Error Code P1133 is defined as HO2S Insufficient Switching Bank 1 Sensor 1.

This error code is a manufacturer-specific trouble code, meaning it does not apply to all vehicles makes. Rather, it applies only to specific vehicle makes, such as Buick, Chevrolet, GMC, GM, Toyota, Isuzu, etc. Specification of the definition, troubleshooting, and repairs still vary from one make and model to another.


The PCM (powertrain control module, also known as ECM or engine control module) monitors lots of systems inside the engine and the vehicle, including the HO2S or (Heated Oxygen Sensor). It monitors the HO2S’s activity for 90 seconds after the start of the closed loop operation. The PCM determines if the HO2S is working properly by counting the number of HO2S-1 Lean/Rich (L/R) and Rich/Lean R/L switches.

Error Code P1133 happens when the PCM determines that the front HO2S was unable to switch enough times.

Common Symptoms

As with other error codes, P1133 activates the Check Engine light and registers the code to the vehicle’s memory system.

Possible Causes

Multiple factors could lead to this error code, such as:

  • Defective HO2S Bank 1 Sensor 1
  • Open or shorted harness in HO2S Bank 1 Sensor 1
  • Poor electrical connection in HO2S Bank 1 Sensor 1
  • Vacuum leak

How to Check

Technicians diagnose this code by starting their inspection at the HO2S Bank 1 Sensor 1. They make sure the sensor is working properly. If not, then it should be replaced.

Next, they check for the condition of the wirings and harness and repair or replace as needed.

Then, they check for vacuum leaks and repair or replace as needed.

How to Fix

The most common cause of this code is a faulty heated oxygen sensor. Thus, it must be checked and must be replaced if proven defective. Other repairs include:

  • Repair vacuum leaks
  • Repair or replace HO2S bank 1 sensor 1 open or shorted harness
  • Repair or replace poor electrical connection in HO2S bank 1 sensor 1

Sometimes, exhaust gas leaks are what is causing this problem. Thus, it must be checked as well and repair, if not replace as needed.