P0134: OBD-II code P0134 means the upstream bank 1 O2 sensor signal is flatlined — the PCM sees no switching activity, so it can't run closed-loop fuel control.

Symptoms

• Check engine light is on; engine may run rich because the PCM defaults to open-loop fuel maps.
• Poor fuel economy from constant open-loop enrichment while the sensor is inactive.
• Rough idle and mild hesitation from the lack of live trim data from the upstream sensor.

Common causes

• A failed O2 sensor with an internally open element producing no signal.
• An unplugged or broken sensor wire, or a corroded connector at the sensor.
• A failed heater causing the sensor to stay cold and never switch (usually with a companion P0030-P0032 code).

What to do?

Read live O2 voltage — it should switch once the sensor warms up. If flatlined at 0.45 V (bias reference), the sensor is open or unplugged. Check the connector and wiring. Replace the sensor if disconnected wiring isn't the cause.