AutoCodes – GMC – P1257 GMC

P1257 GMC Code - Supercharger System Overboost

Quick Answer

With the engine running, the Intake Air Temperature (IAT) sensor input more than 14°F, then the Powertrain Control Module (PCM) detects that the engine torque was above a maximum threshold value (value vanes by commanded gear and rpm) for 6 seconds. Common causes for P1257 GMC include wiring or connector problems, a faulty related sensor or actuator, and control module or calibration issues. 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: P1257 GMC indicates Supercharger System Overboost.
Common causes: Common causes for P1257 GMC include wiring or connector problems, a faulty related sensor or actuator, and control module or calibration issues.
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 4,092

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 Jul 24, 2019

Description
How To Fix
Cost to Diagnose
FAQs
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Code P1257 GMC Description

OBDII Code P1257 GMC - Supercharger System Overboost - AutoCodes.com — P1257 GMC Code - Supercharger System Overboost

How to Fix the DTC P1257 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 P1257 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 P1257 GMC code?

Need more information on how to fix the P1257 GMC code? Get Access to Factory Service Manuals

Need Help with the Code?

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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 P1257 GMC Code

1. What does the OBDII code P1257 mean on a GMC vehicle?

The P1257 code indicates a "Supercharger System Overboost," meaning the engine is receiving more boost pressure from the supercharger than specified by the manufacturer.

2. What are the common symptoms of the P1257 code in a GMC?

Symptoms may include reduced engine performance, a lit check engine light, engine knocking or pinging, and possible engine misfires.

3. What causes the P1257 code to appear on a GMC?

Common causes include a faulty boost pressure sensor, a stuck or malfunctioning bypass valve, issues with the supercharger, or damaged vacuum lines.

4. Can I drive my GMC with the P1257 code?

It’s not recommended, as excessive boost pressure can cause engine damage or lead to severe knocking or overheating.

5. How do I diagnose the P1257 code on a GMC?

Start by checking the boost pressure sensor, bypass valve, and vacuum lines for damage or leaks. Use a scan tool to monitor boost pressure levels and compare them to manufacturer specifications.

6. How can I fix the P1257 code on my GMC?

Possible repairs include replacing a defective boost pressure sensor, repairing or replacing vacuum lines, cleaning or replacing the bypass valve, or inspecting the supercharger for mechanical issues.

7. Does a faulty supercharger cause the P1257 code?

Yes, a malfunctioning supercharger, such as a stuck rotor or damaged components, can result in excess boost pressure and trigger the P1257 code.

8. What tools are needed to troubleshoot the P1257 code on a GMC?

You’ll need an OBDII scan tool, a vacuum gauge, and possibly a multimeter to test electrical components like the boost pressure sensor.

9. How much does it cost to fix the P1257 code on a GMC?

Repair costs vary depending on the issue, ranging from $50 for vacuum line repairs to $1,500 or more for supercharger replacement.

10. Can a software update fix the P1257 code on a GMC?

In some cases, a software update from the manufacturer may address calibration issues with the boost pressure system, but this depends on the specific vehicle and circumstances.