EZCarFix DIY Auto Repair

Symptom Guide

Heater Not Working

Freezing in your own car? The heater runs on engine heat. Here's the chain.

What's happening

The heater blows cold or lukewarm air even when the engine is warm. The heater is just a small radiator using engine heat, so the fault is in the heat supply.

How urgent is it? Low — fix at your convenience

Comfort issue in most climates, but low coolant causing it means the engine is also at risk. Check the level.

Most likely causes

  1. 1

    Low coolant level

    Air in the heater core means no heat. Top cause.

  2. 2

    Thermostat stuck open

    Engine never warms up fully; gauge reads low and heat is weak.

  3. 3

    Clogged heater core

    No flow through the core; both heater hoses won't get hot.

  4. 4

    Blend door failure

    Heat is available but the door won't route air through the core.

How to diagnose it — in order

  1. 1

    Check coolant level when cold; top up and watch for drops.

  2. 2

    Watch the temp gauge: never reaching normal means stuck-open thermostat.

  3. 3

    Feel both heater hoses at the firewall; both should be hot with the heat on.

  4. 4

    If heat changes with the temperature knob, the blend door is suspect.

Trouble codes tied to this symptom

P0128

If your scanner shows one of these, read its dedicated trouble-code page for the full diagnosis.

Repair guides for this symptom

Repair guide

2018-2024 Chevrolet Equinox - Thermostat Replacement

Step-by-step DIY for this repair.

Repair guide

2015-2020 Chevrolet Tahoe - Thermostat Replacement

Step-by-step DIY for this repair.

Repair guide

2010-2024 Toyota 4Runner - Thermostat Replacement

Step-by-step DIY for this repair.

Repair guide

2015-2019 Ford Explorer - Thermostat Replacement

Step-by-step DIY for this repair.

Related symptoms

Engine Overheating Bad Gas Mileage Sweet Smell / Coolant Leak

General diagnostic information, not a factory service procedure. If you're unsure, have a qualified technician confirm the diagnosis before buying parts.