P0731 OBD-II Trouble Code: Gear 1 Incorrect Ratio

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Cost of diagnosing the P0731 code

P0731 is a diagnostic trouble code (DTC) for "Gear 1 Incorrect Ratio". This can happen for multiple reasons and a mechanic needs to diagnose the specific cause for this code to be triggered in your situation. Our certified mobile mechanics can come to your home or office to perform the Check Engine Light diagnostic. Once we are able to diagnose the problem, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. All our repairs are backed by our 12-month / 12,000-mile warranty.

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P0721 code definition

When the automatic transmission in a vehicle cannot shift into first gear, the PCM (powertrain control module) will detect and store a P0721 code, and the Check Engine light will illuminate.

What the P0721 code means

An automatic transmission is designed to detect the driver’s acceleration and deceleration patterns, and to use that information to match the engine’s best torque and power characteristics to select the appropriate gear. The P0721 code occurs when the PCM detects a discrepancy between the speeds of first gear’s input RPM sensor and transmission output RPM sensor. As a result, the vehicle cannot shift into first gear, and it could indicate that the transmission is slipping.

What causes the P0721 code?

A number of scenarios can lead to a P0721 code being detected and stored. These include:

  • Insufficient or contaminated transmission fluid
  • A defective shift solenoid
  • A mechanical problem or blockage within the transmission
  • A defective transmission control module (TCM)
  • A shorted or open transmission control circuit

What are the symptoms of the P0721 code?

In addition to the Check Engine light coming on, drivers may notice a few other symptoms:

  • Inability to start the vehicle in first gear.
  • Slipping transmission
  • Lack of acceleration
  • Decrease in fuel efficiency
  • Inefficient or incorrect shifting

How does a mechanic diagnose the P0721 code?

After identifying the P0721 code with an OBD-II scanner, the mechanic will first check the level and quality of the transmission fluid. If the fluid is low, they will add more and retest the system. If the transmission fluid appears dirty or contaminated, and/or it smells burnt, the mechanic will flush the system, change the transmission fluid, clear the code, and retest once more.

They will then move on to inspect and test the transmission for internal mechanical problems, circuit problems, and issues with the shift solenoid. After each repair, they will clear all codes and retest the vehicle to see if the P0721 code appears again.

Common mistakes when diagnosing the P0721 code

The P0721 code is often misdiagnosed as an engine misfiring issue or a problem with the vehicle’s driveline. If the system is not thoroughly inspected and diagnosed, it can also look like an internal transmission problem when it’s actually a much simpler and less expensive issue.

How serious is the P0721 code?

If your transmission is unable to shift into or out of first gear, the vehicle can become inoperable in very short order. Symptoms may worsen as you continue to drive without resolving the P0721 code, and this can lead to dangerous driving situations, such as being unable to accelerate to get out of the way of oncoming traffic.

What repairs can fix the P0721 code?

Some of the most common repairs to resolve a P0721 code include:

  • Adding more transmission fluid or changing the transmission fluid.
  • Replacing a defective shift solenoid.
  • Replacing or reprogramming a defective TCM.
  • Replacing wiring and/or connectors for a damaged transmission control circuit.

Additional comments for consideration regarding the P0721 code

A detected P0721 code is a serious issue that should be resolved as soon as possible, even if the vehicle is still driving relatively normally. Failure to shift into first gear can result in a number of other transmission problems, and if the problem is caused by a slipping transmission, further shifting problems can be expected soon if this one isn’t dealt with.

In addition to making the car unreliable and/or unsafe to drive, a P0721 code will always be accompanied by a Check Engine light. An Illuminated Check Engine light will prevent your vehicle from passing an OBD-II emissions test, so it’s essential that you get the problem resolved before you need to pass an emissions test to renew your vehicle’s registration.

Need help with a P0721 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6220.

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P0731
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