P0128 means the engine is not reaching its normal running temperature in the time the engine computer expects. Nearly always that is a thermostat stuck open. It is usually safe to drive, but it costs you fuel, it wears the engine, and it can stop the car completing an emissions test. Indicative UK repair cost is £60 to £280.
The tell most owners notice before the light is the heater. If the cabin takes half a journey to warm up, or goes cool again every time you slow down, and the temperature gauge sits below where it used to, that is this fault described in plain language. On older engines the thermostat is a wax element that has failed open. On a lot of newer ones it is a map-controlled thermostat with a heater element inside it, which means the fault can be electrical rather than a jammed valve, and that changes both the diagnosis and the price.
Can I drive with P0128?
Usually OK to drive, get it checked soon. Nothing is going to break this week. The damage is slower and duller than that: an engine that never reaches temperature runs a richer mixture for longer, so economy falls, the oil spends more of its life too thick and then too diluted with unburnt fuel, and a winter of short journeys on a stuck-open thermostat is genuinely harder on an engine than the same winter with a working one. The lukewarm heater is the part that will actually annoy you.
What P0128 means for your MOT
A thermostat is not an MOT item and an engine that runs cool is not a defect. The two routes to a failure here are unlike most codes on this site, and the second one catches people out.
- The lamp. Under the DVSA MOT inspection manual, a lamp indicating a malfunction is a major defect: section 8.2.1.2 for petrol cars first used on or after 1 July 2003, section 8.2.2.2 for diesels first used on or after 1 July 2008.
- The test not finishing. The emissions test is carried out with the engine at normal operating temperature. A thermostat jammed open on a cold day can mean the tester simply cannot get it there, and the manual lists "emissions test unable to be completed" as a major defect of its own. That is a failure with no reading ever taken, which is a frustrating and little-known way to fail an MOT.
Clearing the code the morning of the test is a gamble rather than a fix. The monitor behind P0128 runs on the next cold start and full warm-up, so on a car used normally the light is usually back before you get there.
What usually causes it
- A thermostat stuck open. The most common cause by a wide margin. A wax-element thermostat fails open as it ages. A map-controlled one can fail through its heater circuit or its connector, so the same code can be an electrical fault on a modern engine.
- A faulty coolant temperature sensor. The engine may be warming up perfectly well and simply reporting it wrongly. A sensor reading low also makes the car run rich, so the economy symptom is identical and only measurement separates the two.
- Low coolant level. Air sitting around the sensor reads as a cold engine. A system that is low is low for a reason, so this one is worth following up rather than just topping up.
What it costs to fix in the UK
£60 to £280. A conventional thermostat on an engine where it sits accessibly at the front is the bottom. A map-controlled thermostat that comes as a complete housing is the top, because the part itself is most of the bill. A handful of engines bury the housing under the inlet manifold, and those go past the top of the range on labour alone.
If a garage has quoted you to investigate it, what a car diagnostic costs in the UK is worth reading first, because on this code the diagnosis can cost more than the part.
These are indicative UK ranges for guidance only. Actual causes and prices vary by vehicle, and a stored code points to a system, not always the exact part.
What to check first
- Watch the gauge on a normal journey. An engine that climbs to the middle, holds there on a dual carriageway, then falls back as soon as you come off and sit in traffic is behaving exactly like a thermostat stuck open.
- Feel the heater against the gauge. Lukewarm air at speed in cold weather with a low gauge is the thermostat. Properly hot air with a low gauge points at the sensor or its wiring instead, because the engine is clearly hot and only the reading is wrong.
- Check the coolant level cold. Expansion tank and, where you can safely see it, the radiator itself.
- Put a thermometer on the housing and compare. An infrared thermometer on the thermostat housing against the live coolant reading settles it in a minute. Metal at 85 degrees while the scan tool reports 60 means you are buying a sensor, not a thermostat.
Related codes
- P0420, catalyst efficiency below threshold. Genuinely connected: a catalytic converter on an engine that never gets properly hot does not work properly either, and a long-ignored P0128 is one way to end up with both codes.
- P0113, intake air temperature sensor high input. The other temperature-reading fault, and a cheaper one.
- P0171, system too lean (Bank 1). Another source of an overlapping economy complaint, worth comparing if both are stored.
Get it narrowed down before you spend
A stored code tells you which system is unhappy, not which part has failed. If you want it narrowed down before you spend anything, send the code and your symptoms and a mechanic will read them personally. Quick Diagnosis is £20, and if we can't help, you don't pay.
Written by the First Diagnostic mechanic. Over a decade in the trade: a VW-only workshop running diagnostics and electrical repair, then BMW-only through to full engine rebuilds. Now an independent all-makes workshop, plus building and wiring drift cars for competition, including trackside work as head mechanic. More about who reads your codes.