What does it mean?
This generic powertrain/engine diagnostic code generally applies to all vehicles equipped with OBD-II (e.g., GM, Chevrolet, Honda, Toyota, Ford, etc.), but appears more frequently in Nissan vehicles.
The Transmission Fluid Pressure Sensor/Switch (TFPS) is typically found attached to the side of the valve body inside the transmission, although it may sometimes be found screwed into the side of the transmission case/body itself.
The TFPS converts mechanical transmission pressures into an electrical signal for the Powertrain Control Module (PCM) or Transmission Control Module (TCM). Typically, the PCM/TCM will then inform other controllers using the vehicle’s data communication bus.
The PCM/TCM receives this voltage signal to determine the transmission operating pressure or when a change occurs. This code is set if this “B” input does not match the normal operating voltages stored in the PCM/TCM memory. This can also be due to internal mechanical problems inside the transmission. Refer to a vehicle-specific repair manual to determine what the “B” circuit is for your particular vehicle.
P0849 is typically an electrical circuit issue (TFPS sensor circuit), although, as mentioned earlier, this code can also be set due to mechanical problems (internal leaks, cracked or missing control balls in the valve body, low system operating pressure/main line pressure, stuck valve in the valve body). This cannot be ignored in the troubleshooting phase, especially when it is an intermittent issue.
Troubleshooting steps may vary depending on the manufacturer, TFPS sensor type, and wire colors.
Related Transmission Fluid Pressure Sensor B Circuit Codes:
- P0845 Transmission Fluid Pressure Sensor/Switch, Circuit “B”
- P0846 Transmission Fluid Pressure Sensor/Switch “B” Circuit Performance
- P0847 Transmission Fluid Pressure Sensor/Switch “B” Low
- P0848 Transmission Fluid Pressure Sensor/Switch “B” High
Code Severity and Symptoms
The severity depends on the circuit where the failure occurred. As it can be an electrical failure or a mechanical failure, the PCM/TCM may compensate to some extent. The failure may mean that the PCM/TCM alters the transmission shift if it is electronically controlled.
Symptoms of a P0849 engine code may include:
Malfunction Indicator Light illuminated
Change in shift quality
Vehicle starts in 2nd or 3rd gear (limp mode)
Causes
Typically, the causes for this code to set are as follows:
- Intermittent open in the signal circuit to the TFPS sensor
- Intermittent short to voltage in the signal circuit to the TFPS sensor
- Intermittent short to ground in the signal circuit to the TFPS sensor
- Failed TFPS sensor – likely
- Internal mechanical transmission problem – possible
- Failed PCM – unlikely (programming required after replacement)
Diagnostic and Repair Procedures
A good starting point is always to check the Technical Service Bulletins (TSBs) for your particular vehicle. Your problem may be a known issue with a known fix published by the manufacturer and can save you time and money when diagnosing.
A good example of this would be if there are known codes related to power supply with P0849, or if there is more than one set of pressure sensor/switch codes. If so, start your diagnosis first with the power supply-related trouble code or with the multiple code diagnosis, as this may be the reason for P0849.
Next, locate the Transmission Fluid Pressure Sensor/Switch (TFPS) on your particular vehicle. The TFPS is typically found attached to the side of the valve body inside the transmission, although it is sometimes screwed into the side of the transmission case/body itself. Once located, visually inspect the connector and wiring. Look for chafing, rubbing, bare wires, burns, or melted plastic. Separate the connector and carefully inspect the terminals (the metal parts) inside the connector. See if they look burned or have a greenish tint indicating corrosion, especially if they are mounted outside the transmission case. Use electrical contact cleaner and a plastic bristle brush if cleaning the terminals is necessary. Let it dry and apply electrical grease where the terminals make contact.
If you have a scan tool, clear the diagnostic codes from memory and see if the P0849 code returns. If not, the connections were likely your problem.
This is the most common area of concern for this code, as external transmission connections have the highest incidence of corrosion issues.
If the P0849 code returns, we will need to test the TFPS sensor and its associated circuits. With the key off, disconnect the electrical connector from the TFPS sensor. Connect a black wire from the Digital Voltmeter (DVOM) to the ground or low reference terminal at the TFPS sensor wiring harness connector. Connect the red wire from the Digital Voltmeter to the signal terminal of the TFPS sensor wiring harness. Turn the key to the engine off position. Check the manufacturer’s specifications; the voltmeter should read 12 volts or 5 volts. Wiggle the connections to see if they change. If the voltage is incorrect, repair the power or ground wire or replace the PCM/TCM.
If the previous test was successful, connect one wire of an ohmmeter to the signal terminal of the TFPS sensor and the other wire to the ground or low reference terminal of the sensor. Check the manufacturer’s specifications for the sensor resistance to accurately test the resistance to pressure when no pressure is applied to it. Wiggle the Transmission Fluid Pressure Sensor/Switch connector while monitoring the resistance. If the ohmmeter readings do not pass, replace the TFPS.
If all previous tests were successful and you continue to get a P0849, this would most likely indicate a faulty TFPS sensor, although a faulty PCM/TCM cannot be ruled out, nor can internal transmission malfunctions until the TFPS sensor has been replaced. If in doubt, seek assistance from a qualified automotive diagnostician. PCMs/TCMs must be programmed or calibrated to the vehicle to be installed correctly.