1991 Acura Legend Base, AutomaticFACTORY OEM SERVICE PROCEDURE

Code 41, Code 42: O2 Sensor Heater

NOTE: For wiring diagrams, see WIRING DIAGRAMS article in the ENGINE PERFORMANCE Section. For vacuum diagrams, see VACUUM DIAGRAMS article in the ENGINE PERFORMANCE Section.
  1. Turn ignition off. Remove ACG fuse in underdash fuse box for 10 seconds to reset ECU. Start engine, and connect jumper wire to diagnostic connector. Observe CHECK ENGINE light. If light is not flashing, problem is intermittent. Check for poor and loose connections at O2 sensor connector and connectors C355 and C356 at right shock tower. Test drive, and check again.
  2. If CHECK ENGINE light is flashing Code 41 and/or 42, turn ignition off. Disconnect 4-pin O2 sensor connector(s). Measure resistance across "C" and "D" terminals at sensor. See Fig 1 . If reading is not 10-20 ohms, replace sensor. If reading is within specification, go to next step.
    Fig 1: Identifying O2 Sensor(s) Connector
    G91G16291Courtesy of AMERICAN HONDA MOTOR CO., INC.
  3. Check continuity between ground and all sensor wire terminals. Check continuity between sensor terminal "C" and "A". If continuity exists, sensor is shorted. Replace sensor. If no continuity exists, go to next step.
  4. Reattach O2 sensor connector(s). Attach ECU test harness "A" to main wire harness side only. Turn ignition on. Measure voltage between harness terminals A10 (positive) and A23 (negative) for left O2 sensor or A12 (positive) and A23 (negative) for right O2 sensor. See Figure .
  5. If no battery voltage is present, go to next step. If battery voltage is present, substitute a known good ECU, and recheck. If problem goes away, replace ECU.
  6. If no battery voltage is present, ensure fuse No. 22 in underdash fuse box is good. If fuse is okay, check related circuits for continuity. Repair any problems found. Retest system. If problem is still present, substitute a known good ECU. If condition goes away, replace 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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