Code No. P0038: Oxygen Sensor (Rear) Heater Circuit High Input


  • Power is supplied to the heater power terminal (terminal No. 1) of the oxygen sensor (rear) connector from the engine control relay (terminal No. 2).
  • The heater (terminal No. 2) of the oxygen sensor (rear) connector is controlled by the power transistor in the engine-ECU (terminal No. 35).


  • The power supply to the oxygen sensor (rear) heater is controlled by the ON/OFF control of the power transistor in the engine-ECU.
  • Heating the oxygen sensor (rear) heater enables the oxygen sensor (rear) to provide good response even when the exhaust emission temperature is low.


Check Conditions
  • More than 2 seconds have passed since the engine starting sequence was completed.
  • While the oxygen sensor (rear) heater is on.
  • Battery positive voltage is 11 - 16.5 V.
Judgement Criterion
  • The oxygen sensor (rear) heater current has continued to be higher than 10.5 A for 2 seconds.


  • Failed oxygen sensor (rear) heater
  • Failed engine-ECU


STEP 1. Perform resistance measurement at D-31 oxygen sensor (rear) connector.

  • Disconnect connector, and measure at sensor side.
  • Resistance between terminal No. 1 and No. 2.
OK: 11 - 18 Ω (at 20°C)

Q. Is the check result normal?

Go to Step 2 .
Replace the oxygen sensor (rear).

STEP 2. M.U.T.-III diagnosis code

  • Reconfirmation of diagnosis code.

Q. Is diagnosis code set?

Replace the engine-ECU.
Intermittent malfunction (Refer to GROUP 00 - How to Use Troubleshooting/Inspection Service Points - How to Cope with Intermittent Malfunctions ).