P0128 – Meaning, Causes, Symptoms, & Fixes

Possible Repair Costs for P0128

When it comes to making repairs associated with the P0128 code, one or more of the below repairs may be needed to solve the underlying issue. For each possible repair, the estimated cost of repair includes the cost of the relevant parts and the cost of labor required to make the repair.

  • Loose/damaged gas cap
  • EVAP hoses
  • Fuel tank
  • Charcoal canister
  • Faulty vent control valve
  • Faulty purge control valve

DIY Steps to Diagnose Code P0128

If you’d like to try to fix code P0128 at home without throwing money at parts, you’ll want to follow the steps below for proper diagnosis. Diagnosis can require some specialized equipment beyond what the FIXD Sensor can provide, but, on most vehicles, this is still a beginner-level diagnosis and repair for DIYers.

DIY difficulty level: Easy

This repair is easy enough for beginner-level DIYers to attempt.

Tools/parts needed (our top picks from Amazon):

  • FIXD
  • Basic socket and ratchet set
  • Multimeter
  • Funnel
  • Vehicle specific service manual
STEP 1: USE FIXD TO ENSURE NO OTHER ENGINE CODES ARE PRESENT.

Use FIXD to scan your vehicle to verify P0128 is the only code present. If other codes are present, they must be addressed first.

STEP 2: CHECK COOLANT.

Check the coolant level and condition. If there is excessive rust and poor coolant condition, this can clog up the cooling system or cause the thermostat to stick. If your coolant condition is poor, flush the coolant system and replace the coolant. If the coolant level is low, fill the coolant system and check for leaks.

STEP 3: INPSECT COOLANT TEMPERATURE SENSOR.

The coolant temperature sensor can be checked with a multimeter. The ohm reading will change with the temperature. If the ohm reading is not changing with the temperature, replace the coolant temperature sensor or repair the wiring for the sensor if it is damaged.

STEP 4: REMOVE AND INSPECT THERMOSTAT.

The most common cause for P0128 is the engine coolant thermostat is stuck open. A simple way to diagnose this is to feel the radiator hose and monitor how hot the temperature of the coolant is when it starts flowing through the radiator hose. Nevertheless, you should be extremely careful when doing this as you could be burned. The hose should be barely warm until the thermostat opens. When the thermostat does open, the hot coolant should start to flow and quickly warm up the radiator hose. If the radiator hose heats up slowly, the thermostat is stuck open or opening prematurely and needs to be replaced.

STEP 5: CONSULT A PROFESSIONAL.

If at this point the vehicle is still setting the same code, you may have a more serious problem with your car’s engine coolant system, and you should bring the vehicle to a certified shop to have further diagnostic work performed.

Common P0128 diagnosis mistakes

It is important to complete the entire diagnostic process when diagnosing P0128. Don’t overlook an easy cause such as a loose connector for the engine coolant temperature sensor or low and/or dirty coolant.

Still Need Help Fixing Code P0128?

If you’ve followed the steps above and are still experiencing issues with the coolant temp circuit and code P0128, please contact the FIXD Mechanic Hotline if you’re a FIXD Premium subscriber or find a RepairPal certified shop near you to get the right repairs at a fair price.

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