What Does Code P0128 Mean?
- GM P0128 definition: Coolant Thermostat Temperature Below Regulating Temperature
- Issue Severity: LOW– Continued driving for a short period of time is okay.
- Repair Urgency: Get this code fixed withing the next month to prevent the potential for overheating.
- Diagnosis: It is important to complete the entire diagnostic process when diagnosing the GM P0128 code. This DTC can be triggered by a missing or stuck open thermostat, a faulty coolant temp sensor, or damaged wiring to the coolant temp sensor circuit.
- One of the top 5 most common causes of a check engine light in 7 out of the 27 GMC models we have data on.
The engine control module (ECM) tracks how long it takes for the engine of a vehicle to reach and maintain the correct operating temperature. When the correct engine operating temperature is reached, the powertrain control module orders the fuel system to enter a “closed-loop” where readings from the oxygen sensors are used to maintain the efficient air to fuel ratio of 14.7:1. Most ECMs mandate that the engine coolant temperature sensor record temperatures above 160º F within 15 minutes of the engine starting. Additionally, once the 160º F threshold is achieved, the recorded engine temperature must not fall below 160º F during operation. The PCM will record if either of these two criteria is not met. If either fault is recorded again on the next engine startup, GM P0128 code is triggered.
- Faulty/stuck open thermostat (most common)
- Missing Thermostat
- Faulty coolant temperature sensor
- Faulty wiring for coolant temperature circuit
- Check Engine Light is on
- Higher than normal idle
- Decreased fuel economy
- The temperature gauge is unusually low
How Do I Fix Code P0128?
With a GM P0128 code, the first step is to get it properly diagnosed to figure out what is causing the malfunction in the coolant temperature circuit. Using the live data function of the FIXD sensor and app allows you to read and analyze engine temperatures to properly diagnose a P0128 code.
If your engine is running lean and you’re not comfortable diagnosing this issue at home, we recommend finding a RepairPal-certified shop to pinpoint the problem and give an accurate estimate for repairs.
These shops can not only help you figure out what’s going wrong before you waste time and money on the wrong parts, but they also offer a minimum 12-month, 12,000-mile warranty and stand behind all their estimates with guaranteed fair pricing.
> Find a RepairPal Certified Shop Near You
How Much Does It Cost to Fix Code P0128?
If you take your car to a shop for diagnosis, most shops will start with an hour of “diag time” (the time spent in labor diagnosing your specific issue). Depending on the shop’s labor rate, this typically costs somewhere between $75-$150. Many, if not most, shops will apply this diagnosis fee to any required repairs if you have them perform the repairs for you. From there, a shop will be able to give you an accurate estimate for repairs to fix your GM P0128 code.
Possible Repair Costs for P0128
When it comes to making repairs associated with the GM 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):
- Basic socket and ratchet set
- 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 GM P0128 code, 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.
- Get Your Code Fixed Fast
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Fixed It But The Check Engine Light Is Still On?
Check engine light sometimes need to be reset manually, check out our article:
How to Reset Your Check Engine Light | 4 Ways To Clear It (With or Without a Scanner)
Lifelong automotive enthusiast with a soft spot for offroading. Wrencher turned writer, but I still love to tinker on just about anything with an engine. Dream car: tie between a ‘71 Hemi ‘Cuda and a ’91 GMC Syclone. #GirlDad #SaveTheManuals