AutoCodes – GMC – P18BC GMC

P18BC GMC Code - Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range

Quick Answer

The internal mode switch indicates the Transmission Control Module (TCM) which gear position the vehicle operator has selected. Common causes include Faulty Transmission Internal Mode Switch, Transmission Internal Mode Switch harness is open or shorted, Transmission Internal Mode Switch circuit poor electrical connection. 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: P18BC GMC indicates Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range.
Common causes: Common causes include Faulty Transmission Internal Mode Switch, Transmission Internal Mode Switch harness is open or shorted, Transmission Internal Mode Switch circuit poor electrical connection.
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.

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Code Popularity 1,267

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Mar 23, 2024

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code P18BC GMC Description

The internal mode switch indicates the Transmission Control Module (TCM) which gear position the vehicle operator has selected. The internal mode switch consists of 5 separate hall effect switches. Each hall effect switch is supplied with a 9 V reference circuit and a signal circuit from the TCM. Each signal circuit for each gear selector position will have either a voltage reading of 0.70–0.96 V indicating On or 1.68–2.38 V indicating Off. The voltage values on each internal mode switch circuit will change and are dependent on the position of the gear selector. The state of each internal mode switch A/B/C/P/S signal circuit is displayed on the scan tool. The Diagnostic Trouble Code (DTC) will be set when the TCM has detected the internal mode switch C signal circuit has an invalid voltage range of 1.68 – 2.38 V, for 2.7 s.

OBDII Code P18BC GMC - Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range - AutoCodes.com — P18BC GMC Code - Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range

What does this mean?

What are the Possible Causes of the DTC P18BC GMC?

Faulty Transmission Internal Mode Switch
Transmission Internal Mode Switch harness is open or shorted
Transmission Internal Mode Switch circuit poor electrical connection
Faulty Transmission Control Module (TCM)

How to Fix the DTC P18BC GMC?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P18BC GMC code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the P18BC GMC code?

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Information for Specific GMC Models

P18BC 2014 GMC Sierra — Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range
P18BC 2015 GMC Sierra — Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range
P18BC 2016 GMC Sierra — Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range
P18BC 2017 GMC Sierra — Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range
P18BC 2018 GMC Sierra — Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range

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.

Frequently Asked Questions about P18BC GMC Code

1. What are common symptoms of OBDII code P18BC GMC?

Common symptoms include issues with shifting gears, the vehicle not starting, and the transmission going into limp mode.

2. What are possible causes of the Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range code?

Possible causes include a faulty internal mode switch, damaged wiring or connectors, issues with the transmission control module, or a low battery voltage.

3. How can I diagnose the OBDII code P18BC GMC at home?

You can diagnose this code using an OBDII scanner to read the fault codes, checking the internal mode switch and related wiring for any visible damage, and testing the battery voltage.

4. Can driving with the P18BC code cause further damage to my vehicle?

Continuing to drive with this code present can potentially cause damage to the transmission or other related components, so it is recommended to address the issue promptly.

5. Is it safe to continue driving my vehicle with the Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range code?

It is not recommended to drive the vehicle with this code, as it can lead to transmission issues and potentially unsafe driving conditions.

6. How can I repair the OBDII code P18BC GMC myself?

You can attempt to repair this code by replacing the internal mode switch, checking and repairing any damaged wiring or connectors, or resetting the transmission control module.

7. Will resetting the check engine light fix the P18BC code?

Simply resetting the check engine light will not fix the underlying issue causing the P18BC code. It is important to diagnose and address the root cause of the problem.

8. Can a mechanic help me repair the Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range code?

Yes, a qualified mechanic can assist in diagnosing and repairing the P18BC code, ensuring that the issue is properly resolved.

9. How much does it typically cost to repair the OBDII code P18BC GMC at a repair shop?

The cost of repairing this code can vary depending on the specific issue causing it, but it may range from a few hundred to over a thousand dollars, including parts and labor.

10. Is the OBDII code P18BC GMC a common issue in GMC vehicles?

While it may not be extremely common, the Internal Mode Switch 'C' Circuit Stuck Off Invalid Voltage Range code can occur in certain GMC vehicles, especially those with transmission control module issues.