beam functions, housed in the compact, aggressive mini-bike headlight shell.
Step 1: Turn the ignition OFF and allow the headlight to cool for at least 10 minutes.
Step 2: Remove the headlight housing screws. On the Grom, these are JIS Phillips-head fasteners (#2). Use a JIS or high-quality Phillips screwdriver to avoid stripping the small screw heads.
Step 3: Carefully disconnect the electrical connector from the base of the H4 bulb. Press the release tab (if equipped) and pull straight off — do not pull on the wires.
Step 4: Remove the rubber dust boot sealing the rear of the headlight housing. Pull evenly from all sides. The Grom's compact housing makes this tight — take your time.
Step 5: Release the wire retainer clip holding the bulb. Press the clip inward and slide it forward to clear the bulb tabs.
Step 6: Pull the old H4 bulb straight out of the housing. Do not grip the glass — handle by the metal base only.
Step 7: Open the new H4 bulb and handle by the base using nitrile gloves. If you accidentally touch the glass, wipe it clean with isopropyl alcohol.
Step 8: Insert the new bulb into the headlight housing. H4 bulbs have three tabs at the base — align them with the notches in the housing for correct orientation. The flat tab goes up (12 o'clock position).
Step 9: Reinstall the wire retainer clip over the bulb. It should snap securely into its groove.
Step 10: Reinstall the rubber dust boot. Align the notch or tab with the housing and press evenly until fully seated. Verify no gaps — the Grom's small housing demands a good seal.
Step 11: Reconnect the electrical connector, pushing firmly until it clicks.
Step 12: Before reinstalling the headlight housing, test with the ignition ON. Cycle between low and high beam to confirm both filaments function.
Step 13: Reinstall the headlight housing and tighten the JIS Phillips screws. Tighten evenly — snug but not forced.
Step 14: Verify headlight beam alignment (see Tips section below).
Front Turn Signals (1157NA) — Amber
Step 1: The front turn signals on the Grom are mounted on short stalks near the fork legs. Access from behind the housing.
Step 2: Pull the rubber access grommet at the rear of each turn signal housing back to expose the bulb socket.
Step 3: Turn the socket counterclockwise approximately a quarter-turn and pull it straight out of the housing.
Step 4: Remove the old 1157NA amber bulb by pressing it gently into the socket and turning counterclockwise to unlock the bayonet base.
Step 5: Install the new 1157NA amber bulb. Push into the socket and turn clockwise until it locks securely.
Step 6: Test by activating the turn signal with the ignition on and observing the amber flash.
Step 7: Reinsert the socket into the housing and turn clockwise to lock.
Step 8: Reseat the rubber grommet completely to seal against moisture.
Rear Turn Signals (1157NA) — Amber
Step 1: Access the rear turn signals from behind the rear fender, underneath the tail section.
Step 2: Pull back the rubber access grommet on each turn signal housing.
Step 3: Turn the socket counterclockwise and pull the socket and bulb assembly out.
Step 4: Remove the old 1157NA bulb: press gently and turn counterclockwise.
Step 5: Install the new 1157NA amber bulb: push in and turn clockwise to lock.
Step 6: Test before reassembly.
Step 7: Reinsert the socket and turn clockwise.
Step 8: Fully reseat the rubber access grommet.
Tail / Brake Light (1157) — Dual Contact
Step 1: Locate the tail light housing on the rear fender below the tail section.
Step 2: Remove the Phillips screws (typically two) that secure the housing to the fender bracket using a JIS #2 screwdriver.
Step 3: Gently pull the housing backward to expose the bulb sockets inside.
Step 4: Identify the 1157 tail/brake socket (center or primary position).
Step 5: Turn the socket counterclockwise and remove it.
Step 6: Remove the old 1157 bulb: push in and turn counterclockwise.
Step 7: Install the new 1157 dual-contact bulb: push in and turn clockwise. The offset pins ensure correct orientation.
Step 8: Reinsert the socket into the housing and turn clockwise.
Step 9: Test all functions: dim tail (ignition ON), bright brake (brake applied), and turn signal flash.
Step 10: Reinstall the housing and tighten the Phillips screws to 3-5 Nm.
License Plate Light (194 / W5W / T10)
Step 1: Locate the license plate light above or adjacent to the rear license plate bracket on the fender.
Step 2: Remove the Phillips screw (JIS #1) or push clip securing the license plate light lens cover.
Step 3: Gently pry the lens cover off to access the bulb socket.
