2006-2012 Honda TRX90 TRX90EX/X Service Manual, Page 254Get this manual
IGNITION SYSTEM
TROUBLESHOOTING
Inspect the following before diagnosing the system:Faulty spark plugLoose spark plug cap or spark plug wire connectionWater got into the spark plug cap (leaking the ignition coil secondary voltage) Temporarily exchange the ignition coil with the other good one and perform the spark testIf there is spark, the exchanged ignition coil is faultyNo Spark at Spark Plug Unusual Condition Low peak voltageProbable Cause (Check in numerical order)
Ignition coil primary voltage