DTC P0171: System Too Lean (Bank 1)

OUTLINE OF DIAGNOSIS
Detect the malfunction of fuel system by the amount of main feedback control.

Fig 1: Fuel System Outline Diagram - System Too Lean
Courtesy of SUBARU OF AMERICA, INC.

Fuel system is diagnosed by comparing the target air fuel ratio calculated by ECM with the actual air fuel ratio measured by sensor.

ENABLE CONDITION

ENABLE CONDITIONS

Secondary Parameters	Enable Conditions
Fuel learning system	In operation
Engine coolant temperature	75 <--> 110°C (167 <--> 230°F)
Intake manifold pressure change during	"LESS THAN EQUAL TO" 1.9 kPa
50 msecs	(14.6 mmHg, 0.6 inHg)
Learning value of EVAP conc. during purge	"LESS THAN EQUAL TO" 1.0
Cumulative time of canister purge after engine start	20 seconds or more
Continuous period after canister purge starting	10 seconds or more
Intake air temperature	"GREATER THAN EQUAL TO" Map 3

Map 3

ENGINE SPEED AND MANIFOLD PRESSURE SPECIFICATIONS

Engine speed (rpm)	Idle	800	1200	1600	2000	2400	2800	3200	3600	4000	4400
Pressure kPa (mmHg, inHg)	ha	24.0 (180, 7.1)	27.2 (204, 8.0)	27.2 (204, 8.0)	28.3 (212, 8.3)	29.9 (224, 8.8)	32.5 (244, 9.6)	34.1 (256, 10.1)	34.1 (256, 10.1)	33.1 (248, 9.8)	33.1 (248, 9.8)

GENERAL DRIVING CYCLE
Perform the serial diagnoses at idling after engine warmed up or with a constant vehicle speed.

DIAGNOSTIC METHOD

Judge that the malfunction occurred in fuel system when the malfunction criteria are completed for 50 seconds or more by comparing the diagnostic value (fsobd) with threshold value.

Judgment Value

JUDGMENT VALUE

Malfunction Criteria	Threshold Value
fsobd = aflmd + lambda + kblrc	"GREATER THAN EQUAL TO" fsobdL1
See Map 2
aflmd = measured lambda	fsobdL1 = lean side threshold value of fsobd
lambda = short term fuel trim
kblrc = long term fuel trim

Map 2: Threshold value for fuel system malfunction criteria

FUEL SYSTEM MALFUNCTION THRESOLD VALUE

Amount of air (g/s)	0	3.2	6.4	9.6	12.8	16	19.2
fsobdL1 (%)	40	40	33.2	26.5	26.5	26.5	26.5

Time Needed for Diagnosis: 10 seconds x 5 times

Malfunction Indicator Light Illumination: Twice continuous driving cycles.

Judge OK when the malfunction criteria below are completed for 10 seconds or more.

Judgment Value

JUDGMENT VALUE

Malfunction Criteria	Threshold Value
fsobd = aflmd + lambda + kblrc	< 19%

DTC CLEAR CONDITION
- When the OK idling cycle is completed 40 times in a row
- When "Clear Memory" was performed
MALFUNCTION INDICATOR LIGHT CLEAR CONDITION
- When diagnosed OK with the similar driving for 3 driving cycles in a row
- When "Clear Memory" was performed
FAIL SAFE
None
ECM OPERATION AT DTC SETTING
Memorize the freeze frame data. (For test mode $02)