AutoCodes – Scion – P0128 Scion

P0128 Scion Code - Coolant Thermostat

Quick Answer

The ECM estimates the engine coolant temperature based on the starting temperature, engine loads, and engine speeds. Common causes include Leaking or stuck open thermostat, Low engine coolant level, Insufficient warm up time. 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: P0128 Scion indicates Coolant Thermostat.
Common causes: Common causes include Leaking or stuck open thermostat, Low engine coolant level, Insufficient warm up time.
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 14,454

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Intermediate (2 / 3)

What is this?

Table of contents Created and updated by William Orellana on Aug 23, 2022

Description
Step-by-Step Diagnostic Guide
Possible Causes
How To Fix
Tech Notes
Cost to Diagnose
Comments
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Code P0128 Scion Description

The ECM estimates the engine coolant temperature based on the starting temperature, engine loads, and engine speeds. The Engine Control Module (ECM) then compares the estimated temperature with the actual engine coolant temperature. When the estimated engine coolant temperature reaches 75°C (167°F), the ECM checks the actual engine coolant temperature. If the actual engine coolant temperature is less than 75°C (167°F), the ECM interprets this as a malfunction in the thermostat or the engine cooling system and stores the Diagnostic Trouble Code (DTC).

OBDII Code P0128 Scion - Coolant Thermostat - AutoCodes.com — P0128 Scion Code - Coolant Thermostat

What does this mean?

What are the Possible Causes of the DTC P0128 Scion?

Leaking or stuck open thermostat
Low engine coolant level
Insufficient warm up time
Faulty Engine Coolant Temperature (ECT) sensor
Engine Coolant Temperature sensor harness is open or shorted
Engine Coolant Temperature sensor electrical circuit poor connection
Faulty Engine Control Module (ECM)

How to Fix the DTC P0128 Scion?

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 does this mean?

Code Tech Notes

If the cooling system is not leaking and the fluid level is OK, replacing the thermostat usually takes care of the problem.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the P0128 Scion 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 P0128 Scion code?

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Information for Specific Scion Models

P0128 2005 Scion xB — Coolant Thermostat
P0128 2006 Scion xB — Coolant Thermostat
P0128 2007 Scion xB — Coolant Thermostat

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.