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Technical Description of OBD-II Trouble Code
Low Input in Circuit B of Transmission Fluid Temperature Sensor
What does that signify?
This diagnostic trouble code (DTC) is a generic powertrain code that is applicable to OBD-II equipped vehicles with a transmission fluid temperature sensor (Jeep, Ford, Nissan, Toyota, Honda, Infiniti, Acura, Jaguar, Lexus, etc.). Although it is a generic code, the specific repair steps may vary depending on the make and model of the vehicle.
The transmission fluid temperature (TFT) sensor produces a signal that the powertrain control module (PCM) uses to determine shift points, line pressure, and torque converter clutch (TCC) control. The sensor is typically located in the transmission oil pan.
The TFT sensor receives a reference voltage (usually 5 volts) from the PCM. It then varies its internal resistance in relation to the transmission fluid temperature to send a return voltage signal to the PCM. TFT sensors are a type of negative temperature coefficient (NTC) thermistor. This means that the internal resistance of the sensor is inversely proportional to the oil temperature. The TFT sensor signal voltage decreases as the transmission fluid temperature increases, and vice versa.
Code P2742 is set when the PCM detects a low transmission fluid temperature sensor “B” signal. This usually indicates a shorted circuit. Consult a manufacturer-specific service manual to determine which circuit corresponds to “B” in your vehicle.
An example of a transmission fluid temperature sensor:
Related transmission fluid temperature sensor “B” circuit codes:
Code Severity & Symptoms
This code has a moderate to severe severity level. In some cases, it may indicate a transmission problem. It is advisable to address this code as soon as possible.
Symptoms of a P2742 engine code may include:
Causes
Possible causes for this trouble code may include:
Diagnostic and Repair Procedures
Begin by visually inspecting the transmission fluid temperature sensor and the corresponding wiring. Look for loose connections, damaged wiring, etc. If any damage is found, repair it as necessary, clear the code, and check if it reappears. Next, check for any technical service bulletins (TSBs) related to the issue. If no relevant information is found, proceed with step-by-step diagnosis of the system.
The following is a general procedure, as testing for this code varies between vehicles. To accurately test the system, refer to the manufacturer’s diagnostic flow chart.
Perform some preliminary circuit testing
Use a scan tool to monitor the transmission temperature sensor data parameter. Disconnect the TFT sensor; the scan tool value should drop to a very low value. Next, connect a jumper wire across the terminals. If the scan tool now displays a very high temperature, the connections are sound and the ECM can recognize the input. This indicates that the problem is most likely with the sensor and not a circuit or PCM problem.
Test the sensor
Disconnect the transmission fluid temperature sensor connector. Then, measure the resistance between the two sensor terminals using a digital multimeter set to ohms. Start the engine and observe the meter value; the values should decrease smoothly as the engine warms up (check the engine temperature gauge on the dashboard to ensure that the engine reaches operating temperature). If the engine temperature increases but the TFT resistance does not decrease, the sensor is faulty and should be replaced.
Check the circuit
Check the reference voltage side of the circuit: with the ignition on, use a digital multimeter set to volts to check for a 5-volt reference from the PCM at one of the two transmission fluid temperature sensor terminals. If no reference signal is present, connect the meter set to ohms (with the ignition off) between the reference voltage pin on the TFT and the reference voltage pin on the PCM. If the meter reads “out of limits” (OL), there is an open circuit between the PCM and sensor that needs to be located and repaired. If the meter reads a numeric value, there is continuity.
If everything is functioning properly up to this point, check that there is a 5-volt reference coming out of the PCM at the reference voltage terminal. If there is no 5-volt reference from the PCM, the PCM is likely faulty.
Check the ground side of the circuit
Connect the meter set to ohms (with the ignition off) between the ground terminal on the transmission fluid temperature sensor and the ground terminal on the PCM. If the meter reads “out of limits” (OL), there is an open circuit between the PCM and sensor that needs to be located and repaired. If the meter reads a numeric value, there is continuity. Finally, check that the PCM has a good ground by attaching one meter lead to the PCM ground terminal and the other to chassis ground. Once again, if the meter reads “out of limits” (OL), there is an open circuit between the PCM and ground that needs to be located and repaired.
If everything in the circuit checks out, there may be a problem with the transmission. This is especially true if the transmission fluid temperature codes are set along with other transmission codes.
Professional Service
While we always recommend seeking professional mechanical assistance to resolve this issue, if that is not possible, we offer the option to permanently delete this specific DTC. All other codes will continue to function normally. If you are interested in this option, please upload your ECU file to our portal.
