P1133
HO2S Insufficient Switching Bank 1 Sensor 1
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The P1133 code is a GM-specific diagnostic trouble code indicating that the upstream oxygen sensor on Bank 1 (the side of the engine with cylinder #1) is not switching between rich and lean readings frequently enough. The PCM monitors O2 sensor activity for about 100 seconds and counts rich-to-lean and lean-to-rich transitions. If there aren't enough transitions, it sets this code.
This code is often caused by a worn-out or contaminated oxygen sensor, but vacuum leaks and exhaust leaks near the sensor can also be responsible. The sensor may appear to work — it just isn't responding quickly enough to changes in the air-fuel mixture. This is different from a completely dead sensor (which would set a different code).
While the vehicle is generally safe to drive with this code, it can lead to decreased fuel economy and elevated emissions. If left unaddressed, the catalytic converter may be stressed by improper air-fuel ratios over time. Most DIYers can replace an upstream O2 sensor with basic tools in about 30 minutes, making this one of the more affordable and straightforward repairs.
Severity
Symptoms
- •Check engine light is on
- •Slight decrease in fuel economy
- •Engine may idle roughly or hesitate during acceleration
- •Occasional black smoke from exhaust
- •Vehicle may feel sluggish under load
Likely Causes
The heated oxygen sensor itself may have degraded over time, contaminated by oil or coolant, or simply reached end of life, causing it to not switch between rich and lean fast enough.
An unmetered air leak throws off the air-fuel ratio, preventing the O2 sensor from seeing normal rich-lean oscillations during closed-loop fuel control.
A crack or leak in the exhaust manifold or pipe near the Bank 1 sensor allows outside air in, diluting exhaust readings and reducing sensor switching frequency.
Corroded pins, chafed wires, or a loose connector on the Bank 1 upstream sensor harness can cause intermittent signal loss and trigger this code.
Estimated Cost
Professional Repair
Includes parts + labor
Common Fixes
- Replace the Bank 1 Sensor 1 upstream oxygen sensor
- Inspect and repair any vacuum leaks in the intake system
- Check for and repair exhaust leaks near the sensor
- Clean or replace corroded O2 sensor wiring connectors