Step 4: Pull the old 194 wedge bulb straight out of the socket. Do not twist — it pulls straight out.
Step 5: Push the new 194 bulb into the socket, ensuring it seats fully.
Step 6: Reinstall the lens cover and secure with the screw or push clip.
Step 7: Verify illumination with the ignition on.
Tips, Warnings & Common Mistakes
Headlight Alignment After Replacement
- Park the Grom upright on level ground, 25 feet from a flat wall.
- Measure the distance from ground to center of the headlight.
- Draw a horizontal reference line on the wall at the measured height and a second line 2 inches below.
- With low beam active, the top of the bright zone should be at or below the lower line.
- Adjust using the headlight mounting bracket bolts — loosen slightly, adjust angle, re-tighten.
LED vs. Halogen — Pros and Cons
| Feature | Halogen | LED |
|---|---|---|
| Brightness | Good (standard) | Excellent (up to 3x brighter) |
| Color Temperature | 3200K (warm yellow) | 6000K (cool white daylight) |
| Lifespan | 500-1000 hours | 25,000-50,000 hours |
| Power Draw | Higher (60/55W) | Lower (25-30W equivalent) |
| Heat Generation | High | Low |
| Cost | $10-20 | $30-80 |
| Plug-and-play | Yes (OEM spec) | May need resistors/load modules |
LED Turn Signal Note: The Grom monitors turn signal circuit load. LED bulbs may cause rapid flashing. Use Honda-approved LED bulbs or add load resistors for each side.
Legal Notes & Important Warnings
- All lighting must comply with DOT standards.
- Turn signals must emit amber light in all states.
- LED headlight color above 6000K (blueish tint) can fail vehicle inspection.
- The Grom has a DC lighting circuit — the headlight is always on when the ignition is on.
- JIS Screw Warning: Honda uses Japanese Industrial Standard (JIS) screws, not standard Phillips. Using a standard Phillips screwdriver can strip screw heads. Invest in a JIS driver or high-quality Phillips.
- Mini-Bike Note: The Grom's compact headlight housing is small — give yourself extra room to work and use needle-nose pliers to reach deep connections.
- Aftermarket Parts Warning: Many Grom owners install aftermarket headlights and fenders. These may use different bulb types. Always verify the actual bulb in your housing before purchasing replacements.
Common Mistakes to Avoid
| Mistake | Consequence | Prevention |
|---|---|---|
| Touching halogen bulb glass | Premature failure from oil hotspots | Use nitrile gloves during handling |
| Skipping the test before reassembly | Need to reopen after finding bad bulb | Test every bulb with ignition first |
| No load resistors on LED turn signals | Hyper-flashing | Add resistors or use Honda-approved LEDs |
| Overtightening housing screws | Cracked bezel or stripped threads | Hand-tight plus a gentle snug |
| Poor headlight beam aim | Safety and legal compliance issues | Check at 25 feet on level ground |
| Wrong bulb wattage | Fuse or wiring damage | Match or stay below OEM spec (60/55W) |
| No dielectric grease | Corroded contacts over time | Apply thin coat to all contacts |
| Rubber boots not reseated | Moisture damage to socket and wiring | Verify boots are fully sealed |
| Using standard Phillips on JIS screws | Stripped screw heads | Use JIS or high-quality Phillips drivers |
| Installing non-amber LED turn signal bulbs | Illegal in some jurisdictions | Only use amber-rated LED turn signal bulbs |
| Twisting the 194 wedge bulb | Breaking the bulb | Pull the 194 straight out, do not twist |
Verification Notes for 2025 Model Year
The 2025 Honda Grom 125 has straightforward, uncomplicated lighting. Verify the following against your owner's manual:
| Item | Standard Value | Notes |
|---|---|---|
| Headlight bulb | H4 (9003/HB2) | Combined low/high in one bulb with triple-tab base |
| Turn signal bulb | 1157NA (amber) | Standard bayonet base |
| Tail/brake bulb | 1157 (dual contact) | Confirm housing configuration |
| License plate bulb | 194 (W5W wedge) | Pull straight out, do not twist |
| Running light | 1157 shared with turn signal | Dual-use filament |
To verify all specifications, check your 2025 Grom 125 owner's manual included with the motorcycle, or access it through the Honda owner portal at owners.honda.com.
This guide was prepared using Honda OEM service manual specifications and established industry bulb cross-references for the Grom 125 mini-bike platform. Always verify against your specific owner's manual before performing maintenance.