Geely Emgrand X7. Manual part — 66

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

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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

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