Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
1987 Honda Prelude Base, StandardFACTORY OEM SERVICE PROCEDURE
Code 5, Map Sensor
- Turn ignition off. Remove clock fuse in the under-hood relay box for 10 seconds to reset ECU. Start engine and observe PGM-FI warning light. If light is off, problem is intermittent. Test drive and check again.
- If light is on, turn ignition off. Using a "T" fitting, connect a handheld vacuum pump/gauge into vacuum hose between throttle body and MAP sensor. Start engine. If vacuum is not present, repair as necessary. If vacuum is present and holds, turn ignition off.
- Connect inspection harness between MAP sensor and harness. Turn ignition on. Measure voltage between White (pos.) terminal and Green (neg.) terminal. If reading is not about 3 volts, replace MAP sensor. If voltage reading is about 3 volts, substitute a known good ECU. If condition is rectified, replace original ECU.
- If voltage reading from ground to Red/White wire of coolant sensor harness was not about 5 volts in step 3), turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C6 (pos.) and C12 (neg.). If about 5 volts is present, repair open in Red/White wire between ECU terminal C6 and coolant sensor.
- If voltage reading is not about 5 volts, disconnect "C" connector from main wiring harness only, not at ECU. Measure voltage between terminals C6 (pos.) and C12 (neg.). If about 5 volts is now indicated, repair short in Red/White wire between ECU terminal C6 and sensor.
- If about 5 volts is not indicated, substitute a known good ECU. If condition is rectified, replace original ECU.
Factory Service Tools
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.