Code No. P0037: Oxygen Sensor (rear) Heater Circuit Low Input

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 less than 0.17 A for 2 seconds.

Check Conditions

More than 2 seconds have passed since the engine starting sequence was completed.
While the oxygen sensor (rear) heater is off.
Battery positive voltage is more than 11 V and is less than 16.5 V.

Judgement Criterion

The oxygen sensor (rear) heater voltage is less than 2.0 V for 2 seconds.

FAIL-SAFE AND BACKUP FUNCTION

None

PROBABLE CAUSES

Failed oxygen sensor (rear) heater
Open/short circuit or harness damage 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 OHRL 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. Perform voltage measurement at oxygen sensor (rear) connector.

Disconnect connector, and measure at harness side.
Ignition switch: ON
Voltage between terminal heater power supply line and earth.

OK: System voltage

Q. Is the check result normal?

	Go to Step 3.

	Repair or replace the connector(s), or check and repair the harness of short circuit to earth, and open circuit in power supply line between oxygen sensor (rear) connector and engine control relay connector.

STEP 3. Perform voltage measurement at engine-ECU connector.

Disconnect connector, and measure at harness side.
Ignition switch: ON
Voltage between terminal OHRL line and earth.

OK: System voltage

Q. Is the check result normal?

	Go to Step 4.

	Repair or replace the connector(s), or check and repair the harness of short circuit to earth, and open circuit in OHRL line between oxygen sensor (rear) connector and engine-ECU connector.

STEP 4. Check of damage in OHRL line between oxygen sensor (rear) connector and engine-ECU connector.

Q. Is the check result normal?

	Go to Step 5.

	Repair the connector(s) or wiring harness.

STEP 5. Check of damage in power supply line between oxygen sensor (rear) connector and engine control relay connector.

Q. Is the check result normal?

	Go to Step 6.

	Repair the connector(s) or wiring harness.

STEP 6. M.U.T.-III diagnosis code

Reconfirmation of diagnosis code.

Q. Is diagnosis code stored?

	Replace the engine-ECU.

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