DTC P0131: Heated Oxygen Sensor Circuit Low Voltage (sensor 1)

CIRCUIT OPERATION
• A voltage corresponding to the oxygen concentration in the exhaust gas is sent to the ECM (terminal No. 76) <M/T> or PCM (terminal No. 71) <A/T> from the output terminal (terminal No. 4) of the heated oxygen sensor (front).
• Terminal No. 2 of the heated oxygen sensor (front) is grounded with ECM (terminal No. 92) <M/T> or PCM (terminal No. 57) <A/T>.
TECHNICAL DESCRIPTION
• The heated oxygen sensor (front) detects the concentration of oxygen in the exhaust gas; it converts those data to voltage, and inputs the resulting signals to the ECM <M/T> or PCM <A/T>.
• When the heated oxygen sensor (front) begins to deteriorate, the heated oxygen sensor signal response becomes poor.
• The ECM <M/T> or PCM <A/T> forcibly varies the air/fuel mixture to make it leaner and richer, and checks the response speed of the heated oxygen sensor (front). In addition, the ECM <M/T> or PCM <A/T> also checks for an open circuit in the heated oxygen sensor (front) output line.
DESCRIPTIONS OF MONITOR METHODS
• When the output of the heated oxygen sensor (front) is low, the air-fuel ratio is forcefully made richer. And this time, also the output of the heated oxygen sensor (front) is monitored. If the output is lower than the standard value, a short circuit (short circuit to ground) 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)
• Heated oxygen sensor heater (front) monitor
• Misfire monitor
• Fuel system monitor

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


Check Conditions
After 2 seconds or more pass from the time when the monitor determines normally for detecting an open circuit.
Heated oxygen sensor (front) signal voltage has continued to be 0.2 volt or lower.
Engine coolant temperature is higher than 76°C (169°F).
Volumetric efficiency is higher than 25 percent.
Volume airflow sensor output frequency is 81 Hz or more.
At least 20 seconds have passed since fuel shut off control was canceled.
After the ignition switch is turned ON, the changes in the output voltage of the heated oxygen sensor (front) is lower than 0.078 volt.
Monitoring time: 8 seconds.

Judgement Criterion
• Making the air/fuel ratio 15 percent for 8 seconds richer does not result in raising the heated oxygen sensor (front) output voltage beyond 0.2 volt.
OBD-II DRIVE CYCLE PATTERN
Refer to Diagnostic Function - OBD-II Drive Cycle - Pattern 11 .
TROUBLESHOOTING HINTS (The most likely causes for this code to be set are:
• Heated oxygen sensor (front) deteriorated.
• Short circuit in heated oxygen sensor (front) output line.
• ECM failed. <M/T>
• PCM failed. <A/T>