P0128 on Chrysler: How to Diagnose & Fix
Coolant Thermostat Below Regulating Temperature
6 Chrysler models · 9 documented issues · 8 critical · $100-$8,000 repair
P0128 on Chrysler by model: symptoms, diagnosis and fix
Water pumps fail prematurely due to bearing and seal wear. Air pockets frequently form in the cooling system after repairs, and bleeding air from the 300's cooling system is difficult.
Common Symptoms
- Coolant leak from front of engine or below water pump
- Engine overheating or running hot
- Temperature gauge fluctuating
- Whining or grinding noise from water pump
How to Fix
Replace the water pump with new gaskets. Replace the thermostat at the same time, per TSB. A complete coolant flush and proper bleeding are essential. Use the purge valve in front of upper intake manifold.
Sources for this issue (1)
PT Cruisers are extremely prone to overheating from multiple causes: radiator fan relay failure, fan motor failure, thermostat sticking closed, radiator clogging, and water pump failure. The high-speed fan relay is the most frequent single failure point. Overheating leads to head gasket failure. This is one of the most universally reported problems across all model years.
Common Symptoms
- Temperature gauge climbing above normal, especially at idle
- A/C compressor disengaging (safety cutoff from high temps)
- Radiator fan not running with engine hot
- Steam or coolant smell from engine bay
- Check engine light with coolant temp codes
How to Fix
Diagnose root cause systematically: check fan relay (5149541AA) first, then fan motor, thermostat (Stant 45359), water pump, and radiator. Replace radiator cap (frequently causes overpressure issues). Cooling fan module 5017407AB for complete fan assembly replacement.
What you need to fix it
Only diagnosis- and fitment-reviewed repair parts are linked here.
- Radiator CapCross-ref31518
Fitment confirmed against the ShowMeTheParts catalog for this vehicle's year and engine.
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Sources for this issue (3)
Head gasket failure is common, especially on turbo models. The aluminum cylinder head is susceptible to pitting from acidic coolant which compromises the gasket seal. Turbo models experience higher cylinder pressures that accelerate failure. Overheating from cooling system issues frequently triggers head gasket failure.
Common Symptoms
- White smoke from exhaust (coolant entering combustion)
- Coolant loss with no visible external leak
- Engine overheating
- Milky residue on oil cap (coolant in oil)
- Bubbling in coolant overflow tank
How to Fix
Replace head gasket with Fel-Pro MLS gasket. HS 9946 PT for 2001, HS26206PT-1 for 2002-2009, or individual gasket 26206PT for 2002-2010. Machine cylinder head surface if pitting present. Replace thermostat and flush cooling system. Check head for warpage before reinstalling.
Sources for this issue (2)
The turbocharger on GT and turbo models fails from oil starvation, wastegate actuator failure, and bearing wear. Failures reported as early as 60,000-78,000 miles. Turbo failure is often followed by head gasket failure within months due to changes in combustion pressure dynamics.
Common Symptoms
- Loss of boost / reduced power
- Blue or white smoke from exhaust under acceleration
- Whining or grinding noise from turbo area
- Check engine light with boost pressure codes
- Oil leaking from turbo seals
How to Fix
Replace turbocharger assembly (Mopar 04884234AC; current Mopar listing RL064587AA; confirm by VIN) and turbo oil feed line (4891252AA). Must address root cause: oil feed line restriction, wastegate failure. Replace oil feed and drain lines during turbo replacement. Use only full synthetic oil and change every 3,000 miles. Idle 30-60 seconds before shutdown to cool turbo.
Sources for this issue (4)
Owner reports for these vans point to two separate cooling problems. The first is a water pump leak. Coolant drips from a small hole on the back of the pump, on the passenger side just behind the front wheel. That hole is a vent behind the pump's internal seal, so coolant coming out of it means the seal has failed. Owners report having to keep topping up coolant. The second is the radiator fans not switching on. The engine then overheats in slow traffic or at idle, the A/C cools poorly when the van is not moving, and the engine computer may store code P1491 (radiator fan relay circuit). The fan problem can be the relay, its wiring or fuse, the coolant temperature sensor, or the fan motors themselves. Shops sometimes replace the thermostat during a water pump job. If the engine keeps overheating, the head gasket can be damaged, so stop driving when the gauge climbs.
