Possible causes

– Faulty Rear Heated Oxygen Sensor Bank 1
– Rear Heated Oxygen Sensor Bank 1 harness is open or shorted
– Rear Heated Oxygen Sensor Bank 1 circuit poor electrical connection
– Intake air leaks
– Exhaust gas leaks
– Inappropriate fuel pressure
– Faulty fuel injectors

Tech notes

Replacing the Rear Heated Oxygen Sensor Bank 1 usually takes care of the problem.

When is the code detected?

The Powertrain Control Module (PCM) monitors the HO2S voltage for being fixed above a predetermined voltage. If the PCM detects the voltage is above a predetermined voltage, the P0138 code will set.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– High Fuel Consumption
– Excessive Smoke from Exhaust

P0138 CADILLAC Description

The Powertrain Control Module (PCM) supplies a voltage of about 450 mV between the HO2S high and low signal circuits. The oxygen sensor varies the voltage over a range from about 1,000 mV when the exhaust is rich, down through about 10 mV when the exhaust is lean.
The PCM monitors and stores the heated oxygen sensor (HO2S) voltage information. The PCM evaluates the HO2S voltage samples in order to determine the amount of time the HO2S voltage was out of range. The PCM compares the stored HO2S voltage samples taken within each sample period and determines if majority of the samples are out of the operating range.

CADILLAC Technical Service Bulletins for P0138 code