2.2.7.38 DTC P0420 

1. DTC 

description: 

ECM uses two oxygen sensors (front oxygen sensor and rear oxygen sensor) installed before and 
after the three- way catalytic converters to monitor the conversion efficiency of the three-way 
catalytic converter (TWC). ECM uses front oxygen sensor for Air-fuel ratio close-loop control and 
monitors oxygen content in the exhaust gas not purified by TWC. The Post-catalytic oxygen 
sensor sends voltage signal to ECM indicating the oxygen content in the exhaust gas purified by 
the TWC. ECM compares signals from the two sensors to determine whether the TWC is currently 
under normal working condition. If the calculated TWC conversion efficiency is too low, the fault 
lamp will be lit and the DTC code will be set. 

2.  Conditions For Setting DTC and The Fault Location: 

DTC Code 

DTC 
Detection 
Strategy 

Conditions For Setting The DTC 
(Control Strategy) 

Fault Locations 

P0420 

Hardware 
Circuit 
Inspection 

Coolant temperature is higher than 
70°C (158 °F), and the fuel system is in 
close-loop mode. After driving some 
distance and stop the vehicle, run the 
engine at idle, the system compares the 
front and rear oxygen sensor signals to 
calculate three-way catalyst converter 
oxygen storing time. When the oxygen 
storing time is less than the threshold, 
the system reports a fault. 

1. Front  Oxygen 

Sensor 

2. Rear  Oxygen 

Sensor 

3. Three-way 

Catalytic 
Converter. 

4. Exhaust 

Leak. 

3. Circuit 

sketch 

Refer to 2.2.6 Electrical Schematic Diagram. 

4. Diagnostic 

Steps: 

Note: Before carrying out this diagnosis step, observe the data list on fault diagnosis tester and 
analyze the accuracy of the data, as these will help with quick diagnosis. 

1 

Inspect whether there are control system DTC codes other than DTC P0420. 

A.  Connect fault diagnosis tester to the vehicle diagnostic interface. 

B.  Rotated ignition switch to ON position. 

C.  Press the power button of fault diagnosis tester. 

D.  Select the following menu items: Engine/Read DTC codes. 

Read DTC code 

Results 

DTC Codes Shown 

To Step 

DTC P0420 

Yes 

DTC code other than P0420 

No 

DTC  P0420 

Three-Way Catalytic Converter Conversion Efficiency Low 

263

 

2 

Start engine and turn on the fault diagnosis tester. 

 

 

4 

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

Group 1 Oxygen Sensor Voltage 2 (Rear oxygen sensor) 

 

GC02-0330b

0V

0V

0V

0V

1V

1V

1V

1V

 

Whether front oxygen sensor and rear oxygen sensor signal voltage is matching Normal TWC in 
the figure? 

 

3 

Keep the engine speed at 2,500 rpm for more than 2 min to warm up the engine, until 
the engine coolant temperature reaches 80  (176 

℃

°F). 

5 

Observe front oxygen sensor and rear oxygen sensor output voltages. 

Yes 

Refer to 2.2.7.3 Intermittent Fault 
Inspection for intermittent fault. 

No 

Next 

Next 

Next 

No 

Refer to 2.2.7.14 DTC Chapter Index. 

Yes 

Normal TWC 

Front oxygen 

sensor 

Rear oxygen 

sensor 

Deteriorated TWC 

264

Serious Warning: Propane gas is flammable gas. It is strictly prohibited to operate propane 
gas near a fire, otherwise it will cause a fire. 

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  Inspect whether the sensor voltage data is changed significantly, as the signal voltage will 

increase rapidly. 

Front Oxygen Sensor Signal 
Voltage 

Rear Oxygen Sensor Signal 
Voltage 

To Step 

Obvious Change 

No Change 

A 

No Change 

Obvious Change 

B 

Obvious Change 

No Change 

C 

No Change 

No Change 

D 

 

 

8 

Refer to 2.7.6.3 “Replacement of Three-way Catalytic Converter” to replace the 
three-way catalytic converter. 

 

9 

Inspect whether there is exhaust leakage. 

6 

Carry out the oxygen sensor signal test. 

7 

Inspect whether there is exhaust leakage. 

Next 

Go to step 11 

Yes 

Repair the faulty part. Go to step 11 

No 

B 

Inspect the cause for engine air-fuel 
ratio too lean/too rich. Refer to 2.2.7.4 
Fault Symptom Table. 

A 

C 

D 

Refer to 2.4.7.2 “Replacement of 
Front Oxygen Sensor” to replace the 
front oxygen sensor. Go to Step 11. 

Go to step 9 

265

 

10 

Replace the rear oxygen sensor. Refer to 2.4.6.1 Replacement of Rear Oxygen Sensor.

 

11 

Use a fault diagnosis tester to confirm whether the DTC Code is stored again. 

A.  Connect fault diagnosis tester to the data link connector. 

B.  Rotated ignition switch to ON position. 

C.  Clear DTC code. 

D.  Start and run the engine at idle speed to warm up the engine for at least 5min. 

E.  Road test the vehicle for at least 10min. 

F.  Read control system DTC code again to confirm that the system has no DTC code. 

 

12 Troubleshooting 

No 

Refer to 2.2.7.3 Intermittent Fault 
Inspection for intermittent fault. 

Yes 

Next 

Yes 

Repair the faulty part. Go to step 11 

No 

266