# P0128 — Coolant Temperature Below Thermostat Regulating Temperature

> ClearTheCode diagnostic code reference (not OEM service data).

Source: https://clearthecode.com/codes/P0128

**System:** Powertrain · Cooling System

**Severity:** minor

## Description

Engine isn't reaching operating temperature in expected time. Almost always a stuck-open thermostat. Replace the thermostat.

## Diagnostic snapshot

- **Difficulty:** 6/10 — Intermediate diagnostics
- **Est. repair cost:** $120 – $450

## Diagnostic workflow

1. Confirm P0128 with a live scan — note pending vs stored and freeze frame data.
2. Inspect wiring/connectors and related sensors before replacing modules.
3. Clear codes and road-test; re-scan after two drive cycles if the monitor must set.

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