Chevrolet P1133: HO2S Insufficient Switching (B1S1) — Fix
On Chevrolet vehicles, P1133 means the upstream oxygen sensor (bank 1, sensor 1) is not switching between rich and lean often enough — a lazy sensor.
- Applies to
- Chevrolet vehicles
- Est. repair cost
- $40 – $350
- Time to fix
- 30 minutes – 2 hours
What it means
Causes include an aged O2 sensor, an exhaust leak, or a persistent mixture problem. P1xxx codes are manufacturer-specific — P1133 is defined by Chevrolet (not part of the generic SAE J2012 set), so this definition applies to Chevrolet vehicles. Model years can vary, so confirm against the Chevrolet service manual. Fix the fault, then clear the code with a scan tool.
Symptoms
- Check engine light on
- Slightly worse fuel economy
- May fail emissions
How to fix it
- 1
Check for exhaust leaks
1. Inspect the exhaust near the sensor for leaks.
- 2
Replace the O2 sensor
1. Replace a slow/lazy upstream O2 sensor, then clear the code and drive a cycle.
Frequently asked questions
Is P1133 a bad O2 sensor?
Usually — a sensor that's too slow to switch is worn. Rule out an exhaust leak first, then replace the sensor.
Last updated July 19, 2026
Guidance only — always consult a qualified professional or the official service manual before carrying out repairs.