P0991 code definition
A P0991 trouble code means that the Powertrain Control Module (PCM) has detected a problem in the transmission fluid pressure sensor circuit. The transmission fluid pressure sensor helps with shift actuation and proper torque converter operation though use of electronic pressure control solenoids controlled by the PCM. Related codes include P0987, P0988, P0989, and P0999 codes.
What the P0991 code means
A P0991 trouble code means that the PCM has detected a variation in the transmission fluid pressure out of the normal range as determined by the manufacturer. The result is a stored P0991 trouble code and the Check Engine Light illuminating. The required pressure is determined by the PCM using a combination of factors, including the position of the throttle, vehicle speed, engine load, and engine speed.
What causes the P0991 code?
Some common causes behind the storage of a P0991 trouble code includes:
- Malfunctioning electronic pressure control solenoid
- Transmission fluid pressure sensor
- Mechanical failure of the transmission
- Blockage within the hydraulic fluid passages within the transmission
- Dirty or contaminated transmission fluid
- Low transmission fluid condition
What are the symptoms of the P0991 code?
Symptoms include failure of the torque converter to operate properly, the PCM placing the transmission in Limp-in mode, and various transmission problems. These problems include trouble shifting, the transmission slipping, and the transmission overheating. Vehicle owners can also expect a reduction in their vehicles fuel economy in some cases.
How does a mechanic diagnose the P0991 code?
The mechanic needs to have a scanner, digital volt/ohmmeter, manual pressure gauge, hydraulic pressure diagram, and a manufacturer's wiring diagram to properly diagnose a P0991 trouble code. In addition, the mechanic needs to complete the following steps:
Pull out the transmission dipstick and inspect the fluid, paying particular attention to the fluid's smell and condition
If the fluid smells burned or looks strange, drop the transmission pan to inspect it for material from the clutch or other debris, the presence of which signifies the need for a rebuild
Next, if the transmission is low on fluid, the mechanic should fill it to an appropriate level to find any leaks
Inspect the wiring, connectors, and components, looking for anything damaged, corroded, or shorted
Connect the scanner and download any stored codes and freeze frame data. Clear any codes and fill the transmission fluid to the proper level
Start the vehicle and allow it to reach a normal operating temperature before test driving it to test the system and see if the code returns
If the code returns, use the manual pressure pump and the hydraulic pressure diagram to check the pressure level of the hydraulic pump. The mechanic should check their findings against the manufacturer's specifications to see if the pump has gone bad
If the code still persists after replacing the pump, check the reference voltage and ground signal at the transmission fluid pressure sensor and the electronic pressure control solenoid. The mechanic should also disconnect any related control modules to prevent damage or memory loss before testing the continuity and resistance of any open circuits
Clear the code again and if it returns, check the PCM for fault
Clear the P0991 trouble code and retest the system again to see if the code returns
Common mistakes when diagnosing the P0991 code
The most common mistake made by mechanics when diagnosing a P0991 trouble code is to mistakenly replace a high pressure pump instead of faulty electronic pressure control solenoids, transmission pressure sensors, and wiring where the problem really lies.
How serious is the P0991 code?
A P0991 trouble code can result in general problems with a vehicle's transmission, including slipping, trouble shifting, and even failure in extreme cases. Most often, transmission problems are directly related to dirty or contaminated transmission fluid or a lingering low fluid condition.
What repairs can fix the P0991 code?
Once the mechanic has diagnosed the P0991 trouble code, they can attempt to repair it by taking the following steps:
Start by rebuilding the transmission and instal a new torque converter if necessary
The mechanic should then fill the transmission fluid to a level that allows them to repair any leaks
If the hydraulic pump is not working properly, the mechanic should replace it. Hydraulic pump replacement requires the mechanic to remove the transmission and partially disassemble it to the reach the pump
Replace any faulty transmission fluid pressure sensors
Replace the electronic pressure control solenoids.
Replace the PCM if it has gone bad.
Need help with a P0991 code?
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