P0138

DTC P0138 HO2S Circuit High Voltage (Bank 1, Sensor 2) (Catalyst Motor):

Heated Oxygen Sensor (HO2S) (Bank 1, Sensor 2):

CIRCUIT DESCRIPTION
In order to control emissions of Hydrocarbons (HC), Carbon Monoxide (CO) and Oxides of Nitrogen (NOx), a three-way catalytic converter is used. The catalyst within the converter promotes a chemical reaction which oxidizes the HC and CO present in the exhaust gas, converting them into harmless water vapor and carbon dioxide. The catalyst also reduces NOx, converting it to nitrogen. The Vehicle Control Module-A (VCM-A) has the capability to monitor this process using Heated Oxygen Sensor (HO2S) 2. HO2S 2, located in the exhaust stream past the three-way catalytic converter, produces an output signal which indicates the oxygen storage capacity of the catalyst; this in turn indicates the catalyst's ability to convert exhaust emissions effectively. If the catalyst is functioning correctly, the HO2S 2 signal will be far less active than that produced by HO2S 1. If a problem exists which causes the VCM-A to detect high HO2S 2 signal voltage for an extended period of time, the VCM-A will set DTC P0138. DTC P0138 determines if the HO2S or circuit is shorted to high be checking for a rich condition during steady throttle.

DTC P0138 WILL SET WHEN
^ No Throttle Position (TP) sensor DTCs.
^ No Evaporative Emissions (EVAP) System DTCs.
^ No Intake Air Temperature (IAT) sensor DTCs.
^ No Manifold Absolute Pressure (MAP) DTCs.
^ No fuel trim DTCs.
^ No Exhaust Gas Recirculation (EGR) DTCs.
^ No Engine Coolant Temperature (ECT) sensor DTCs.
^ "Closed Loop."
^ Air/fuel ratio is between 14.5 and 14.8.
^ Throttle position is between 5% and 50%.
^ Above met for 5 seconds.
^ HO2S voltage is greater than 0.993 volt

ACTION TAKEN (VCM-A WILL DEFAULT TO)
The VCM-A will turn "ON" the Malfunction Indicator Lamp (MIL) (Service Engine Soon) after 2 consecutive failures.

DTC P0138 WILL CLEAR WHEN
The VCM-A will turn the MIL "OFF" after three consecutive trips without a fault condition present. A history DTC will be cleared if no fault conditions have been detected for forty warm-up cycles [coolant temperature has risen 22°C (40°F) from start-up coolant temperature and engine coolant temperature exceeds 71°C (160°F) that same ignition cycle] or the Tech 1 clearing feature has been used.

DTC CHART TEST DESCRIPTION
Number(s) below refer to circled number(s) on the diagnostic chart.
1. Determines if conditions necessary for setting DTC P0138 exist.
2. Disconnecting HO2S 2 and jumpering CKTs 1670 and 1671 to ground should cause the Tech 1 to display HO2S 2 voltage below 0.2 volt (200 mV). If the signal voltage is still high, the VCM-A is faulty.

DIAGNOSTIC AIDS
Check for:
^ Rich exhaust - If DTC P0131, P0132, or P0134 are present, diagnose these DTC(s) prior to DTC P0138. An overly rich exhaust may load the catalyst causing high HO2S 2 signal voltages.
^ False rich indication due to silicon contamination of HO2S 2. This will be indicated by DTC P0138 accompanied by a powdery white deposit on the sensor.
^ Faulty HO2S 2 (catalyst monitor) - If HO2S 2 is internally shorted the HO2S 2 voltage displayed on a Tech 1 will be over 1 volt. Try disconnecting the HO2S 2 and jumpering CKT 1671 to engine ground; if the displayed voltage goes from over 1000 mV to around 450 mV, replace HO2S 2.
^ Intermittent test - Observe HO2S 2 on Tech 1 while moving related connectors and wiring harness with key "ON." If the failure is induced, the HO2S 2 display will change. This may help to isolate the location of the malfunction.
^ Damaged harness - Inspect VCM-A harness for a short to voltage in CKT 1670.