GSIC - Global Service Information Center

### DTC P0171 System Too Lean (Fuel Trim)

### DTC P0172 System Too Rich (Fuel Trim)

## for Preparation Click here

## DESCRIPTION

The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim includes the short-term fuel trim and long-term fuel trim.
The short-term fuel trim is the short-term fuel compensation used to maintain the air-fuel ratio at its ideal theoretical value. The signal from the heated oxygen sensor indicates whether the air-fuel ratio is rich or lean compared to the ideal theoretical value, triggering a reduction in the fuel volume if the air-fuel ratio is rich, and an increase in the fuel volume if it is lean.
The long-term fuel trim is the overall fuel compensation in order to balance the short-term fuel trim for a continual deviation from the central value by individual engine differences, operating environment and age deterioration.
If both the short-term fuel trim and the long-term fuel trim are lean or rich beyond a standard level, it is detected as a malfunction in the SFI system. The ECM illuminates the MIL and a DTC is set.

DTC No. | DTC Detection Condition | Trouble Area

P0171

When air-fuel feedback is stable after warming up engine, fuel trim is considerably in error on lean side (2 trip detection logic)

1. Air induction system
2. Injector blockage
3. Mass air flow meter
4. Engine coolant temperature sensor
5. Fuel pressure
6. Gas leakage in exhaust system
7. Open or short in A/F sensor (sensor 1) circuit
8. A/F sensor (sensor 1)
9. A/F sensor heater (sensor 1)
10. A/F relay
11. MAIN relay
12. PCV value and hose
13. PCV hose connection
14. ECM

P0172

When air-fuel feedback is stable after warming up engine, fuel trim is considerably in error on rich side (2 trip detection logic)

1. Injector leakage or blockage
2. Mass air flow meter
3. Engine coolant temperature sensor
4. Ignition system
5. Fuel pressure
6. Gas leakage in exhaust system
7. Open or short in A/F sensor (sensor 1) circuit
8. A/F sensor (sensor 1)
9. A/F sensor heater
10. A/F relay
11. MAIN relay
12. ECM

HINT:

1. When DTC P0171 is set, the actual air-fuel ratio is lean. When DTC P0172 is set, the actual air-fuel ratio is rich.
2. If the vehicle runs out of fuel, the air-fuel ratio is lean and DTC P0171 may be set. The MIL then illuminates.
3. If the total of the short-term fuel trim value and long-term fuel trim value is within +- 34%, the system is functioning normally.

## MONITOR DESCRIPTION

Under closed-loop fuel control, fuel injection amounts that deviate from the ECM's estimated fuel amount will cause a change in the long-term fuel trim compensation value. This long-term fuel amount will cause a change in the long-term fuel trim compensation value. This long-term fuel trim is adjusted when there are persistent deviations in the short-term fuel trim values. And, the deviation from the simulated fuel injection amount by the ECM affects the smoothed fuel trim learning value. The smoothed fuel trim learning value is the combination of smoothed short term fuel trim (fuel feedback compensation value) and smoothed long-term fuel trim (learning value of the air-fuel ratio). When the smoothed fuel trim learning value exceeds the DTC threshold, the ECM interprets this as a fault in the fuel system and sets a DTC.
Example:
When the smoothed fuel trim leaning value is more than + 34% or less than - 34%, the ECM interprets this as a failure in the fuel system.

## WIRING DIAGRAM

Refer to DTC P2195 (Click here).

## INSPECTION PROCEDURE

HINT:

  Intelligent tester only:

  Malfunctioning areas can be found by performing the Active Test / A/F Control operation. The A/F Control operation can determine if the A/F sensor, heated oxygen sensor or other potential trouble areas are malfunctioning or not.

(a) Perform Active Test using the intelligent tester.

HINT:

  The A/F Control operation lowers the injection volume by 12.5% or increases the injection volume by 25%.

