Technical Description of OBD-II Trouble Code
Intermittent Circuit Issue with Transmission Fluid Pressure Sensor / Switch “E”
What does that signify?
This generic trouble code is applicable to all vehicles equipped with OBD-II (e.g. GM, Chevrolet, Honda, Toyota, Ford, etc.), but it is more commonly found in Nissan vehicles.
The Transmission Fluid Pressure Sensor/Switch (TFPS) is typically located on the side of the valve body inside the transmission, although it may sometimes be screwed into the side of the transmission case/body itself.
The TFPS converts the mechanical transmission pressures into an electrical signal for the Powertrain Control Module (PCM) or Transmission Control Module (TCM). The PCM/TCM then communicates this signal to other controllers through the vehicle’s data communication bus.
The PCM/TCM uses this voltage signal to determine the transmission’s operating pressure and when a shift is occurring. This code is triggered when the “E” input does not match the normal operating voltages stored in the PCM/TCM’s memory. It may also indicate internal mechanical issues within the transmission. Consult a vehicle-specific repair manual to identify the “E” circuit for your particular vehicle.
P0991 typically indicates an electrical circuit (TFPS sensor circuit) issue, although as mentioned earlier, it can also be caused by mechanical problems (internal leaks, cracked or missing check balls in the valve body, low system operating pressure/main line pressure, stuck valve in the valve body). It is important not to overlook these possibilities during the troubleshooting process, especially when dealing with an intermittent problem.
Troubleshooting steps may vary depending on the manufacturer, type of TFPS sensor, and wire colors.
Related transmission fluid pressure sensor “E” circuit codes:
Code Severity & Symptoms
The severity of the issue depends on which circuit the failure has occurred in. Since it could be an electrical or mechanical failure, the PCM/TCM can compensate to some extent. The failure may result in the PCM/TCM modifying the shifting of the transmission if it is electronically controlled.
Symptoms of a P0991 engine code may include:
Causes
The common causes for this code to be triggered are:
Diagnostic and Repair Procedures
It is always a good idea to check for technical service bulletins (TSB) specific to your vehicle. There may be a known issue with a known fix provided by the manufacturer, which can save time and money during diagnosis.
For example, if there are any known power-related codes set along with the P0991, or if there is more than one pressure sensor/switch code set, start the diagnosis with the power-related fault code or the multiple code diagnosis, as they may be the reason for the P0991.
Next, locate the Transmission Fluid Pressure Sensor/Switch (TFPS) on your vehicle. It is usually attached to the side of the valve body inside the transmission, although it may sometimes be screwed into the side of the transmission case/body itself. Once located, visually inspect the connector and wiring for any signs of scraping, rubbing, bare wires, burn spots, or melted plastic. Separate the connector and carefully examine the terminals inside for any signs of burning or corrosion (green tint). If necessary, clean the terminals using electrical contact cleaner and a plastic bristle brush. After cleaning, let them dry and apply electrical grease to the contact points.
If you have a scan tool, clear the diagnostic trouble codes from memory and check if the P0991 code returns. If it does not, then the issue was most likely related to the connections.
This is the most common area of concern for this code, as the external transmission connections are prone to corrosion issues.
If the P0991 code does reappear, further testing of the TFPS sensor and its associated circuits is necessary. With the Key Off, disconnect the electrical connector at the TFPS sensor. Connect a Digital Voltmeter (DVOM) black lead to the ground or low reference terminal at the TFPS sensor wiring harness connector. Connect the red lead of the Digital Voltmeter to the signal terminal at the TFPS sensor wiring harness connector. Turn the Key On Engine Off. Check the manufacturer’s specifications; the voltmeter should read either 12 volts or 5 volts. Wiggle the connections to see if there are any changes. If the voltage is incorrect, repair the power or ground wire, or replace the PCM/TCM.
If the previous test passes, connect one lead of an ohmmeter to the signal terminal at the TFPS sensor and the other lead to the ground or low reference terminal at the sensor. Check the manufacturer’s specifications for the resistance of the sensor to accurately test its resistance to pressure when no pressure is applied. Wiggle the connector at the Transmission Fluid Pressure Sensor/Switch while monitoring the resistance. If the ohmmeter readings do not pass, replace the TFPS.
If all previous tests pass and the P0991 code continues to appear, it is likely that the TFPS sensor has failed. However, a failed PCM/TCM cannot be ruled out, nor can internal transmission malfunctions, until the TFPS sensor is replaced. If unsure, seek assistance from a trained automotive diagnostician. PCM/TCMs must be programmed or calibrated to the vehicle for correct installation.
