P0191 is a common OBD-II code. It indicates that the fuel rail pressure sensor is receiving a signal that is outside of its normal range. This is a generic code, which indicates that it has the same meaning for the Scion iQ as it would any other vehicle.
P0191 is typically caused by a bad fuel pressure sensor, fuel pressure regulator, or an issue with the fuel rail pressure sensor wiring harness.
P0191 Symptoms: Scion iQ
When P0191 is present, there typically are noticeable symptoms. Drivability is usually affected.
Here are the most common symptoms of P0191:
If there isn’t enough fuel pressure getting to the fuel injectors, the engine may stall out or hesitate in certain situations.
Air/Fuel Mixture Issues
Without a reliable reading from the fuel pressure sensor, your iQ may end up with the wrong air/fuel mixture. If it runs too rich, you may get black smoke and P0172. If it runs too lean, you’re likely to have p0171 accompany P0191.
P0191 can be caused when the fuel pressure regulator is bad. If the fuel pressure at the injectors is inconsistent, it’ll be really hard for your iQ to maintain a steady idle speed. This can lead to the engine dying when coming to a stop as well.
Poor Fuel Mileage
Fuel mileage can suffer when the air/fuel mixture isn’t correct.
P0191 Causes: Scion iQ
Here are the most common issues that cause P0191 to be thrown:
Fuel Pressure Sensor
The fuel pressure sensor is responsible for telling your iQ’s ECM/PCM what the actual fuel pressure of the vehicle is. If it goes bad (or if the fuel pressure really is out of the normal range) P0191 can and will appear.
A bad fuel pressure sensor can force the vehicle to overwork the fuel pump, thinking that it’s constantly starving for fuel when it is not.
Wiring issues can cause P0191 to be thrown. Take a look at the wiring harness around the fuel pressure sensor. Does it look broken or damaged? Is the connection to the sensor solid? If it looks bad, it may need to be replaced.
Fuel Pressure Regulator
The fuel pressure regulator is responsible for making sure that the amount of fuel pressure that reaches the fuel rail is at a constant level (which will be less than pushed by the fuel pump). If it goes bad, it can restrict the flow of fuel to the point that P0191 can be thrown.
A bad fuel pump would act exactly like a bad fuel pressure regulator. You’d notice hesitation and stalling as the fuel pump struggles to keep up with the demands of the engine. This is typically noticed under heavy engine load. Or, right after heavy engine load when the fuel pressure isn’t recovered enough for a proper idle.
See Also: Scion iQ: Bad Fuel Pump
P0191 Diagnosis: Scion iQ
Here’s a great video from Airtex:
A good scanner can help you look at the conditions that were present when the code was set. You can also use it to check the fuel pressure reading as you drive. If the reading seems off, you’ll need to manually check the fuel pressure.
If the manual fuel pressure reading is normal, then you know that you’ve got an issue with the fuel pressure sensor (or the wiring going to and from it). If the fuel pressure reading is low, than you’re going to need to figure out if it’s the fuel pump or fuel pressure regulator that is causing P0191.
P0191 can be a difficult code to track down, but should not be ignored. Doing so very well could leave your Scion iQ stranded. If there is anything that you would like to add, please leave a comment below. Good luck.