1987 Honda Prelude Base, AutomaticFACTORY OEM SERVICE PROCEDURE

Code 12, EGR System

  1. Check harness and connections between EGR valve and ECU. Verify vacuum supply to EGR control solenoid. Check vacuum hoses for cracks, splits or looseness. Repair as necessary. Warm engine to normal operating temperature (cooling fan on). Disconnect vacuum hose from EGR valve. Using a hand-held vacuum pump, apply vacuum to EGR valve. If vacuum does not hold and engine does not stall, replace faulty EGR valve or clean blocked EGR passages.
  2. If vacuum held and engine stalled in step 1), remove control box cover. Disconnect vacuum hose between air chamber and solenoid valve. Connect vacuum gauge to air chamber. If there is not about 8 in. Hg, replace CVC. If correct vacuum is present, reconnect hose between vacuum chamber and EGR solenoid.
  3. If vacuum is now present at EGR valve, check for short in Red wire between ECU terminal A10 and EGR control solenoid. If wire is okay, replace solenoid. If vacuum is not present, disconnect 4-wire connector from control box. Jumper battery voltage to Black/Yellow wire terminal and ground to Red wire terminal at control box (solenoid side of harness). Check EGR valve for about 8 in. Hg. If correct vacuum is present, go to step 5).
  4. Connect and disconnect ground from Red terminal of solenoid harness. If EGR solenoid does not click, replace solenoid. If solenoid does click, repair blockage in vacuum port of solenoid.
  5. Measure voltage between ground and Black/Yellow wire terminal of harness. If battery voltage is not present, repair open in Black/Yellow wire between solenoid valve and No. 12 fuse. If battery voltage is present, turn ignition off. Connect test harness between EGR lift sensor and sensor connector.
  6. Turn ignition on. Measure voltage between Red (pos.) and Green (neg.) terminals of harness. Go to step 8) if about 5 volts is indicated. If 5 volts is not indicated, measure voltage between ground and Red (pos.) harness terminal. If 5 volts is indicated, repair open in Green/White wire between ECU terminal C12 and sensor.
  7. If 5 volts is not indicated, turn ignition off. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C13 (pos.) and C12 (neg.). If voltage is about 5 volts, repair open in White/Yellow wire between ECU terminal C13 and sensor. If 5 volts is not indicated, substitute a known good ECU. If condition is rectified, replace original ECU.
  8. Measure voltage between White (pos.) and Green (neg.) terminals of harness. If about one volt is not indicated, replace EGR valve or repair short in Yellow wire between ECU terminal C8 and EGR valve lift sensor. If about one volt is indicated, turn ignition off.
  9. Connect system checker harness between ECU and ECU connector. Turn ignition on. Measure voltage between terminals C8 (pos.) and C12 (neg.). If about one volt is not indicated, repair open Yellow wire between ECU terminal C8 and sensor.
  10. If about one volt is not indicated, disconnect "A" connector of system checker harness from ECU only, not main harness. Measure voltage between terminals A10 (pos.) and A18 (neg.). If battery voltage is not present, repair open Red wire between ECU terminal A10 and EGR control solenoid.
  11. If battery voltage is indicated, substitute a known good ECU. If condition is rectified, replace original ECU.
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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.

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