P0842 Low rate sensor / switch transmission fluid pressure
OBD-II DTC Datasheet
Transmission fluid pressure sensor / switch "A" - low signal level
What does this mean?
This Generic Transmission / Engine DTC usually 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 it can sometimes be found screwed into the side of the transmission case / housing itself.
TFPS converts the mechanical transmission pressure into an electrical signal for the powertrain control module (PCM) or transmission control module (TCM). Typically, the PCM / TCM will then inform the other controllers using the vehicle's data bus.
The PCM / TCM receives this voltage signal to determine the operating pressure of the transmission or when a gear change is occurring. This code is set if this "A" input does not match the normal operating voltages stored in the PCM / TCM memory. It can also be due to internal mechanical problems within the transmission. Refer to your specific vehicle repair manual to determine which circuit “A” your specific vehicle has.
P0842 is usually an electrical circuit problem (TFPS sensor circuit). This should not be overlooked during the troubleshooting phase, especially when dealing with an intermittent problem.
Troubleshooting steps may vary depending on manufacturer, TFPS sensor type, and wire colors.
Related Transmission Fluid Pressure Sensor Circuit Codes:
- P0840 Transmission fluid pressure sensor / switch "A"
- P0841 Transmission Fluid Pressure Sensor / Switch "A" Circuit Range / Performance
- P0843 A high transmission fluid pressure sensor / switch "A"
- P0844 Transmission Fluid Pressure Sensor / Switch "A" Circuit Intermittent
Code severity and symptoms
The severity depends on which circuit the failure occurred. Since this is an electrical failure, the PCM / TCM can compensate for it to some extent. A malfunction could mean the PCM / TCM is modifying the transmission shift when electronically controlled.
Symptoms of a P0842 engine code may include:
- Fault indicator light is on
- Change the quality of the shift
- The car starts to move in 2nd or 3rd gear (limping in mode).
reasons
Usually the reason for installing this code is:
- Short to ground in the signal circuit to the TFPS sensor - possible
- Faulty TFPS sensor / internal short circuit - likely
- Faulty PCM - unlikely (programming required after replacement)
Diagnostic and repair procedures
A good starting point is always to check the Technical Service Bulletins (TSB) for your particular vehicle. Your problem may be a known issue with a known manufacturer-released fix and could save you time and money while troubleshooting.
Then locate the Transmission Fluid Pressure (TFPS) sensor / switch on your specific vehicle. The TFPS is usually found attached to the side of the valve body inside the transmission, although it can sometimes be found screwed into the side of the transmission case / housing itself. Once found, visually inspect the connector and wiring. Look for scratches, scuffs, exposed wires, burn marks, or molten plastic. Disconnect the connector and carefully inspect the terminals (metal parts) inside the connector. See if they look burnt or have a green tinge indicating corrosion, especially if they are attached outside the gearbox housing. If you need to clean the terminals, use an electrical contact cleaner and a plastic bristle brush. Allow to dry and apply electrical grease where the terminals touch.
If you have a scan tool, clear the DTCs from memory and see if P0842 returns. If this is not the case, then there is most likely a connection problem.
This is the most common area of concern in this code, as external transmission connections have the most corrosion problems.
If the P0842 code does return, we will need to test the TFPS sensor and associated circuits. With the key off, disconnect the electrical connector at the TFPS sensor. Connect the black lead from the digital voltmeter (DVOM) to the ground or low reference terminal on the TFPS sensor harness connector. Connect the red lead from the DVM to the signal terminal on the TFPS sensor harness connector. Turn on the engine, turn it off. Check manufacturer specifications; the voltmeter should read 12 volts or 5 volts. Rock the connections to see if they have changed. If the voltage is not correct, repair the power or ground wire or replace the PCM / TCM.
If the previous test was successful, connect one lead of the ohmmeter to the signal terminal on the TFPS sensor and the other lead to ground or the low reference terminal on 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 on the transmission fluid pressure sensor / switch while checking the resistance. If the ohmmeter reading does not pass, replace the TFPS.
If all previous tests have passed and you continue to receive P0842, it will most likely indicate a failed TFPS sensor, although a failed PCM / TCM and internal communication failures could not be ruled out until the TFPS sensor was replaced. If you are unsure, seek help from a qualified automotive diagnostician. To install correctly, the PCM / TCM must be programmed or calibrated for the vehicle.
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