B1789 OBD2 reports a sensor fault, replacement of the sensor is unlikely to resolve the underlying problem. The fault is most likely to be caused by the systems that the sensor is monitoring, but might even be caused by the wiring to the sensor itself.
OBD 2 B1789 code and tailpipe testing are two different approaches to identify vehicles in need of repair. The OBD system looks for broken or malfunctioning emissions control components while tailpipe tests sample a vehicle's exhaust to see if it is above or below certain prescribed limits. Given the robust nature of today's emissions control components, it is entirely possible for an individual component to malfunction without leading to an immediate increase in emissions at the tailpipe. In such cases, other components (like the catalyst) can temporarily compensate for the part that is broken however, these other components can only do double duty for so long before they, too, begin to malfunction. Because of its ability to monitor individual components, OBD is able to give motorists an early warning that repairs are needed; it is because of this early warning capability that OBD will sometimes fail vehicles that would otherwise pass a tailpipe test.
Accelerator Pedal Position Sensor 1 Circuit Low and P2127 - Accelerator Pedal Position Sensor 2 Circuit Low would lead you to suspect a defective accelerator pedal position sensor.
In-line Ford engines, along with those of most other manufacturers, begin the numbering of cylinders at the front and proceed in numerical order toward the back. In the V-engine design, Ford follows a similar design with the number one cylinder at the front left of the engine. In the V-6 configuration, cylinder 4 is at the front right of the engine and in a V-8, cylinder number 5 is in that location. Other manufacturers sometimes use an alternating pattern in the V-engines.