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 3, 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. Connect test harness between MAP sensor and sensor harness. Turn ignition on. Measure voltage between Red (pos.) and Green (neg.) terminals. If reading is about 5 volts, go to step 4). If not, measure voltage between Red (pos.) terminal and ground. If voltage is still not 5 volts, go to step 3). If voltage is about 5 volts, repair open in Blue/White wire between ECU terminal C14 and MAP sensor.
- Turn ignition off. Connect system checker harness between ECU and ECU harness. Measure voltage between terminal C15 (pos.) and C14 (neg.) with ignition on. If voltage reading is about 5 volts, repair open Red/White wire between ECU terminal C15 and MAP sensor. If voltage is not about 5 volts, substitute a known good ECU. If condition is rectified, replace original ECU.
- Measure voltage between White (pos.) and Green (neg.) terminals. If reading is not about 3 volts, replace MAP sensor or repair short in White/Blue wire between ECU terminal C11 and MAP sensor.
- If measured voltage is about 3 volts, turn ignition off. Connect system checker harness between ECU and ECU connectors. Turn ignition on. Measure voltage between terminals C11 (pos.) and C14 (neg.). If there is not about 3 volts present, repair open White/Blue wire between ECU terminal C11 and MAP sensor.
- If there are about 3 volts present, 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.