Error Code P1151 is defined as Lack of Upstream Heated Oxygen Sensor Switch Sensor Indicates Lean Bank 2.

This error code is a manufacturer-specific trouble code, meaning it does not apply to all vehicles makes. Rather, it applies exclusively to Ford vehicles.


When the HO2S or Heated Oxygen Sensor indicates lean condition at the end of a test, that means it is trying to correct for an over-rich condition. The test fails when the fuel control system is unable to detect switching for the calibrated amount of time.

Common Symptoms

As with other error codes, this code activates the Check Engine light and registers the code to the vehicle’s system. Other than that, there are no drivability problems that come with this error code.

Possible Causes

Multiple factors lead to this error code, such as:

  • Defective upstream O2 sensor bank 2
  • High or low fuel pressure
  • Dirty or defective MAF (mass airflow) sensor
  • Vacuum leak on engine
  • Leak on fuel injector or fuel pressure regulator
  • Defective PCM

Also, specific components in the system could be damaged or experiencing issues, which results in this error code. This would mean one or more of the following happened:

For Electrical Components:

  • Short to VPWR in the harness or HO2S
  • Water in the harness connector
  • Open or shorted in HO2S circuit
  • Corrosion or poor mating terminals and wiring
  • Faulty HO2S
  • Faulty PCM

For the Fuel System:

  • Excessive fuel pressure Leaking/contaminated fuel injectors
  • Low fuel pressure or running out of fuel
  • Leaking fuel pressure regulator
  • Vapor recovery system

Base Engine:

  • Oil overfill
  • Cam Timing
  • Cylinder compression
  • Exhaust leaks before or near the HO2S(s)

Induction System:

  • Air leaks after the MAF
  • Vacuum Leaks

PCV System:

  • Improperly seated engine oil dipstick

EGR System:

  • Leaking gasket
  • Stuck EGR valve
  • Leaking diaphragm or EVR

How to Check

Diagnosis for this code is pretty simple and straightforward; you can refer to the list above and go from there. Inspect, repair, or replace as needed. For more comprehensive diagnosis, refer to the following:

Performa a self-test. If there are no codes other than the P1151, then proceed to next step. If there are other codes present, then follow normal diagnostic procedures for those codes.

To start further diagnosis, remove the upper intake and then replace only the port gaskets (9H486) and isolator bolt assemblies (9S479). New isolator bolt assemblies use green colored rubber material, never install the old isolator bolt assemblies with the black rubber material. Also, never replace any other intake parts or gaskets. If you’re not sure, then refer to the Workshop Manual, or better yet, let your mechanic do the job.

While the upper intake manifold is removed, check the EGR or exhaust gas recirculation orifices that are pressed into each primary port of the lower intake manifold. If there are other orifices plugged with soot, then clean and unplug them using a pick solvent.

How to Fix

Follow the repairs above and repair or replace defective components as needed.

If after replacing the Oxygen sensor and gaskets on the manifold still have leaks, then you may be experiencing clogged fuel injectors. Try using high octane gas for a couple of weeks to see if this fixes the problem.

For intake leaks, make sure you check the hoses for damages.

Note that warm ambient temperature could also lead to this error code, and using winter grade fuels which have very high volatility, may boil in the fuel lines (vapor lock) during a short hot soak. Oxygenated fuels can easily aggravate this condition. If there are no non-oxygenated fuels available, then you can opt for mid/high octane fuels, as they have lower volatility and can help alleviate this problem.

When testing for vacuum leaks, never use smoke detection test, especially propane gas, carburetor, or brake cleaner. The last two can cause the IMRC (intake manifold runner control) bearing to fail. Smoke leaks from the IAC (idle air control) or EGR valve and IMRC seal are usually normal. Never replace these parts. Also, never replace the lower aluminum intake manifold.