OBD-1 ECU’s are the best era ECU HANDS DOWN. They’re coming up just shy of 30 years and are sadly beginning to show their age sadly. They have several common issues you may or have yet to experience but are mostly repairable and can breathe new life into the ECU.
Leaky / Bad Capacitors
The #1 issue is capacitors going bad.
They can leak onto and stain the main board that can be damaging. They can grow a weird turquoisey mold/fuzz. They can omit a fishy smell (if smelt up close or warmed up from desoldering from the main board). They can have a hand in causing the fuel pump to not activate or intermittently activate. They can make the vehicle seem sensitive to cold and warm conditions such as no start-ups after the car is warmed up.
Fuel Pump issues
If you’ve experienced intermittent or no fuel pump activation, it can be due to bad capacitors and/or a bad IC17 chip. From my experience, the “integrated circuit” 17 zip aka IC17 chip seems to be more of the culprit than the capacitors.
Burnt Q31 Transistor
The purge control solenoid or PCS (an emissions related solenoid found on the back left side of OBD1 D-series intake manifolds) is what the Q31 transistor regulates. Usually when an engine swap is performed on a 92-95 Civic, it’s easy to overlook and mistakenly plug the PCS into the IAT (intake air temp) sensor found on the left side intake runner since the share the same style round 2-wire connector. The voltage for either of these guys are completely different and the result is the burning out of the IAT sensor tip and the Q31 transistor on the ECU. Sometimes the Q31 can burn out so bad it will damage the board making it impossible to replace. The good thing is it can be removed completely for engine swap purposes if emissions isn’t a priority.
Burned Q31 Examples: