Error Code P1131 is described as Lack of Upstream Heated Oxygen Sensor Switch – Sensor Indicates Lean – Bank No. 1.


Error Code P1131 refers to insufficient switching  of oxygen sensor bank 1 or the sensor 1 is lean. This trouble code indicates that the system is correcting the ‘rich’ for an overly ‘lean’ condition in engine’s air fuel ratio. The Heater Oxygen Sensor voltage is less than 0.45V. This is because the upstream O2 sensor (one closes to the engine), is not switching rich to lean and vice versa as often as it should.

If the diagnostic trouble code shows Error Code that starts with P1xxx, this means it’s a manufacturer specific code. Thus, you need to be specific in your search and indicate the make of your vehicle.

Oxygen SensorIn a nutshell, Error Code P1131 on your Ford vehicle indicates a trouble in your O2 sensor. The O2 sensor’s job is to measure the amount of oxygen in the exhaust, and determine whether the air fuel ratio in the cylinder is too rich or too lean. An optimized air fuel ratio mixture means better fuel economy and fewer exhaust emission. This error code refers to lean condition, which means there is too much air in the air fuel ratio mixture.

Combustion engines, such as the one we have in our vehicle, runs by a mixture of air and fuel at around 14.7 to 1—14.7 parts air to 1 part fuel. If the ratio goes lower than 14.7 parts, this is called “rich” mixture. If it goes above 14.7 parts, it is referred as “lean” mixture.

To define:

  • Rich Mixture = Too much fuel, not enough air
  • Lean Mixture = Too much air, not enough fuel

And to keep the engine running properly, the ECM (Engine Control Module) measures the oxygen content in the exhaust through O2 sensors and compensates any imbalances of the right mixture; by injecting more or less fuel.

Common Symptoms

  • Check Engine light is on
  • Engine misfire

Possible Causes

  • Vacuum leak after MAF
  • Rich fuel mixture
  • Restricted fuel filter, or anything that affects the fuel mixture
  • Restricted air filter
  • Bad PCV valve or PCV system hose
  • Low fuel pressure
  • Low engine coolant level
  • Leaking intake manifold gasket
  • Incorrect engine thermostat (stuck open or too cold)
  • Plugged injector

How to Diagnose

  1. Start with a vacuum test to determine whether there are any leaks in the system.
  2. Check the fuel injectors in the bank 1 cylinders.
  3. Determine when was the last time the fuel filters were changes, just in case you see it clogged enough to interfere the normal flow.
  4. Inspect the Exhaust Gas Recirculation known as the EGR valve to determine whether it is clogged or stuck open.
  5. On most vehicles, the engine idle is usually determined by the computer of the vehicle, but for some, you need to have it checked to make sure it is always on par with the recommended settings, especially if the vehicle just had a tune up.
  6. Lastly, the Oxygen sensor itself may be in bad condition, especially if it is cold. If you can’t find any other causes of the error code from your diagnosis, then you most probably have a defective O2 sensor.

You can also check the O2 sensor using a DVOM (Digital Volt Ohm Meter). Connect the device to the signal wire from the O2 sensor, warm the engine up to its operating temperature, and then disconnect the brake booster hose to induce a lean condition. The O2 sensor voltage should drop down to near zero.  Then, reconnect the host and spray some aerosol carb cleaner to the engine air intake; the O2 sensor voltage should spike to around 9/10 of volt from the rich condition. If the sensor responds as described, it is operational.

Then, using a thermometer, measure the thermostat opening temperature. If it doesn’t reach 195° F or if the coolant level is low, you will get rich condition.

How to Repair

If the vehicle has a lot of miles on it, the easiest way to get rid of this error code is to replace the O2 sensor as part of its maintenance. Or replace at around 100,000 miles.

As said in the diagnosis above, you need to conduct a vacuum leak test first. Eliminate this possibility before proceeding to the air filter. When the code is still on, then you can also change your fuel filter.

Measure your thermostat opening temperature with a thermometer. If the engine is not reaching 195 degrees F or if the coolant level is low, it will cause a rich condition and set the code you describe.

Replace faulty parts, then clear the code and see if the problem comes back.

Parting Tips

Do NOT buy and replace any parts of your vehicle until you have fully determined the actual cause of the problem through thorough testing. Last thing you want is to throw any functional parts. You should know that Error Codes don’t indicate specific component failures, rather, they say general system or circuit has gone abnormal. Thus, it is important to perform manual test to pin point the root cause of the problem.

For more guides and troubleshooting tips of different Error Codes, check out our website. For professional help, Contact Us.