P0128 on Jaguar: How to Diagnose & Fix
Coolant Thermostat Below Regulating Temperature
1 Jaguar model · 1 documented issue · 1 critical
P0128 on Jaguar by model: symptoms, diagnosis and fix
Owners of 4.2 V8 XJs report the plastic thermostat housing cracking and leaking coolant, with the low-coolant warning or overheating as the first sign. Owners also name the expansion (overflow) tank and its small pipe connections as a leak point; Jaguar service message SSM51717 confirms the expansion tank stub pipe can split on XJ (X350) up to the end of the 2009 model year. Applies to: 2004-2009 XJ (X350 generation) with the 4.2 V8; owner reports include a 2004 Vanden Plas, a 2005 XJ8L and a 2006 Super V8. The expansion tank stub pipe message (SSM51717) covers all X350 XJs up to the end of 2009 MY. Summarized from the owner reports linked here; not a manufacturer finding. Your vehicle's equipment and history may differ; a technician must confirm the cause. Mileage when it happened, as stated in the linked owner reports: 20,000 miles on a 2004 and 108,000 miles on a 2005.
How to Fix
If a low-coolant warning appears or temperature climbs, have a qualified technician pressure-test the cold cooling system and inspect the thermostat housing, expansion tank and hoses. Owners report replacing a diagnosed cracked thermostat housing with its gaskets, or a diagnosed leaking expansion tank. The new tank pictured is ONLY the latter branch, not a complete cooling-system repair kit. SSM51717 historically reported an improved stub-pipe design and assured replacement stock from 2010; current VIN-specific parts still need confirmation. One owner traced continued loss and overheating to a replacement water-pump impeller; that individual outcome is not a reason to replace every pump.
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 (4)
- Jag-lovers: Unfortunate X350 coolant leakforum
- Jaguar Forums (XJ X350/X358): Coolant leak and overheating (Wayback copy)forum
- Jaguar Forums (XJ X350/X358): What issues plague the 2005-2008 Super V8 or XJR? (Wayback copy)forum
- Jaguar SSM51717, Coolant expansion tank stub pipe leaking up to 2009 MY (XJ X350, XK X150, XF X250)manufacturer
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 Jaguar?
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 Jaguar it is documented across 1 model: XJ.
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 Jaguar?
Repair costs vary widely with the root cause and Jaguar model; each issue on this page carries its own estimate where one is published.
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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.