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P1521

Transmission Engaged At High Throttle Angle.

  • Buick
  • Powertrain
  • Manufacturer code
  • Low severity

How this code is built

P
SystemPowertrain
1
Defined byThe manufacturer
5
AreaVehicle speed, idle control and auxiliary inputs
21
FaultThe specific fault in that area

What it means

On Buick models, P1521 means the powertrain control module detected the transmission being put into gear (for example from Park or Neutral into Drive or Reverse) while the throttle angle was high. This is mainly a record of an abusive or abnormal engagement event rather than a typical electrical fault. It can also be set if the throttle position signal is wrong or the engine idle is unusually high at the moment of engagement.

Symptoms

  • Harsh or sudden engagement when selecting Drive or Reverse
  • Engine revving high as the gear is selected
  • Malfunction indicator lamp on, or a stored code with no other symptoms
  • High or fluctuating idle speed
  • Possible clunk from the driveline on engagement

Possible causes

  • Driver selecting Drive or Reverse with the throttle pressed or the engine revved (the event the code is designed to record)likely
  • Throttle position sensor giving a false high reading or erratic signalpossible
  • Sticking or dirty throttle body, or throttle linkage or cable not returning to idlepossible
  • High idle caused by an air leak, idle control fault or vacuum leakpossible
  • Faulty or misadjusted park/neutral or gear selector switch signal making the PCM misjudge when the gear is engagedpossible
  • Wiring or connector fault on the throttle position sensor circuit, or a PCM faultrare

How to diagnose

  1. Read the stored codes and freeze frame data, noting throttle angle, engine speed and gear selection at the time of the fault
  2. Ask the driver about habits such as revving in Park or Neutral before engaging a gear, then clear the code and see if it returns under normal use
  3. Check for other stored throttle, idle, or transmission codes and deal with those first
  4. Inspect the throttle body, linkage or cable, and wiring and connectors on the throttle position sensor for dirt, damage or sticking
  5. With a scan tool, watch the throttle position reading at closed throttle and while moving the pedal slowly, looking for a wrong idle value or glitches
  6. Check engine idle speed and look for vacuum or intake air leaks
  7. Verify the park/neutral or gear selector switch signal matches the lever position and check its adjustment
  8. Check the transmission fluid level and condition, as harsh engagement may have other causes; procedures vary by model and transmission

How to fix

  • Advise the driver to select gear at idle with the foot off the throttle, and clear the code
  • Clean the throttle body or free up and adjust a sticking linkage or cable as the manufacturer specifies
  • Replace a faulty throttle position sensor or repair damaged wiring and connectors
  • Repair any vacuum or intake air leaks and correct high idle faults
  • Adjust or replace the park/neutral or gear selector switch if its signal is wrong
  • Correct transmission fluid level or fault if it is contributing to harsh engagement

Can I keep driving?

Low severity

The car can normally be driven while you get it checked, as the code usually records an abnormal engagement event. If engagement is harsh or the idle is high, avoid revving when selecting gear and have it looked at soon to prevent driveline wear.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.

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