1. Connect the intelligent tester to the DLC3.
2. Start the engine and turn the intelligent tester ON.
3. Warm up the engine by running the engine at 2,500 rpm for approximately 90 seconds.
4. On the intelligent tester, select the following menu items: Powertrain / Engine and ECT / Active Test / A/F Control.
5. Select the following monitor items: AFS B1 S1 and O2S B1 S2.
6. Perform the A/F Control operation with the engine in an idling condition (press the right or left button).

Standard:

  Tester Display (Sensor) | Injection Volume | Status | Voltage
  AFS B1 S1 (A/F) | + 25% | Rich | Less than 3.0 V
  AFS B1 S1 (A/F) | - 12.5% | Lean | More than 3.35 V
  O2S B1 S1 (HO2) | + 25% | Rich | More than 0.55 V
  O2S B1 S1 (HO2) | - 12.5% | Lean | Less than 0.4 V

NOTICE:

  The A/F sensor output has a few seconds of delay and the heated oxygen sensor output has about 20 seconds of delay at maximum.

Case | A/F Sensor (Sensor 1) Output Voltage | HO2 Sensor (Sensor 2) Output Voltage | Main Suspected Trouble Areas
1 | Injection Volume +25% -12.5% |  | Injection Volume +25% -12.5% |  | -
Output Voltage More than 3.35 V Less than 3.0 V |  | Output Voltage More than 0.55 V Less than 0.4 V |

2

Injection Volume
+25%
-12.5%

Injection Volume
+25%
-12.5%

1. A/F sensor
2. A/F sensor heater
3. A/F sensor circuit

Output Voltage Almost no reaction |  | Output Voltage More than 0.55 V Less than 0.4 V |

3

Injection Volume
+25%
-12.5%

Injection Volume
+25%
-12.5%

1. HO2 sensor
2. HO2 sensor heater
3. HO2 sensor circuit

Output Voltage More than 3.35 V Less than 3.0 V |  | Output Voltage Almost no reaction |

4

Injection volume
+25%
-12.5%

Injection Volume
+25%
-12.5%

1. Injector
2. Fuel pressure
3. Gas leakage from exhaust system
   (Air-fuel ratio extremely lean or rich)

Output Voltage Almost no reaction |  | Output Voltage Almost no reaction |

1. The following A/F Control procedure enables the technician to check and graph the output voltage of both A/F sensor and heated oxygen sensor.
   To display the graph, select the following menu items on the tester: View / Line graph.

HINT:

1. Read freeze frame data using the intelligent tester. Freeze frame data records the engine conditions when malfunctions are detects. 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.
2. A low A/F sensor voltage could be caused by a rich air-fuel mixture. Check the conditions that would cause the engine to run with the rich air-fuel mixture.
3. A high A/F sensor voltage could be caused by a lean air-fuel mixture. Check the conditions that would cause the engine to run with the lean air-fuel mixture.

1.CHECK ANY OTHER DTCS OUTPUT (IN ADDITION TO DTC P0171 OR P0172)

1. Connect the intelligent tester to the DLC3.

1. Turn the ignition switch ON and turn the tester ON.

1. Select the following menu items: Powertrain / Engine and ECT / DTC.

1. Read DTCs.

   Result:

     Display (DTC Output) | Proceed to
     P0171 or P0172 | A
     P0171 or P0172 and other DTCs | B

   HINT:
     If any DTCs other than P0171 or P0172 are output, troubleshoot those DTCs first.

| B |

GO TO RELEVANT DTC CHART

A |

2.CHECK PCV HOSE

OK:
  PCV hose is connected correctly, and is not damaged.

| NG |

REPAIR OR REPLACE PCV HOSE

OK |

3.CHECK AIR INDUCTION SYSTEM

1. Check the air induction system for vacuum leakage.

   OK:
     No leakage from air induction system.

| NG |

REPAIR OR REPLACE AIR INDUCTION SYSTEM

OK |

4.PERFORM ACTIVE TEST (A/F CONTROL)

1. Connect the intelligent tester to the DLC3.

1. Start the engine and turn the tester ON.

1. Warm up the engine at an engine speed of 2,500 rpm for approximately 90 seconds.

1. On the tester, select the following menu items: Powertrain / Engine and ECT / Active Test / A/F Control.

1. Perform the A/F Control operation with the engine in an idling condition (press the right or left button to change the fuel injection volume).

