Infiniti · Cooling · Minor code

P0128 on Infiniti: How to Diagnose & Fix

Coolant Thermostat Below Regulating Temperature

1 Infiniti model · 1 documented issue · 1 critical · $600-$700 repair

P0128 on Infiniti by model: symptoms, diagnosis and fix

Two FX50S owners on InfinitiScene report the radiator itself leaking at around 93,000-100,000 miles. A 2009 FX50S owner with just under 100,000 miles chased a small coolant leak near the bottom of the radiator. The radiator turned out to have a small crack in each side tank and was replaced. A 2011 FX50S owner with about 93,000 miles saw smoke from under the hood and coolant spraying out of the front grille on the passenger side. Coolant had pooled under the car and the expansion tank was empty. On the drive home his temperature gauge climbed toward the last tick mark on hills. He found no road debris, and the hoses looked fine and were dry. Another forum member suggested vibration or a weak spot that gave way under pressure. A third FX50 owner had a cracked coolant filler neck instead, which the dealer replaced under warranty. These are owner reports. We found no Infiniti bulletin or recall for this problem.

Common Symptoms

Temperature gauge climbing toward the top, especially on hills or in traffic
Smoke or coolant smell from under the hood
Coolant spraying or dripping from behind the front grille or under the radiator
Coolant level dropping or an empty expansion tank

How to Fix

With the engine cold, check the coolant level and the expansion tank. Look for wet coolant or dried residue on the radiator side tanks, along its lower edge and behind the front grille. Also check the coolant filler neck, since one owner's leak came from a cracked neck and not the radiator. A cooling-system pressure test can confirm a leak that is hard to see. If the gauge went high, a forum member advised checking the oil for coolant and running a compression or leak-down test before assuming the engine is fine. The fix owners describe is a new radiator. On the 2011 car the shop also replaced the thermostat and coolant, and forum members recommended doing the hoses and thermostat at the same time. The radiator also carries transmission fluid, so ask the shop to check and top up the transmission fluid after the swap. One owner had jerky shifts after his radiator job, and a member suspected the fluid was never refilled. That owner was quoted about $600 at an independent shop ($400 for a Denso radiator plus $200 labor). The dealer quoted $700 for the radiator. He found the same Denso radiator online for about $132. We found no recall, warranty extension or service bulletin that covers this repair.

What you need to fix it

Only diagnosis- and fitment-reviewed repair parts are linked here.

DENSO replacement radiator with transmission-oil cooler
OEMDENSO 221-3425
radiator branch after a core, tank or integrated-cooler fault is confirmed Fitment/availability: DENSO 221-3425 is cataloged for the 2009-2013 Infiniti FX50 5.0L VK50VE and includes the transmission-oil cooler. Confirm VIN and listing pa…
PartsGeek
Genuine Infiniti VK50VE thermostat
OEMInfiniti 21200-AD201 / 21200-1LA1A
Do not select the VQ35HR/VQ37VHR thermostats also shown on the model page; confirm VIN.
InfinitiPartsDeal
Genuine Infiniti driver-side cooling-fan motor
OEMInfiniti 21486-1BH0A
driver-side motor-only branch after command, power/ground and current testing proves that motor failed Fitment/availability: Infiniti 21486-1BH0A is the driver-side fan motor cataloged for 2009-2013 FX50 Premium/Sport VK50VE. It is not t…
InfinitiPartsDeal
Genuine Infiniti complete motor and fan assembly with shroud
OEMInfiniti 21481-1BH0B
Confirm VIN and connector; do not order it together with the driver mot
InfinitiPartsDeal

As an Amazon Associate, we earn from qualifying purchases; other part links may also earn au7o a commission. Confirm fitment by VIN before buying.

Owner tips & cautions

TipReplace aging upper and lower radiator hoses during major cooling work to avoid repeat labor.
TipDo not continue driving if the temperature climbs; the V8 can suffer major engine damage from overheating.

Typical repair cost:$600 - $700

Sources for this issue (2)

Research This Issue

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Community Reported

Medium ConfidenceUpdated Nov 7, 2026

All Infiniti FX50 known issues →

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 Infiniti?

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 Infiniti it is documented across 1 model: FX50.

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 Infiniti?

Published repair estimates on Infiniti range from $600 to $700 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.