2.12.7.24 DTC P0137 P0138 P0140 

1. DTC 

description: 

DTC  P0137 

Rear Oxygen Sensor Circuit Short to Low Voltage 

 

DTC  P0138 

Rear Oxygen Sensor Circuit Short to High Voltage 

 

DTC  P0140 

Rear Oxygen Sensor Circuit Open 

After the vehicle started, ECM works in open-loop mode, in which ECM ignores the heated type 
oxygen sensor signal voltage when calculating the Air-Fuel ratio. The control module provides 
approximately 450mV reference voltage to the heated oxygen sensor. When the engine starts 
running, the heated type oxygen sensor will be heated and begin to generate a 0-0.1V voltage. 
This voltage fluctuates. Once the control module detects that the heated oxygen sensor voltage 
exceeds a threshold voltage, it will immediately enter into the closed-loop mode. Control module 
determines the Air-fuel ratio using the received heated oxygen sensor voltage signal. If the heated 
type oxygen sensor voltage increases to above the reference voltage (tend to 1 V), the mixture is 
too thick. If the heated oxygen sensor voltage decreases to below the reference voltage (tend to 
0mV), it indicates that the air fuel mixture is too thin. 

• 

ECM provides a signal through ECM harness connector EN01 terminal No.48 to rear oxygen 
sensor wiring harness connector EN03 terminal B. 

• 

ECM provides an internal low reference voltage through ECM harness connector EM01 
terminal No.73 to front oxygen sensor wiring harness connector EM03 terminal No.A. 

2.  Conditions For Setting DTC and The Fault Location: 

DTC 
Code 

DTC Detection 
Strategy 

Conditions For Setting The DTC 
(Control Strategy) 

Fault 
Locations 

P0137 

Short to Ground When 
Cold 

1.  Engine Running Longer than 60s. 

2.  Water temperature is more than 

70 (158 ºF)

℃

 

3.  Rear Oxygen Sensor Short to 

Ground 

4.  Duration Longer Than 25s. 

P0138 

Post-catalytic Oxygen 
Sensor Voltage Too 
High 

1.  Engine Running Longer than 60s. 

2.  Water temperature is more than 

70 (158 ºF)

℃

 

3.  Rear oxygen sensor signal is 

terminated at the power supply. 

4.  Duration Longer Than 13s. 

P0140 

1. Oxygen  Sensor 1.  Engine Running Longer than 60s. 

1. Sensor 

Circuit 

2. Sensor 

3. ECM. 

771

Signal Circuit 
Open 

2. Oxygen  Sensor 

With High 
Resistance at High 
Temperature 

2.  Water temperature is more than 

70 (158 ºF)

℃

 

3. Rear oxygen sensor signal 

exceeds a reasonable range. 

4.  Duration Longer Than 165s. 

3. Circuit 

sketch 

NL02-2065b

BAT

15

A

E

F2

1

86

87

30

85

发动机主继电器

D

EN03

C

EN03

B

EN03

A

EN03

后氧传感器

23

EN01 O

2

BHTR

48

EN01 O

2

HI

73

EN01

44

EN01

V5RTN3

MPR

ECM

 

4. Diagnostic 

Steps: 

Step 1

 

Connect a fault diagnosis tester. 

 

Step 2

 

Start engine and turn on the fault diagnosis tester. 

 

Step 3

 

(d)  Select on the fault diagnosis tester: Engine / Read Data Flow / Group 1 Oxygen 
Sensor Voltage 2 (Rear Oxygen Sensor). 

 

Step 4

 

Observe oxygen sensor output voltage, the data should be within 0.16-0.7V 
unchanged. 

Next 

Next 

Next 

Engine 
main relay 

Rear oxygen sensor

772

 

Step 5

 

Carry out the oxygen sensor signal test. 

(a)  If the voltage data is consistently lower than 0.45 V (mixture too thin), carry out the 

following inspection steps: 

z  Spray proper amount of propane gas into the intake. 

z  Check whether the rear oxygen sensor voltage data has a significant change, as it will cause 

signal voltage to rise rapidly. 

(b)  If the voltage data is consistently higher than 0.45 V (mixture too thick), carry out the 

following inspection steps: 

z  Put the transmission gear into neutral. 

z  Apply hand brake. 

z  Press the accelerator pedal so the engine speed suddenly increases to 4,000 rpm and then 

quickly release the accelerator pedal. 

z  Repeat the previous steps more than 3 times. 

z  Check whether the rear oxygen sensor voltage data has a significant change, as it will cause 

signal voltage to decrease rapidly. 

During the above test, the oxygen sensor signal voltage should have a significant change. 

Is voltage changed significantly? 

 

(a)  Connect fault diagnosis tester to the vehicle diagnostic interface. 

(b)  Turn ignition switch to ON position. 

(c)  Press the fault diagnosis tester power button. 

(d)  Select the following menu items: Engine/Read DTC codes. 

(e)  Read DTC codes. 

DTC Codes Shown 

To Step 

DTC except for P0137,P0138,P0140 

No 

P0137、P0138、P0140 Yes 

Step 6

 

Inspect there are no other control system DTC codes output. 

Yes 

Inspect the cause for engine air-fuel ratio too lean/too rich. 
Refer to 2.12.7.4 Fault Symptom Table. 

No 

Yes 

Intermittent fault refers to 2.12.7.3 List of 
Intermittent Fault. 

No 

773

 

Step 7

 

Inspect the sealing of exhaust system. 

(a)  Inspect whether the three-way catalytic converter looks good (signs of excessive heat and 

gasket missing, etc.). 

(b)  Inspect whether the exhaust pipe is intact and whether gasket is intact. 

 

Step 8

 

Measure rear oxygen sensor signal circuit. 

 

(a)  Rotated ignition switch to OFF 

position . 

(b) Disconnect the rear oxygen 

sensor harness connector EN03. 

(c)  Rotated ignition switch to ON 

position . 

(d) Measure voltage between rear 

oxygen sensor wiring harness 
connector EN03 terminal B and a 
reliable ground. 

Standard Voltage: 0.35-0.5V 

(e)  Connect the rear oxygen sensor 

harness connector EN03. 

NL02-2066b

A

B

C

D

EN03 后氧传感器线束连接器

B

Is voltage the specified value? 

 

Step 9

 

Measure the rear oxygen sensor ground circuit. 

 

No 

Go to step 11 

Yes 

No 

Replace the damaged components. Go to 
step 15. 

No 

Refer to 2.12.7.14 DTC Chapter Index. 

Yes 

EN03 rear oxygen sensor harness connector 

774