2AZ-FE ENGINE CONTROL SYSTEM – SFI SYSTEM ES–87

Oxygen Sensor Heater Control Circuit Low

DTC P0037
(Bank 1 Sensor 2)

Oxygen Sensor Heater Control Circuit High

DTC P0038
(Bank 1 Sensor 2)

Oxygen Sensor Heater Circuit Malfunction

DTC P0141
(Bank 1 Sensor 2)
DESCRIPTION
Refer to DTC P0136 (see page ES-129). ES
HINT:
• When any of these DTCs are set, the ECM enters fail-safe mode. The ECM turns off the Heated
Oxygen (HO2) Sensor heater in fail-safe mode. Fail-safe mode continues until the ignition switch is
turned OFF.
• The ECM provides a pulse width modulated control circuit to adjust the current through the heater. The
HO2 sensor heater circuit uses a relay on the B+ side of the circuit.

Reference (System Diagram of Sensor 2):

HO2 Sensor ECM

EFI MAIN
EFI MAIN Heater
from +B HT1B HT1B
Battery EFI NO. 1
Sensor
E2 OX1B OX1B

EX1B

MREL

Ground
A133556E01

DTC No. DTC Detection Conditions Trouble Areas

• Open in HO2 sensor heater circuit
Heated Oxygen (HO2) sensor heater current less than 0.3 A • HO2 sensor heater (sensor 2)
P0037
(1 trip detection logic) • Integration relay (EFI MAIN relay)
• ECM
• Short in HO2 sensor heater circuit
Heated Oxygen (HO2) sensor heater current more than 2 A • HO2 sensor heater (sensor 2)
P0038
(1 trip detection logic) • Integration relay (EFI MAIN relay)
• ECM
• Open or short in HO2 sensor heater circuit
Cumulative heater resistance correction value exceeds
• HO2 sensor heater (sensor 2)
P0141 threshold
• Integration relay (EFI MAIN relay)
(2 trip detection logic)
• ECM
 ES–88 2AZ-FE ENGINE CONTROL SYSTEM – SFI SYSTEM

MONITOR DESCRIPTION
The sensing position of the Heated Oxygen (HO2) sensor has a zirconia element which is used to detect
the oxygen concentration in the exhaust gas. If the zirconia element is at the appropriate temperature,
and the difference between the oxygen concentrations surrounding the inside and outside surfaces of the
sensor is large, the zirconia element generates voltage signals. In order to increase the oxygen
concentration detecting capacity of the zirconia element, the ECM supplements the heat from the exhaust
with heat from a heating element inside the sensor.
Heated oxygen sensor heater range check (P0037 and P0038):
The ECM monitors the current applied to the O2 sensor heater to check the heater for malfunctions. If the
current is below the threshold value, the ECM determines that there is an open circuit in the heater. If the
current is above the threshold value, the ECM determines that there is a short circuit in the heater.
The ECM constantly monitors the current applied to the heater. If the ECM detects an open or short
ES circuit, the ECM turns the MIL on and sets a DTC.
If a malfunction is detected, the ECM cuts off the current applied to the heater.
Example:
The ECM sets DTC P0038 when the current in the HO2 sensor heater is more than 2 A. Conversely,
when the heater current is less than 0.3 A, DTC P0037 is set.
Heated oxygen sensor heater performance (P0141):
After the accumulated heater ON time exceeds 100 seconds, the ECM calculates the heater resistance
using the battery voltage and the current applied to the heater.
If the resistance is above the threshold value, the ECM determines that there is a malfunction in the HO2
sensor heater and set DTC P0141.

MONITOR STRATEGY
P0037: Heated oxygen sensor heater range check (Low electrical
current)
Related DTCs P0038: Heated oxygen sensor heater range check (High electrical
current)
P0141: Heated oxygen sensor heater performance
Required Sensors/Components (Main) Heated oxygen sensor heater
Required Sensors/Components (Related) -
Continuous: P0037 and P0038
Frequency of Operation
Once per driving cycle: P0141
1 second: P0037 and P0038
Duration
10 seconds: P0141
Immediate: P0037 and P0038
MIL Operation
2 driving cycles: P0141
Sequence of Operation None

TYPICAL ENABLING CONDITIONS

All:
Monitor runs whenever following DTCs not present None

P0037:
Battery voltage 10.5 to 20 V

P0038:
Battery voltage 10.5 to 20 V

P0141 (Heater performance monitor check):

All of following conditions met: -
Battery voltage 10.5 V or more
Fuel cut OFF
Time after fuel cut ON to OFF 30 seconds or more
 2AZ-FE ENGINE CONTROL SYSTEM – SFI SYSTEM ES–89

Accumulated heater ON time 100 seconds or more

TYPICAL MALFUNCTION THRESHOLDS

P0037:
Heater current Less than 0.3 A

P0038:
Heater current More than 2 A

P0141 (Heater performance monitor check):

Accumulated heater resistance Varies with sensor element temperature (Example: More than 23 Ω)

COMPONENT OPERATING RANGE ES

0.4 to 1 A (when engine idles, HO2 sensor warmed up and battery
Heated Oxygen (HO2) sensor heater current
voltage 11 to 14 V)

WIRING DIAGRAM
Refer to DTC P0136 (see page ES-136).

