Transmission Fluid Pressure Sensor/Switch “C” Circuit Low
This code indicates that the PCM has detected a transmission fluid pressure which is not within a predetermined limit. When this occurs, a code is stored and a malfunction indicator illuminated. Some vehicles will also enter “limp-in” mode
Code Set Parameters
Transmission fluid pressure levels that exceed (or fall below) a predetermined limit, set by the vehicle manufacturer, will cause a code to be stored in the PCM and a service engine soon lamp to be illuminated.
A service engine soon lamp is usually accompanied by the distinctive smell of overheated transmission fluid. Smoke from the transmission area is also possible, as is transmission slippage, or lack of transmission engagement.
Common causes include a low transmission fluid condition (caused by a transmission fluid leak), a defective transmission high pressure pump, a faulty or slipping transmission, a faulty transmission temperature sensor, overheating engine, or faulty PCM. When this code is set, it is imperative that the transmission be checked prior to further vehicle operation.
Misdiagnosis may range from one extreme to the other. Often, when this code is stored, a problem is incorrectly diagnosed as only a transmission fluid leak when a low fluid condition has allowed vital internal transmission components to be damaged. On the other end, the transmission does not necessarily need to be rebuilt just because it has overheated once.
- The automatic transmission is equipped with a fluid pressure sensor (usually a two-wire variable resistance sensor) which provides the PCM with data
- The sensor has a voltage wire (typically it is a 5-volt reference signal) and a ground wire
- The PCM uses the reference voltage wire to monitor transmission temperature
- As transmission fluid pressure increases, sensor resistance decreases and reference voltage increases
- When transmission fluid pressure is low, sensor resistance is high, driving reference voltage to the PCM down
- The PCM receives input reference voltage readings as transmission pressure variations and reacts accordingly
- If transmission fluid pressure is either lower or higher than desired, a code is stored and a service engine soon lamp is illuminated.
If the transmission fluid pressure code is set and the engine is not overheating, then it is very likely that a low or substandard transmission fluid condition is present
- To perform a thorough diagnosis, any transmission leaks should be repaired and the transmission should be filled with the proper amount and type of fluid (if the fluid smells really “burnt” and you are experienced enough to detect it, you may go ahead and condemn the transmission)
- The transmission should then be operated normally and rechecked
- However, leaks that originate from the front seal (behind the torque converter) will likely require removal of the transmission, at which time you may want to consider a transmission rebuild if the mileage dictates.
If the transmission appears to be in good working order and doesn’t appear to be overheating, suspect a faulty transmission fluid pressure sensor
There are several tools which will be instrumental in attempting to successfully diagnose the conditions which contribute to this code being stored
- A suitable OBD-II scanner (or code reader) and a digital volt/ohmmeter will be most helpful in trying to perform a successful diagnosis
- Gaining access to access a manufacturer’s wiring schematic will also prove to be necessary to successfully diagnosing this code.
Perform a careful visual inspection of all PCM wiring and connectors
- Repair or replace damaged, disconnected, shorted, or corroded wiring, connectors, and components as necessary
- Always retest the system after repairs are completed to ensure success.
If all system wiring, connectors, and components (Including fuses) appear to be in normal working order, connect the scanner (or code reader) to the diagnostic connector and record all stored codes and freeze frame data
- This information can be extremely helpful in diagnosing intermittent conditions that may have contributed to this code being stored
- Continue by unplugging the electrical connector from the transmission temperature sensor and testing for reference voltage
- This is usually 5-volts but consult your manufacturer’s service manual to be sure
- If there is a 5-volt signal present with the ignition turned to the run position, then check the sensor ground wire
- If both the reference signal and the ground signal are present, test the sensor resistance using the manufacturer’s temperature to resistance chart
- Compare your findings to the manufacturer’s specifications and replace the sensor if it fails to comply.
If the sensor and voltage at the sensor are within acceptable specifications, disconnect the PCM electrical connector and test circuit continuity and resistance between the sensor and the PCM
- Repair or replace system circuitry and components as required and retest the system.
Should all system circuitry and sensors check out, suspect a faulty PCM but keep in mind that PCM failure is rare and PCM replacement will require reprogramming.