P1521

What it means

On GM vehicles P1521 means the powertrain control module saw the transmission engage into a drive range (Drive or Reverse) while the throttle angle was high. This is often a driver-induced event, such as revving in Park or Neutral and then selecting a gear. It can also be set when the throttle position signal or the range (park/neutral) signal is wrong, so the module wrongly sees the two together.

Symptoms

• Harsh or sudden engagement when selecting Drive or Reverse
• Engine flaring or racing as the gear is selected
• Check engine light on, possibly with a transmission warning
• Erratic or unstable throttle position reading on a scan tool
• Delayed or inconsistent gear selection or range indication

Possible causes

• Driver selecting Drive or Reverse with the throttle held open (revving in Park or Neutral then dropping into gear)likely
• Faulty or erratic throttle position sensor signal or throttle body fault giving a false high throttle anglepossible
• Transmission range (park/neutral) switch out of adjustment or faulty, so gear selection is misreportedpossible
• Wiring or connector fault on the throttle position or range switch circuits, such as corrosion, chafing or poor terminal tensionpossible
• Sticking throttle cable or linkage, or carbon build-up holding the throttle open at idle (depends on engine and model)possible
• Internal transmission fault causing abnormally harsh or high-pressure engagementrare
• Outdated control module calibration where a known software update addresses this code for the specific modelrare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including throttle angle, engine speed, gear range and vehicle speed when the code set
2. Ask the driver whether the engine is revved in Park or Neutral before selecting a gear, and clear the code to see if it returns with normal driving
3. Inspect the throttle position sensor, range switch and related wiring and connectors for damage, corrosion, moisture or loose terminals
4. With a scan tool, watch the throttle position reading from closed to wide open and check it is smooth, with no spikes or dropouts and a correct idle value
5. Compare the range selector position shown on the scan tool with the actual lever position in each gear, and check the range switch alignment against the model's procedure
6. Check the throttle body, cable or linkage for sticking, binding or excess carbon, and that the throttle returns fully to idle
7. Check transmission fluid level and condition, and look for any related transmission codes
8. Check for a calibration update or technical bulletin for the specific model and engine

How to fix

• Advise the driver to let the engine settle to idle before selecting a gear, then clear the code and confirm it does not return
• Replace or repair a faulty throttle position sensor or throttle body, and carry out any relearn the model requires
• Adjust or replace the transmission range switch following the model's procedure
• Repair damaged wiring, connectors or terminals on the throttle and range switch circuits
• Free, clean or replace a sticking throttle linkage or cable, and clean the throttle body where the manufacturer allows
• Update the control module calibration only if a bulletin confirms it as the fix, or repair the transmission if testing shows an internal fault

Can I keep driving?

This code for other makes

• P1521BuickTransmission Engaged At High Throttle Angle
• P1521CadillacTransmission Engaged At High Throttle Angle
• P1521ChevroletTransmission Engaged At High Throttle Angle
• P1521ChryslerTransmission Engaged At High Throttle Angle
• P1521GMCTransmission Engaged At High Throttle Angle
• P1521OldsmobileTransmission Engaged At High Throttle Angle
• P1521OtherVariable Intake Solenoid #1 Circuit Malfunction
• P1521SaturnTransmission Engaged At High Throttle Angle
• P1521VolkswagenIntake Manifold Changeover Valve 2 Circuit Electrical Malfunction

Nearby codes

• P1520GMGear Indicator System
• P1522GMPark Neutral To Drive Reverse At High RPM