CONFIRMATION DRIVING PATTERN

These DTCs are detected when the engine idles for 110 seconds or more.

INSPECTION PROCEDURE
HINT:
Sensor 2 refers to the sensor mounted behind the Three-Way Catalytic Converter (TWC) and located far
from the engine assembly.
Read freeze frame data using the intelligent tester. Freeze frame data records the engine condition when
malfunctions are detected. When troubleshooting, freeze frame data can help determine if the vehicle was
moving or stationary, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other
data from the time the malfunction occurred.

1 INSPECT HEATED OXYGEN SENSOR (HEATER RESISTANCE)

(a) Disconnect the B19 Heated Oxygen (HO2) sensor

Component Side : connector.
(b) Measure the resistance of the HO2 sensor connector.
Standard resistance
+B HT1B Tester Connections Specified Conditions

Sensor 2
2 1
Front View 1 (HT1B) - 2 (+B) 11 to 16 Ω at 20°C (68°F)
4 3
1 (HT1B) - 4 (E2) 10 kΩ or higher
E2 OX1B
HO2 Sensor (c) Reconnect the HO2 sensor connector.
A115661E02
NG REPLACE HEATED OXYGEN SENSOR

OK
 ES–90 2AZ-FE ENGINE CONTROL SYSTEM – SFI SYSTEM

2 CHECK TERMINAL VOLTAGE (+B OF HO2 SENSOR)

(a) Disconnect the B19 HO2 sensor connector.

Wire Harness Side:
(b) Turn the ignition switch ON.
Heated Oxygen Sensor Connector
(c) Measure the voltage between the terminals of the B19
HO2 sensor connector and body ground.
+B Standard voltage
B19
Tester Connections Specified Conditions
Sensor 2 B19-2 (+B) - Body ground 9 to 14 V

Front View (d) Reconnect the HO2 sensor connector.

ES A072895E07
OK Go to step 5

NG

3 INSPECT INTEGRATION RELAY (EFI MAIN RELAY) (See page ES-84)

NG REPLACE INTEGRATION RELAY

OK
 2AZ-FE ENGINE CONTROL SYSTEM – SFI SYSTEM ES–91

4 CHECK HARNESS AND CONNECTOR (HO2 SENSOR - EFI RELAY)

(a) Check the EFI No. 1 fuse.

Wire Harness Side: (b) Disconnect the B19 HO2 sensor connector.
(c) Remove the integration relay from the engine room No. 1
relay block.
HO2 Sensor Connector
(d) Check the resistance.
Standard resistance (Check for open)
+B
Tester Connections Specified Conditions
B19
B19-2 (+B) - 1A-4 (Engine room No. 1
Below 1 Ω
R/B)

Standard resistance (Check for short) ES

Integration Relay
Tester Connections Specified Conditions
B19-2 (+B) or 1A-4 (Engine room No.
10 kΩ or higher
1A 1 R/B) - Body ground

(e) Reconnect the HO2 sensor connector.

(f) Reinstall the integration relay.
4 NG REPAIR OR REPLACE HARNESS OR
CONNECTOR
Engine Room
No. 1 Relay
Block

EFI No. 1

A127734E01

OK

CHECK ECM POWER SOURCE CIRCUIT

 ES–92 2AZ-FE ENGINE CONTROL SYSTEM – SFI SYSTEM

5 CHECK HARNESS AND CONNECTOR (HO2 SENSOR - ECM)

(a) Disconnect the B19 HO2 sensor connector.

Wire Harness Side:
(b) Disconnect the B30 ECM connector.
HO2 Sensor Connector (c) Measure the resistance.
Standard resistance (Check for open)
Tester Connections Specified Conditions
HT1B
B19 B19-1 (HT1B) - B30-47 (HT1B) Below 1 Ω

Standard resistance (Check for short)

Front View Tester Connections Specified Conditions

ES B19-1 (HT1B) or B30-47 (HT1B) -

Body ground
10 kΩ or higher

B30 ECM Connector (d) Reconnect the HO2 sensor connector.

HT1B (e) Reconnect the ECM connector.
NG REPAIR OR REPLACE HARNESS OR
CONNECTOR

Front View
A104950E05

OK

6 CHECK WHETHER DTC OUTPUT RECURS

(a) Connect the intelligent tester to the DLC3.

(b) Turn the ignition switch ON.
(c) Turn the tester ON.
(d) Clear DTCs (see page ES-35).
(e) Start the engine.
(f) Allow the engine to idle for 2 minutes or more.
(g) Select the following menu items: DIAGNOSIS /
ENHANCED OBD II / DTC INFO / CURRENT CODES.
(h) Read DTCs.
Result
Display (DTC Output) Proceed To
No output A
P0037, P0038 and/or P0141 B

B REPLACE ECM

CHECK FOR INTERMITTENT PROBLEMS