The Honda MSX125 Grom is one of the best-selling 125cc manual bikes in the UK. But its PGM-FI system causes MOT failures and no-start issues. Based on UK MOT data and our PGM-FI sensor series - tailored for UK riders.
Why MSX125 PGM-FI is Different in UK
UK MSX125 (2013-2025) must pass annual MOT. PGM-FI MIL light ON = MOT failure. UK models also have O2 sensor + EVAP system for Euro 5. Top 5 issues from UK MOT failure data and MSX125 UK Owners Facebook group (15k members).
Problem #1: MOT Failure - MIL Light ON - Bad O2 Sensor - Code 21-1 - Most Common UK Failure
Symptom: MIL light stays ON, fails MOT. High fuel consumption, 70 MPG instead of 100 MPG. UK MSX125 Euro 5 models 2021+ have O2 sensor that fails early due to UK rain + road salt.
UK MOT Rule: If MIL light is ON at MOT, automatic fail. Many UK testers fail MSX125 for this.
Fix: Test O2 heater 5-10Ω, signal should swing 0.1-0.9V at 3,000 RPM. Replace with NTK OZA624-E4 (UK equivalent) £30-40 on eBay UK or Wemoto. Clear code with DLC jumper under seat. Tip: UK road salt corrodes O2 connector - clean with contact cleaner.
UK Search: "MSX125 MOT failure MIL light O2 sensor"
Problem #2: No Start After Winter Storage - Bad CKP Sensor (Pulser Coil) - MIL 19 - Very Common in UK
Symptom: After UK winter (bike stored in shed/garage Nov-March), engine cranks but no spark. MSX125 left in cold damp shed = CKP coil moisture + corrosion.
Fix: Test CKP resistance: Should be 180-300Ω. Many UK bikes show OL (open) after winter. Replace pulser coil (Honda 30300-K26-901) £25-35 on CMSNL or David Silver Spares UK. UK tip: Store bike with dehumidifier or cover CKP area with ACF-50 anti-corrosion spray.
Problem #3: Poor Idle, Stalling in Traffic - Dirty TP Sensor & IACV - MIL 8-1 - London Traffic Issue
Symptom: In London stop-start traffic, idle hunts 1,200-2,500 RPM, stalls at lights. Worse with aftermarket exhaust (common UK mod).
Fix: UK MSX125 has IACV (Idle Air Control Valve) that clogs with UK stop-start soot. Clean throttle body + IACV with carb cleaner. TP voltage: 0.4-0.6V closed, 3.5-4.5V WOT. UK riders with Scorpion or Akrapovic exhaust need fuel controller or remap or get TP codes.
Problem #4: Hard Cold Start -5°C to 5°C - Bad EOT Sensor - MIL 7-1 - UK Winter Problem
Symptom: UK winter morning -2°C, bike needs 8-10 cranks. EOT reads warm when actually freezing = ECU gives too little fuel.
Fix: Test EOT: At 0°C ~5-7kΩ, at 20°C ~2.5kΩ, at 80°C ~0.3kΩ. UK tip: Use 5W-30 oil in winter (not 10W-40) for easier start. Replace EOT sensor (37870-K26-901) £18. Many UK riders fit Oxford Heaterz grips and forget EOT needs correct reading.
Problem #5: Sputtering at 50-60 MPH, Won't Hit 70 MPH - Clogged Fuel Pump + MAP Sensor - MIL 1-1 - UK A-Road Issue
Symptom: On UK A-roads, bike sputters at 50-60 MPH, won't reach top speed 65-70 MPH. UK fuel with E10 ethanol (since 2021) causes fuel pump clog and tank rust on older MSX.
Fix: Check fuel pressure: 43 psi. UK E10 ethanol absorbs water in winter = rust. Many UK owners fit fuel filter sock mod. Test MAP: Idle ~1.2V, WOT ~4V. UK tip: Use E5 super unleaded (97 RON) in winter, not E10, to prevent water absorption.
UK-Specific Tips for MSX125 PGM-FI
- E10 Fuel (UK since Sept 2021): E10 causes more water absorption + fuel pump issues. Use E5 super unleaded in winter storage. Add fuel stabiliser (Sta-Bil) if storing.
- Road Salt: UK road salt corrodes O2 and CKP connectors. Wash bike weekly in winter, spray connectors with ACF-50 or WD40 Specialist.
- MOT: MIL light ON = fail. Fix sensors before MOT. Some UK testers also fail if aftermarket exhaust has no baffle + O2 delete.
- Parts: Use Wemoto, David Silver Spares, CMSNL UK for OEM parts - cheaper than Honda UK dealer. Multimeter from Halfords £15 does same tests.
- DLC Connector: Under seat, near battery. Jump with wire to read MIL codes - same as US Grom.
UK MIL Code Reference for MSX125
1-1 MAP, 1-2 IAT, 7-1 EOT, 8-1 TP, 8-2 CKP, 19-1 CKP, 21-1 O2, 29-1 IACV, 54-1 Bank Angle
Related: Our full PGM-FI series: MAP Sensor | TP Sensor | EOT Sensor | CKP No Spark Fix | O2 Sensor | US Grom Edition
Labels: Honda Service Guides, Troubleshooting, UK Edition, MSX125, MOT Guide