Code 4, Vacuum Switch
- Turn off ignition. Remove "EFI/ECU" fuse in under-hood relay panel for 10 seconds to reset ECU. Start engine and allow to idle for 30 seconds. Check if warning light is on. If not on, problem is intermittent. Test drive and check light again.
- If light is on, turn ignition off. Disconnect 8-wire connector at control box. Check for continuity between terminals No. 3 and 4 of control box. See Fig 1
. If continuity exists, go to step 4). If continuity does not exist, install "T" fitting in manifold vacuum hose between manifold and vacuum switch (located inside control box). Connect vacuum gauge to "T" fitting.
- Start engine. If vacuum is less than 1.2 in. Hg, check vacuum hose routing. If vacuum is greater than 1.2 in. Hg, replace vacuum switch or repair open in Black/Yellow or White wire in control box.
- If continuity existed in step 2), turn ignition on. Measure voltage between Black/Yellow harness terminal and ground. If battery voltage is not present, repair open in Black/Yellow wire between 8-wire harness connector and No. 12 fuse.
- If battery voltage is present, turn ignition off. Reconnect 8-wire connector to control box. Connect system checker harness between ECU and ECU connectors. Turn ignition on. Measure voltage between terminals A10 (pos.) and B8 (neg.). If battery voltage is not present, repair open in Light Blue wire between ECU terminal A10 and vacuum switch.
- If battery voltage is not present, substitute a known good ECU. If condition is rectified, replace original ECU.
Courtesy of AMERICAN HONDA MOTOR CO., INC.