P0842 Transmission Fluid Pressure Sensor / Switch A Circuit Low
OBD-II Trouble Code Technical Description
ASE Certified Master Technician
Transmission Fluid Pressure Sensor / Switch "A" Circuit Low
What does that mean?
This generic powertrain/engine diagnostic trouble code typically applies 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 usually found attached to the side of the valve body inside the transmission, although sometimes it may be found 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). Typically, the PCM/TCM will then inform other controllers utilizing the vehicle's data communication bus.
The PCM/TCM receives this voltage signal to determine transmission operating pressure or when a shift is occurring. This code is set if this "A" input does not match normal operating voltages stored in the PCM/TCM's memory. It may also be due to internal mechanical issues inside the transmission. Refer to a vehicle specific repair manual to determine which is the "A" circuit for your particular vehicle.
P0842 is typically an electrical circuit (TFPS sensor circuit) issue. This cannot be overlooked in the troubleshooting stage, especially when dealing with an intermittent problem.
Troubleshooting steps may vary depending upon manufacturer, type of TFPS sensor and wire colors.
Related trans fluid pressure sensor A circuit codes:
- P0840 Transmission Fluid Pressure Sensor/Switch "A" Circuit
- P0841 Transmission Fluid Pressure Sensor/Switch "A" Circuit Range/Performance
- P0843 Transmission Fluid Pressure Sensor/Switch "A" Circuit High
- P0844 Transmission Fluid Pressure Sensor/Switch "A" Circuit Intermittent
Code Severity & Symptoms
Severity depends upon which circuit the failure has occurred in. Since this is an electrical failure, the PCM/TCM can compensate to some degree. The failure may mean that the PCM/TCM modifies the shifting of the transmission if electronically controlled.
Symptoms of a P0842 engine code may include:
- Malfunction Indicator Light On
- Change in shift quality
- Vehicle starts off in 2nd or 3rd gear (limp in mode)
Typically the causes for this code to set are:
- Short to ground in the signal circuit to the TFPS sensor - possible
- Failed TFPS Sensor / internally shorted – likely
- Failed PCM - unlikely (programming required after replacement)
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 issue with a known fix put out by the manufacturer and 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 sometimes it may be found 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. Pull the connector apart and carefully inspect the terminals (the metal parts) inside the connector. See if they look burned or have a green tint indicating corrosion, especially if they are attached outside the transmission case. Use electrical contact cleaner and a plastic bristle brush if cleaning of the terminals is needed. Let dry and apply electrical grease where the terminals contact.
If you have a scan tool, clear the diagnostic trouble codes from memory, and see if P0842 code returns. If it does not, then the connections were most likely your problem.
This is the most common area of concern for this code, as the external transmission connections have the greatest number of issues with corrosion.
If the P0842 code does return, we 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 Key On Engine Off. Check manufacturer's specifications; 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 prior 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 manufacturers specifications on the resistance of the sensor to accurately test the resistance to pressure when there is no pressure 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 prior tests have passed and you continue to get a P0842, this would most likely indicate a failed TFPS sensor, although a failed PCM/TCM could not be ruled out, nor could internal transmission malfunctions until the TFPS sensor had 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.
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