Code No. P0038: Oxygen Sensor (rear) Heater Circuit High Input <Vehicles with dual oxygen sensors>

OPERATION

Power is supplied to the heater power terminal of the oxygen sensor (rear) connector from the engine control relay.
The heater of the oxygen sensor (rear) connector is controlled by the power transistor in the engine-ECU connector terminal OHRL.

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 JUDGEMENT

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.

FAIL-SAFE AND BACKUP FUNCTION

None

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. Perform resistance measurement at oxygen sensor (rear) connector.

Disconnect connector, and measure at sensor side.
Resistance between heater power supply terminal and OHFL line terminal.

OK: 4.5 - 8.0 Ω (at 20°C)

Q. Is the check result normal?

	Go to Step 2.

	Replace the oxygen sensor (rear).

STEP 2. Check of short to power supply in power supply line between engine control relay connector and oxygen sensor (rear) connector.

Q. Is the check result normal?

	Go to Step 3.

	Repair the connector(s) or wiring harness.

STEP 3. Check short to power supply in OHRL line between oxygen sensor (front) connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 4.

	Repair the connector(s) or wiring harness.

STEP 4. 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 ).