Technical Description of OBD-II Trouble Code
The circuit for the Transmission Fluid Pressure Sensor/Switch “A” is experiencing low pressure.
What does that mean?
This generic powertrain/engine diagnostic trouble code is applicable to all OBD-II equipped vehicles, including but not limited to Chevrolet, Acura, Honda, Saturn, Jeep, Toyota, etc.
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 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 using the vehicle’s data communication bus.
The PCM/TCM uses this voltage signal to determine transmission operating pressure and when a shift is occurring. This code is triggered if the “A” 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 “A” circuit for your particular vehicle.
P0842 typically indicates an electrical circuit (TFPS sensor circuit) issue. This should not be overlooked during troubleshooting, 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 A circuit codes:
Code Severity & Symptoms
The severity depends on which circuit the failure has occurred in. Since this is an electrical 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 P0842 engine code may include:
Causes
The typical causes for this code to be triggered are:
Diagnostic and Repair Procedures
A good starting point is always to check for technical service bulletins (TSB) for your particular vehicle. Your issue may be a known problem with a known fix provided by the manufacturer, which can save you time and money during diagnosis.
Next, locate the Transmission Fluid Pressure Sensor/Switch (TFPS) on your particular vehicle. The TFPS is usually found 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. Look for scraping, rubbing, bare wires, burn spots, or melted plastic. Disconnect the connector and carefully inspect the terminals (the metal parts) inside the connector. Check for signs of burning or a green tint indicating corrosion, especially if they are attached outside the transmission case. If cleaning of the terminals is necessary, use electrical contact cleaner and a plastic bristle brush. Allow them to dry and apply electrical grease where the terminals make contact.
If you have a scan tool, clear the diagnostic trouble codes from memory and see if the P0842 code returns. If it does not, then the connections were most likely the problem.
This is the most common area of concern for this code, as the external transmission connections are prone to corrosion issues.
If the P0842 code does return, you will need to test the TFPS sensor and its associated circuits. 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 they change. If the voltage is incorrect, repair the power or ground wire, or replace the PCM/TCM.
If the previous test passed, 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 the resistance to pressure when no pressure is applied to it. 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 have passed and you continue to get a P0842, this most likely indicates a failed TFPS sensor. However, a failed PCM/TCM cannot be ruled out, nor can internal transmission malfunctions until the TFPS sensor has been replaced. If unsure, seek assistance from a trained automotive diagnostician. PCM/TCMs must be programmed or calibrated to the vehicle in order to be installed correctly.
