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

OPERATION

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).

FUNCTION

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.

TROUBLE JUDGMENT

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 more than 11 V and is less than 16.5 V.

Judgement Criterion

The oxygen sensor (rear) heater current is more than 10.5 A for 2 seconds.

PROBABLE CAUSES

Failed oxygen sensor (rear) heater
Short circuit in oxygen sensor (rear) heater circuit or loose connector contact
Failed engine-ECU

DIAGNOSIS PROCEDURE

STEP 1. Connector check: D-31 oxygen sensor (rear) connector

Q. Is the check result normal?

	Go to Step 2 .

	Repair or replace the connector.

STEP 2. 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: 4.5 - 8.0 Ω (at 20°C)

Q. Is the check result normal?

	Go to Step 3 .

	Replace the oxygen sensor (rear).

STEP 3. Connector check: A-33X engine control relay connector

Q. Is the check result normal?

	Go to Step 4 .

	Repair or replace the connector.

STEP 4. Check harness between A-33X (terminal No. 2) engine control relay connector and D-31 (terminal No. 1) oxygen sensor (rear) connector.

note

Before checking harness, check intermediate connectors C-31, and repair if necessary.

Check power supply line for short circuit.

Q. Is the check result normal?

	Go to Step 5 .

	Repair the damaged harness wire.

STEP 5. Connector check: B-08 engine-ECU connector

Q. Is the check result normal?

	Go to Step 6 .

	Repair or replace the connector.

STEP 6. Check harness between D-31 (terminal No. 2) oxygen sensor (rear) connector and B-08 (terminal No. 35) engine-ECU connector.

note

Before checking harness, check intermediate connectors A-11, C-29 and repair if necessary.

Check earthing line for short circuit.

Q. Is the check result normal?

	Go to Step 7 .

	Repair the damaged harness wire.

STEP 7. 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 ).