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Engine-Codes – Generic – P18AC

P18AC Code - Transmission Range Control Valve 2 Position Switch Circuit Stuck On

Quick Answer

The P18AC code indicates that there is a malfunction in the Transmission Range Control Valve 2 Position Switch Circuit, specifically that it is stuck in the on position. Common causes include Low transmission fluid levels, leading to inadequate hydraulic pressure, Faulty Transmission Range Control Valve 2 Position Switch, Open or shorted wiring in the Transmission Range Control Valve 2 Position Switch harness. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means
P18AC indicates Transmission Range Control Valve 2 Position Switch Circuit Stuck On.
Common causes
Common causes include Low transmission fluid levels, leading to inadequate hydraulic pressure, Faulty Transmission Range Control Valve 2 Position Switch, Open or shorted wiring in the Transmission Range Control Valve 2 Position Switch harness.
Severity
High - diagnose this code as soon as possible.
Can I drive?
Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.
What is this?
Table of contents Created and updated by William Orellana on Oct 02, 2025
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Code P18AC Description

The P18AC code indicates that there is a malfunction in the Transmission Range Control Valve 2 Position Switch Circuit, specifically that it is stuck in the on position. This switch is crucial for the proper operation of the transmission system, as it communicates the current gear selection to the Transmission Control Module (TCM). The TCM relies on accurate input from various sensors, including the automatic transmission input and output speed sensors, throttle position sensor, and others, to manage shifting and optimize performance. When the switch is stuck, the TCM may not receive the correct signals, leading to improper gear selection and shifting issues.
OBDII Code P18AC - Transmission Range Control Valve 2 Position Switch Circuit Stuck On
P18AC Code - Transmission Range Control Valve 2 Position Switch Circuit Stuck On

Common Causes of P18AC

  1. Low transmission fluid levels, leading to inadequate hydraulic pressure
  2. Faulty Transmission Range Control Valve 2 Position Switch
  3. Open or shorted wiring in the Transmission Range Control Valve 2 Position Switch harness
  4. Poor electrical connection within the Transmission Range Control Valve 2 Position Switch circuit
  5. Defective Control Valve Solenoid Body affecting transmission operation

Symptoms of P18AC

  1. Engine Light ON (or Service Engine Soon Warning Light)
  2. Erratic or harsh shifting between gears
  3. Transmission may get stuck in a single gear
  4. Decreased fuel efficiency
  5. Unusual noises from the transmission during operation

How to Fix P18AC

  1. Begin by scanning the vehicle's onboard computer to confirm the P18AC code and check for any additional codes.
  2. Inspect the transmission fluid level and condition; top off or replace if necessary.
  3. Examine the Transmission Range Control Valve 2 Position Switch and its wiring for any signs of damage, corrosion, or disconnection.
  4. Test the switch for proper operation using a multimeter; replace it if it is found to be faulty.
  5. If the switch is functioning correctly, inspect the Control Valve Solenoid Body and replace it if it shows signs of failure.

Cost to Fix P18AC

The cost to diagnose and repair the P18AC code can vary significantly based on the specific issue and the vehicle make and model. Typical parts, such as the Transmission Range Control Valve 2 Position Switch, may range from $50 to $150, while labor costs can vary from $80 to $150 per hour depending on the shop type—dealerships usually charge more, while independent shops may offer lower rates. Overall, you might expect to spend between $200 and $600 for a complete diagnosis and repair, depending on the extent of the repairs needed.

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Code P18AC Information for Specific Makes

  • P18AC Buick — Transmission Range Control Valve 2 Position Switch Circuit Stuck On
  • P18AC Cadillac — Transmission Range Control Valve 2 Position Switch Circuit Stuck On
  • P18AC Chevrolet — Transmission Range Control Valve 2 Position Switch Circuit Stuck On
  • P18AC GMC — Transmission Range Control Valve 2 Position Switch Circuit Stuck On
About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

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