1. Monitor the output voltage of A/F and HO2 sensors (AFS B1 S1 and O2S B1 S2) displayed on the tester.

   HINT:
   1. The A/F Control operation decreases the fuel injection volume by 12.5% or increases the injection volume by 25%.
   2. Each sensor reacts in accordance with increases and decreases in the fuel injection volume.

   Standard:

     Tester Display (Sensor) | Injection Volume | Status | Voltage
     AFS B1 S1 (A/F) | + 25% | Rich | Less than 3.0 V
     AFS B1 S1 (A/F) | - 12.5% | Lean | More than 3.35 V
     O2S B1 S1 (HO2) | + 25% | Rich | More than 0.55 V
     O2S B1 S1 (HO2) | - 12.5% | Lean | Less than 0.4 V

   Result:
   Status AFS B1 S1 | Status O2S B1 S2 | A/F Condition and A/F Sensor Condition | Misfire | Suspected Trouble Areas | Proceed to
   Lean / Rich | Lean / Rich | Normal | - | - | C

   Lean

   Lean

   Actual air-fuel ratio lean

   May occur

   1. PCV valve and hose
   2. PCV hose connections
   3. Injector blockage
   4. Gas leakage from exhaust system
   5. Air induction system
   6. Fuel pressure
   7. Mass Air Flow (MAF) meter
   8. Engine coolant Temperature (ECT) sensor

   A

   Rich

   Rich

   Actual air-fuel ratio rich

   -

   1. Injector leakage or blockage
   2. Gas leakage from exhaust system
   3. Ignition system
   4. Fuel pressure
   5. MAF meter
   6. ECT sensor

   A

   Lean

   Lean / Rich

   A/F sensor malfunction

   -

   1. A/F sensor

   B

   Rich

   Lean / Rich

   A/F sensor malfunction

   -

   1. A/F sensor

   B

   1. Lean: During A/F Control, the A/F sensor output voltage (AFS) is consistently more than 3.35 V, and the HO2 sensor output voltage (O2S) is consistently less than 0.4 V.
   2. Rich: During A/F Control, the AFS is consistently less than 3.0 V, and the O2S is consistently more than 0.55 V.

| B |

Go to step 11

| C |

Go to step 15

A |

5.READ VALUE OF DATA LIST (COOLANT TEMP)

1. Connect the intelligent tester to the DLC3.

1. Turn the ignition switch ON and turn the tester ON.

1. Select the following menu items: Powertrain / Engine and ECT / Data List / Coolant Temp.

1. Read the Coolant Temp value twice, when the engine is cold and also when warmed up.

   Standard:
     With cold engine: Same as ambient air temperature.
     With warm engine: Between 75 and 95°C (167 and 203°F)

| NG |

REPLACE ENGINE COOLANT TEMPERATURE SENSOR

OK |

6.READ VALUE OF DATA LIST (MAF)

1. Connect the intelligent tester to the DLC3.

1. Turn the ignition switch ON and turn the tester ON.

1. Select the following menu items: Powertrain / Engine and ECT / Data List / MAF and Coolant Temp.

1. Allow the engine to idle until the Coolant Temp value reaches 75°C (167°F) or more.

1. Read the MAF with the engine in an idling condition and at an engine speed of 2,500 rpm.

   Standard:
     MAF while engine idling: Between 1 and 4 g/s (shift position: N; A/C: OFF).
     MAF at engine speed of 2,500 rpm: Between 4 and 15 g/s (shift position: N; A/C: OFF).

| NG |

REPLACE MASS AIR FLOW METER

OK |

7.CHECK FUEL PRESSURE

1. Check the fuel pressure (Click here).

| NG |

REPAIR OR REPLACE FUEL SYSTEM

OK |

8.CHECK FOR EXHAUST GAS LEAKAGE

OK:
  No gas leakage.

