2.12.7.26 DTC P0171 P0172 P1167 P1171 P2187 P2188 

1. DTC 

description: 

DTC  P0171 

Mixture Too Thin 

 

DTC  P0172 

Mixture Too Thick 

 

DTC  P1167 

Pre-catalytic Oxygen Indicating Mixture Too Thick During Deceleration 

 

DTC  P1171 

Pre-Catalytic Oxygen Indicating Mixture Too Thin During Acceleration 

 

DTC  P2187 

Mixture Too Thin When Idling 

 

DTC  P2188 

Mixture Too Thick When Idling 

Engine Control Module (ECM) controls the close-loop Air-fuel ratio Measure system that 
achieves optimal combination of performance, fuel economy and emission control. In the 
close-loop mode, the engine control module monitors heated oxygen sensor (HO2S) signal voltage 
and adjusts fuel supply according to the signal. Changes in fuel supply will change the value of 
long-term and short-term fuel supply adjustment. Short-term fuel supply adjustment will respond 
to heated type oxygen sensor signal voltage and rapidly change. These changes will fine tune the 
fuel supply. Long-term fuel supply adjustment will respond to the trend in shortterm fuel supply 
adjustment. Long-term fuel adjustment adjusts the fuel supply in order to return to the center of 
the short-term fuel adjustment value and controls the short-term fuel adjustment. The ideal fuel 
adjustment value is around 0%. A positive value indicates that engine control module is increasing 
fuel supply to compensate the lean Air-fuel mixture. A negative value indicates that engine control 
module is decreasing fuel supply to compensate the thick Air-Fuel mixture. 

2.  Conditions For Setting DTC and The Fault Location: 

DTC 
Code 

DTC Detection Strategy

Set(Control Strategy) 
Conditions 

Fault Locations 

P0171 

P0172 

P1167 

P1171 

P2187 

1. Fuel  Adjustment 

Value Higher Than 
Maximum Limit 

2. Fuel  Adjustment 

Value Lower Than 
Minimum Limit 

3. Fuel  Adjustment 

Value Higher Than 

1.  Engine slows down and 

enters (DFCO) working 
condition. 

2. ECM detected oxygen 

sensor signal voltage is 
higher than 0.55 V. 

3. Engine 

enters 

power-enriched (PE) 

1.  Fuel Injectors 

2.  Canister 

3.  MAP 

4.  TPS 

5.  HO2S（front）

783

P2188 Maximum 

Limit 

(Low-Load Zone) 

4. Fuel  Adjustment 

Value Lower Than 
Minimum Limit 
(Low-Load Zone) 

working condition. 

4. ECM detected oxygen 

sensor signal voltage is 
lower than 0.35V. 

5.  Duration longer than 12s. 

3. Circuit 

figure 

Refer to 2.12.6 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. 

Step 1

 

Inspect there are no other control system DTC codes output. 

(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. 

Are there DTC codes other than P0171, P0172, P1167, P1171, P2187, P2188? 

 

Step 2

 

Read the intake manifold absolute pressure sensor data. 

(a)  Turn ignition switch to OFF position, connect fault diagnosis tester. 

(b)  Start vehicle . 

(c)  Read the intake manifold absolute pressure sensor data. 

(d)  Read the fault diagnosis tester for atmospheric pressure value, and compare it with table 

2.12.1.3 Altitude and Atmospheric Pressure Correlation. 

Is fault diagnosis tester atmospheric pressure reading normal? 

 

Step 3

 

Read the throttle position sensor data stream. 

(a) Start 

the 

vehicle. 

(b)  Warm up the engine with normal idle speed and throttle opening is less than 10%. 

No 

Refer to 2.12.7.18 DTC P0107 P0108 

Yes 

Yes 

Refer to 2.12.6.7 DTC Code Section Index 
 

No 

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(c)  Use fault diagnosis tester to read throttle position sensor data. 

Is throttle position sensor data normal? 

 

4 

Read the front oxygen sensor data stream. 

(a) Start 

the 

vehicle. 

(b)  Warm up the engine with normal idle speed. 

(c)  Read the front oxygen sensor data stream with a fault diagnosis tester. 

Front oxygen sensor standard value: 0.2-0.8 V 

Is front oxygen sensor data normal? 

 

Step 5

 

Observe the long-term fuel adjustment parameters. 

(a) Start 

the 

vehicle. 

(b)  Warm up the engine. 

(c)  Observe the long-term fuel adjustment parameter with a fault diagnosis tester. 

Is the long-term fuel adjustment parameter normal? 

 

Step 6

 

Inspect engine system and its components. 

(a)  Turn the ignition switch to OFF position. 

(b)  Inspect the vacuum hose crack, kink or connections. 

(c)  Inspect the intake manifold, throttle body and fuel injector vacuum leakage. 

(d)  Inspect the crankshaft ventilation system leakage. 

(e)  Inspect Fuel Contamination. 

(f)  Inspect the working station of fuel system when the air-fuel ratio is too lean. 

(g) Inspect injector nozzle spray fuel too thin. 

(h) Inspect the fuel system working at Air-Fuel ratio too thick. 

(i) Inspect injector spray fuel too thick. 

Yes 

The system is normal. 

No 

No 

Refer to 2.12.7.22 DTC P0131 P0132 
P0133 P0134. 

Yes 

No 

Refer to 2.12.7.21 DTC P0122 P0123 

Yes 

785

(j) Inspect intake manifold collapse or obstruction. 

(k) Inspect whether there is excessive fuel in the crankcase. 

(l) Inspect evaporative emission control systems working condition. 

(m) Inspect working condition of other fault lamps in the instrument. 

Is engine System normal? 

 

Step 7

 

Repair engine system and its components. 

 

Step 8

 

The system is normal. 

5. Maintenance 

guide 

: 

Refer to 2.12.8.5 “Replacement of Fuel Injectors” to replace the fuel injectors 

Refer to “2.4.6.3 Replacement of Canister Solenoid Valve” to replace the canister solenoid valve. 

Next 

Yes 

The system is normal. 

No 

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