DTC P0171: System too Lean

System too Lean Circuit
• Refer to, DTC P0201 - P0204 Injector Circuit Malfunction.
CIRCUIT OPERATION
• Refer to, DTC P0201 - P0204 Injector Circuit Malfunction.
TECHNICAL DESCRIPTION
• If a malfunction occurs in the fuel system, the fuel trim value becomes too large.
• The ECM <M/T> or PCM <A/T> checks whether the fuel trim value is within a specified range.
DESCRIPTIONS OF MONITOR METHODS
• If an excessively lean state of air-fuel ratio is detected from the learned air-fuel ratio (long-term fuel trim) and the feedback air-fuel ratio (short-term fuel trim), a malfunction is determined to have occurred.
MONITOR EXECUTION
Continuous
MONITOR EXECUTION CONDITIONS (Other monitor and Sensor)

Other Monitor (There is no temporary DTC stored in memory for the item monitored below)
• Misfire monitor

Sensor (The sensor below is determined to be normal)
• Volume airflow sensor
• Barometric pressure sensor
• Intake air temperature sensor
• Engine coolant temperature sensor
• Throttle position sensor
DTC SET CONDITIONS Logic Flow Chart

DTC SET CONDITIONS

Check Conditions
Engine coolant temperature is lower than approximately 100°C (212°F) when the engine is started.
Intake air temperature is lower than 60°C (140°F) when the engine is started.
Under the closed loop air/fuel ratio control.
Engine coolant temperature is higher than 76°C (169°F).
Volume airflow sensor output frequency is 81 Hz or more.

Judgment Criteria
• Long-term fuel trim has continued to be higher than +12.5 percent for 5 seconds.
or
• Short-term fuel trim has continued to be higher than +10.0 percent for 5 seconds.

Check Conditions
• Engine coolant temperature is lower than approximately 100°C (212°F) when the engine is started.
• Intake air temperature is lower than 60°C (140°F) when the engine is started.
• Under the closed loop air/fuel ratio control.
• Engine coolant temperature is higher than 76°C (169°F).
• Volume airflow sensor output frequency is 81 Hz or less.

Judgment Criteria
• Long-term fuel trim has continued to be higher than +12.5 percent for 5 seconds.
or
• Short-term fuel trim has continued to be higher than +15 percent for 5 seconds.

Check Conditions
• Engine coolant temperature is higher than approximately 100°C (212°F) when the engine is started.
• Intake air temperature is higher than 60°C (140°F) when the engine is started.
• Under the closed loop air/fuel ratio control.
• Engine coolant temperature is higher than 76°C (169°F).
• Volume airflow sensor output frequency is 81 Hz or more.

Judgment Criteria
• Long-term fuel trim has continued to be higher than +12.5 percent for 5 seconds.
or
• Short-term fuel trim has continued to be higher than +20.0 percent for 5 seconds.

Check Conditions
• Engine coolant temperature is higher than approximately 100°C (212°F) when the engine is started.
• Intake air temperature is higher than 60°C (140°F) when the engine is started.
• Under the closed loop air/fuel ratio control.
• Engine coolant temperature is higher than 76°C (169°F).
• Volume airflow sensor output frequency is 81 Hz or less.

Judgment Criteria
• Long-term fuel trim has continued to be higher than +12.5 percent for 5 seconds.
or
• Short-term fuel trim has continued to be higher than +25.0 percent for 5 seconds.

Check Conditions
• Engine coolant temperature is higher than 76°C (169°F).
• Under the closed loop air/fuel ratio control.

Judgment Criteria
• Long-term fuel trim has continued to be +12.5 percent for 2 seconds.
or
• Short-term fuel trim has continued to be +25.0 percent for 2 seconds.
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 18 .
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:
• Volume airflow sensor failed.
• Injector failed.
• Incorrect fuel pressure.
• Air drawn in from gaps in gasket, seals, etc.
• Heated oxygen sensor failed.
• Engine coolant temperature sensor failed.
• Intake air temperature sensor failed.
• Barometric pressure sensor failed.
• Use of incorrect or contaminated fuel.
• ECM failed. <M/T>
• PCM failed. <A/T>