The Honda Grom (MSX125 in Europe) is the most popular manual mini-moto in the USA. But its PGM-FI system has 5 common problems that cause no-start, poor idle, and MIL lights. Based on our complete PGM-FI sensor series.
Why Honda Grom PGM-FI is Different in USA
US Grom (2014-2025) uses same PGM-FI sensors as Indonesian Supra GTR / CB150R: MAP, TP, IAT, EOT, CKP, O2. But US models have stricter emission + O2 sensor. Here are top 5 issues reported on Grom forums (Reddit r/hondagrom, GromTalk).
Problem #1: No Start, No Spark - Bad CKP Sensor (Pulser Coil) - MIL 19
Symptom: Engine cranks strong, but no spark at plug. Most common on 2014-2018 Grom with 5,000+ miles.
Fix: Test CKP resistance: Should be 180-300Ω. Many US riders find 0Ω when hot = coil expands and breaks. Replace pulser coil (Honda part 30300-K26-901). Cost $35-50 on Amazon. See our CKP Sensor Explained for test procedure with multimeter.
US Search Term: "Grom cranks but won't start no spark"
Problem #2: Poor Idle, Stalling, Jerking - Dirty TP Sensor & Throttle Body - MIL 8-1
Symptom: Idle hunts 1,200-2,000 RPM, jerks when opening throttle from stop. Very common after installing aftermarket intake / exhaust without tune.
Fix: Clean throttle body with CRC cleaner, check TP voltage: 0.4-0.6V closed, 3.5-4.5V wide open. If jumps, replace throttle body assembly. Do TP reset: Key ON, open/close throttle fully 3x. Common fix on Grom with Chimera intake.
Problem #3: High Fuel Consumption, Black Smoke - Bad O2 Sensor - MIL 21-1
Symptom: US Grom should get 100-130 MPG. If getting 70 MPG and black soot on exhaust, O2 sensor dead. US models 2018+ have O2 sensor in header.
Fix: Test O2 heater: 5-10Ω, signal should fluctuate 0.1-0.9V at 3,000 RPM when hot. Many US riders delete O2 with aftermarket exhaust and get MIL - need O2 eliminator or tune. Replace with Denso 234-9021 equivalent $25.
Problem #4: Hard Cold Start in Winter - Bad EOT Sensor - MIL 7-1
Symptom: In US winter (30-40°F), Grom needs 5-10 cranks to start. EOT reads warm when actually cold = ECU gives too little fuel.
Fix: Test EOT resistance: At 32°F (0°C) ~5-7kΩ, at 68°F (20°C) ~2.5kΩ, at 176°F (80°C) ~0.3kΩ. If reads low, replace EOT sensor (Honda 37870-K26-901) $20. Location: cylinder head, near spark plug.
Problem #5: Sputtering at High RPM, Won't Rev Over 7k - Clogged Fuel Pump & Bad MAP Sensor - MIL 1-1
Symptom: Grom hits 6,000 RPM and sputters, won't reach 8,500 RPM redline. Very common with old fuel or rusty tank (Grom steel tank rusts in US if left with ethanol fuel).
Fix: Check fuel pump pressure: Should be 43 psi (294 kPa). If low, replace fuel filter (inside pump assembly) or pump (Quantum $60). Test MAP sensor: At idle ~1.2V, at WOT ~4V. If stuck, clean airbox drain (Grom airbox collects water).
US-Specific Tips for Grom PGM-FI
- Ethanol Fuel: US fuel has 10% ethanol - causes rust in tank and injector clog. Use ethanol-free or add stabilizer. Drain float? Grom is FI, no float, but clean injector with Techron.
- Aftermarket Parts: Chimera, Yoshimura, MNNTHBX intakes require fuel controller (Power Commander, aRacer) or ECU reflash, else TP/MAP codes.
- DLC Connector: Under seat, near battery. Jump with wire to read MIL codes - same as Indonesian Honda.
- Tools: US riders use $25 multimeter from Harbor Freight - same tests as our guides.
Quick MIL Code Reference for US Grom
1-1 MAP, 1-2 IAT, 7-1 EOT, 8-1 TP, 8-2 CKP, 19-1 CKP, 21-1 O2, 29-1 IACV
Related: Our full PGM-FI series: MAP Sensor | TP Sensor | EOT Sensor | CKP No Spark Fix | O2 Sensor Fuel Economy
Labels: Honda Service Guides, Troubleshooting, Engine & Transmission, USA Edition, Grom MSX125