Emissions

P1133 — HO2S Insufficient Switching, Sensor 1

Severity: low Safe to drive

What P1133 means

The engine control module watches the front heated oxygen sensor, which sits ahead of the catalytic converter. Normally its voltage swings quickly between rich and lean as the ECM trims fuel delivery. P1133 (the GM definition) is set when those swings are too slow or too infrequent. Typical causes are an aging sensor, an exhaust leak, or an underlying rich or lean condition.

Symptoms

  • Check Engine light on
  • Worse fuel economy
  • Failed emissions test
  • Rough idle
  • Reduced performance

Possible causes

  1. Aging or contaminated upstream oxygen sensor
  2. Exhaust leak ahead of the sensor
  3. Vacuum leak or fuel system fault causing a rich or lean mixture
  4. Sensor contaminated by oil, coolant or soot
  5. Damaged sensor wiring or connector
  6. Heater circuit fault in the sensor

How to diagnose

  1. Read all codes and freeze-frame data and look for related mixture or misfire codes.
  2. Inspect the exhaust manifold, gaskets and pipes ahead of the sensor for leaks or soot trails.
  3. Check the sensor wiring and connector for damage, corrosion, oil or water.
  4. With a scan tool, watch the sensor live at operating temperature: it should switch rapidly between rich and lean.
  5. Review short and long term fuel trims and find the cause of any shift: vacuum leak, fuel pressure, MAF sensor.
  6. Test the heater circuit resistance and supply per the factory wiring diagram.
  7. Compare the upstream sensor with the downstream one if the diagnosis is unclear.

Common fixes

  • Repair the exhaust leak
  • Replace the upstream oxygen sensor
  • Repair the wiring or connector
  • Correct the rich or lean cause (vacuum leak, MAF, injectors, fuel pressure)
  • Clear the codes and confirm the fault does not return

P1133 on different brands

Chevrolet · manuals
On Chevrolet, P1133 refers to insufficient switching of heated oxygen sensor HO2S 1 on bank 1.
Chrysler
Chrysler lists this code as heated oxygen sensor insufficient switching.
Honda · manuals
On Honda, the code points to insufficient switching of the right-hand front heated oxygen sensor.
Ford · manuals
Ford's definition ties the code to bank 1 fuel control shifted lean.

Questions and answers

Can I drive with a P1133 code?

Yes, usually. The car stays drivable, but a bad mixture raises fuel use and can eventually damage the catalytic converter, so diagnose it soon.

Does P1133 always mean I need a new oxygen sensor?

No. Check for exhaust leaks and fuel trim problems first, because a mixture issue can mimic a slow sensor. Replace the sensor if everything else checks out and its signal is sluggish.

Why does P1133 have different meanings on different brands?

It is a manufacturer-specific code, so each automaker can define it differently. Check the definition for your make and engine before diagnosing.

This is general information about the code. The meaning can differ between makes and models: a scan tool and the repair manual for your car will show the exact cause.