| NG |

REPAIR OR REPLACE EXHAUST SYSTEM

OK |

9.CHECK FOR SPARK AND IGNITION

HINT:

  If the spark plugs or ignition system malfunctions, engine misfire may occur. The misfire count can be read using the intelligent tester. Select the following menu items: Powertrain / Engine and ECT / Data List / CYL #1 (to CYL #4).

| NG |

REPAIR OR REPLACE IGNITION SYSTEM

OK |

10.INSPECT FUEL INJECTOR ASSEMBLY (INJECTION AND VOLUME)

HINT:

  If the injectors malfunction, engine misfire may occur. The misfire count can be read using the intelligent tester. Select the following menu items: Powertrain / Engine and ECT / Data List / CYL #1 (to CYL #4).

| NG |

REPLACE FUEL INJECTOR ASSEMBLY

OK |

11.INSPECT AIR FUEL RATIO SENSOR (HEATER RESISTANCE)

1. Disconnect the A18 A/F sensor connector.

1. Measure the resistance between the terminals of the A/F sensor connector.

   Standard resistance:

     Tester Connection | Condition | Specified Condition
     1 (HT) -2 (+B) | 20°C (68°F) | 1.8 to 3.4 Ω
     1 (HT) -4 (AF-) | Always | 10 kΩ or higher

| NG |

REPLACE AIR FUEL RATIO SENSOR

OK |

12.INSPECT RELAY (MARKING: A/F)

1. Remove the relay from the engine room relay block.

1. Check the resistance of the relay.

   Standard resistance:

     Tester Connection | Specified Condition
     3 - 5 | 10 kΩ or higher
     3 - 5 | Below 1 Ω (when battery voltage is applied to terminals 1 and 2)

| NG |

REPLACE RELAY

OK |

13.CHECK WIRE HARNESS (A/F SENSOR - ECM)

1. Disconnect the A18 sensor connector.

1. Disconnect the E12 ECM connector.

1. Measure the resistance of the wire harness side connectors.

   Standard resistance:

     Tester Connection | Specified Condition
     A18-1 (HT) - E12-1 (HA1A) | Below 1 Ω
     A18-3 (AF+) - E12-21 (A1A+) | Below 1 Ω
     A18-4 (AF-) - E12-31 (A1A-) | Below 1 Ω
     A18-1 (HT) or E12-1 (HA1A) - Body ground | 10 kΩ or higher
     A18-3 (AF+) or E12-21 (A1A+) - Body ground | 10 kΩ or higher
     A18-4 (AF-) or E12-31 (A1A-) - Body ground | 10 kΩ or higher

| NG |

REPAIR OR REPLACE HARNESS AND CONNECTOR

OK |

14.REPLACE AIR FUEL RATIO SENSOR

NEXT |

15.PERFORM CONFIRMATION DRIVING PATTERN

(a) Connect the intelligent tester to the DLC3.
(b) Turn the ignition switch ON and turn the tester ON.
(c) Clear DTCs.
(d) Change the ECM from normal mode to check mode using the tester.
(e) Start the engine and warm it up with all the accessories turned OFF.
(f) Drive the vehicle at between 60 and 120 km/h (38 and 75 mph) and at an engine speed of between 1,400 and 3,200 rpm for 3 to 5 minutes.

HINT:

  If the system is still malfunctioning, the MIL will be illuminated during step (f).

NOTICE:

  If the conditions in this test are not strictly followed, no malfunction will be detected.

NEXT |

16.CHECK IF DTC OUTPUT RECURS (DTC P0171 OR P0172)

1. On the intelligent tester, select the following menu items: Powertrain / Engine and ECT / DTC.

1. Read DTCs.

   Result:

     Display (DTC Output) | Proceed to
     No output | A
     P0171 or P0172 | B

| B |

Go to step 5

A |

END