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P1151 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.


An indication that the HO2S or Heated Oxygen Sensor is lean indicates it is trying to compensate for an over-rich condition. Fuel control systems fail the test when they cannot detect switching for the calibrated amount of time.


The error code P1151 is defined as an indicator that indicates the bank 2 is lean due to a lack of elevated oxygen sensors.

Error code 00644 relates to the make and model of a particular vehicle.  It is applicable only to Fords.


  • There is a problem with the O2 sensor bank 2 upstream
  • Low fuel pressure or high fuel pressure
  • MAF sensor (mass airflow) clogged or faulty
  • An engine vacuum leak has been detected
  • Fuel pressure regulator or fuel injector leak
  • The PCM is defective


No problems with drivability are likely to occur.


Look for damage to the wiring harness.  Inspect the contacts where the sensors plug in.  The harness fails more often than a typical wiring harness because it contains an oxygen sensor on a hot exhaust component.

Almost any car can be found with its upstream oxygen sensor.  You can find the oxygen sensor by following the exhaust from the manifold to bank 2.

There could be a vacuum leak in the system

There may be a problem with the fuel pressure.  Fuel pressure is too low because of a plugged fuel filter, a faulty fuel pump, or a faulty fuel pressure regulator.  The P1131 error code would also appear if fuel pressure was too low (same code, just a different engine side).

What repairs can fix the code?

It is possible that you have clogged fuel injectors if you have replaced the oxygen sensor and gaskets on the manifold but still have leaks. Check to see if the problem is resolved after using high-octane gas for a few weeks.

Ensure that you look for any damage to the intake hoses if there is an intake leak.

This error code can also be caused by warm ambient temperatures, and fuels specially made in winter, high volatility can cause the fuel to boil (vapor lock) during a hot soak. The condition can be easily aggravated by oxygenated fuel. You could use mid-high octane fuel if there are no non-oxygenated fuels available, as they will have less volatility and can help mitigate this issue.

Related codes

None listed.


There are countless issues that could be the reason for P1151. A faulty oxygen sensor, fuel injector, or vacuum leak will more than likely cause the problem. The HO2S is an oxygen sensor that is built into the exhaust manifold. It helps the engine control system to maintain a proper air/fuel mixture by sensing the oxygen content in the exhaust gas that comes out of the engine. You may have a problem with the oxygen sensor if you are seeing this code and replaced the sensor and gaskets. This problem is more likely to occur during the colder months and can be caused by high-octane gas.

P1151 Code – What Does It Mean & How To Fix It