Common Symptoms
- Coolant dripping from the vent hole on the back of the water pump (passenger side, behind the front wheel)
- Coolant level dropping and needing regular top-ups
- Engine overheating at idle or in slow traffic
- Radiator and condenser fans not running when hot or with the A/C on
- A/C not cooling well when the van is stopped
- Check engine light with code P1491 (radiator fan relay circuit)
- Temperature gauge fluctuating
- Heater blowing cold air
How to Fix
First, work out which fault you have. If coolant is low and there is a wet trail at the back of the water pump (passenger side, behind the wheel), the pump seal has failed. Replace the water pump. Do not plug the vent hole. A mechanic in the cited thread said a pump leaking from the hole under the rubber cover needs replacing, not just a new cover. While the pump is out, count its mounting bolts. There should be five, and a missing bolt can also leak because some bolts thread into a coolant passage. A shop may also say the water pump inlet tube is about to leak and replace it in the same visit. If coolant is full but the engine overheats at idle or in traffic, check whether the radiator fans run when the engine is hot or the A/C is on. Ask the shop to check four things: 1. Is the radiator fan fuse powered on both sides with the key on? 2. Is the fan relay (at the front of the van near the radiator) bolted tightly to the body? A mechanic in the cited thread says the relay relies on that mounting to shed heat and points to a service bulletin about broken relay mounting bolts. 3. Do the fans run if the relay is jumped, or if 12 V is applied directly to each fan motor? 4. Are the relay's trigger, power and ground circuits good? Owners report P1491 in this situation, and one owner's fans started working again after the coolant temperature sensor was replaced. A mechanic notes the 2-wire sensor can be unplugged to test whether the fans respond. These are age and wear repairs. The cited sources mention no recall or warranty extension. Costs from owner reports: a mechanic estimated about $250-300 for a water pump replacement. One 2005 owner paid about $610 for a water pump, inlet tube, thermostat, coolant flush and leak diagnosis ($234 parts, $376 labor). Fan-relay repairs vary with which part is at fault.
Call to confirm they can assess your vehicle and perform the stated work. Search results do not confirm qualification, eligibility or coverage.
Sources for this issue (5)
- Coolant leak on back of water pump housing (2004 Town and Country 3.8L)owner-forum
- water pump leaking? (2005 Chrysler Town & Country)owner-forum
- Radiator fans will not work (2003 Town and Country Limited 3.8L)owner-forum
- 2carpros: 2005 Town & Country coolant leak - water pump, inlet tube and thermostat invoiceforum
- 2carpros: Town & Country radiator fan relayforum
The 3.6L Pentastar's water pump is a recognized wear item on 2011-2016 vans. The internal shaft seal fails first and coolant escapes from the weep hole at the front of the pump; if it is ignored the bearing follows and the pump can shed its drive, producing rapid overheating. Owners report the leak between roughly 80,000 and 120,000 miles. A very common misdiagnosis on these vans is a leaking plastic thermostat housing, which drips into the same area and looks identical from above - replacing the pump without checking the housing leaves the van still leaking. Note this is the Pentastar pump; the 3.3L/3.8L vans have an entirely different cooling-system failure pattern.
Common Symptoms
- Coolant drip from the front of the engine / weep hole
- Low coolant warning and repeated top-ups
- Temperature gauge climbing in traffic
- Whining or grinding from the front of the engine
- Sweet coolant smell after shutdown
- Steam from the engine bay
How to Fix
No recall. Replace the water pump with an updated Mopar or quality OE-equivalent unit, and inspect (usually replace) the plastic thermostat housing at the same time since it is the other common leak in that area and the labor overlaps. Refill with the correct OAT coolant, bleed the system properly, and confirm no residual leak with a pressure test before returning the van to service. Overheating on this engine risks head damage, so do not defer a weep-hole drip.
Sources for this issue (2)
P0128 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0128 mean on a Chrysler?
P0128 stands for "Coolant Thermostat Below Regulating Temperature." This code means the engine isn't reaching its proper operating temperature within the expected time, so the coolant stays below the thermostat's regulating temperature. The computer estimates how warm the coolant should be (based on run time, airflow, and intake air temperature) and compares it to the actual reading. When the engine warms too slowly or never gets hot enough, P0128 sets. The most common cause is a thermostat stuck open, which can cause reduced heater output, poorer fuel economy, and higher emissions. On Chrysler it is documented across 6 models: 300, PT Cruiser, Town & Country, Cirrus, Pacifica, Voyager.
What are the most common causes of P0128?
The most common causes of P0128 are: Thermostat stuck open or opening too early, Faulty engine coolant temperature sensor, Low coolant level, Cooling fan running constantly (overcooling), Wrong-temperature-rating thermostat installed, Corroded or loose coolant temperature sensor connector, Air pockets in the cooling system, Faulty PCM/ECM (rare). The specific cause varies by vehicle.
How much does it cost to fix P0128 on a Chrysler?
Published repair estimates on Chrysler range from $100 to $8,000 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.