Possible causes

– Faulty Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1
– Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit fuse
– Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit open shorted to ground
– Heated Oxygen Sensor (H2OS) Bank 1 Sensor 1 circuit poor electrical connection
– Faulty Powertrain Control Module (PCM)

Tech notes

The code means that there is a problem with the heater element circuit of the heated oxygen sensor. The control module monitors how long it take the sensor to warm up and start sending an adequate signal. The code is triggered when the sensor is taking too long to warm up. Before replacing the sensor, check for the condition of the heated oxygen sensor fuse and connectors. If the sensor and connector are OK, replacing the O2 Sensor 1 usually takes care of the problem

When is the code detected?

If the Powertrain Control Module (PCM) detects that the process took too much time for the HO2S to enter into normal operating range, a DTC sets. The time the process takes the HO2S to reach operating temperature is based on the amount of air that flows into the engine.

Symptoms

– Engine Light ON (or Service Engine Soon Warning Light)
– Possible higher than usual fuel Consumption

P0135 HUMMER Description

The Powertrain Control Module (PCM) supplies a bias voltage (approximately 450 mV) on the Heated Oxygen Sensor (HO2S) signal high and low circuits. When you turn the ignition to the ON position, battery voltage is supplied to the HO2S heater. As the heater reaches the operating temperature, the HO2S voltage responds by changing from a bias voltage range to the normal operation. Typically, as the HO2S reaches the operating temperature, the HO2S voltage goes from a bias voltage to a voltage below 300 mV. Depending on the exhaust gas content, it is possible for the HO2S voltage to go above 450 mV.
The PCM runs the heater test only on a cold start (depends on the cumulative air flow) and only once an ignition cycle. When you start the engine the PCM monitors the HO2S voltage. When the HO2S voltage goes above or below the bias range threshold, the PCM determines how much time it took.

HUMMER Technical Service Bulletins for P0135 code