Eight symptoms, each paired with the most likely cause and a real cost range. Use the severity strip on the left to see which ones can wait and which mean stop driving now.
Severity scale
Check the reservoir level first. If it is low, look for a leak before topping it off.
Clogged radiator core
$100 to $250 (flush)
Stuck thermostat
$150 to $300 (replace)
Failed water pump
$300 to $700 (replace)
Low coolant level
Free to $50 (top off)
Coolant colour identifies the chemistry: green is IAT, orange is OAT, pink or red is HOAT or PHOAT, blue is HOAT-2EHA.
Radiator leak
$150 to $400 repair / $400 to $1,800 replace
Hose or clamp failure
$80 to $200
Water pump leak
$300 to $700
Heater core leak
$500 to $1,300
An overlooked radiator symptom. The heater core shares coolant with the radiator, so radiator deposits often clog it first.
Clogged heater core (often fixed by flush)
$100 to $250
Heater core replacement (if flush fails)
$500 to $1,300
Failed thermostat (engine never warms)
$150 to $300
Blend door actuator (mechanical, not coolant)
$200 to $500
Brown is recoverable with a flush. Milky is a serious head gasket signal and needs a leak-down test now, not a flush.
Brown or rusty: corrosion in radiator
$130 to $250 (descaling flush)
Milky: oil mixing (head gasket alert)
$1,500 to $3,000+
Gritty sediment: advanced corrosion
$400 to $1,800 (replace)
Coolant has a distinctive ethylene-glycol sweetness. If you smell it inside the cabin, suspect the heater core, not the radiator.
Slow radiator pinhole leak
$150 to $400 / $400+ replace
Hose seep at clamp
$80 to $200
Reservoir cap pressure failure
$10 to $25
Heater core leak (if cabin smells)
$500 to $1,300
Bent fins are usually cosmetic unless 30%+ of the surface is blocked. Cracked tanks cannot be reliably repaired.
Bent fins from road debris (cosmetic)
$50 to $150 (fin straightening)
Cracked plastic side tank
$400 to $1,800 (replace)
Surface corrosion on metal core
Inspect for through-corrosion
A bad cap is the cheapest cooling-system fix you will ever buy. But check why pressure is too high before assuming the cap is at fault.
Worn pressure cap
$10 to $25 (replace)
Cracked overflow tank
$80 to $250
Head gasket leaking into cooling system
$1,500 to $3,000+
If bubbles persist after a proper bleed, do a chemical block test for combustion gas in the coolant.
Air pocket from recent service
Free (bleed system)
Failed head gasket
$1,500 to $3,000+
Cracked engine block (rare)
$3,000 to $7,000+
Stop, wait or drive
Gauge in the normal range
Coolant topped up, no visible leak. Drive carefully to a shop within a day or two.
Slow drift toward hot
Pull over, let the engine cool for an hour, drive short distances only with the heater on full.
Gauge in the red zone
Stop now. Driving an overheating engine for five more minutes can cost you a $3,000 head gasket repair.
Frequently asked
Coolant under the car at the front of the engine and a temperature gauge that climbs gradually are the two strongest radiator-specific signals. Sudden overheating is more often a thermostat or water pump. A weak heater alone often comes from the heater core or a low coolant level.
If the temperature gauge stays in the normal range and you have no visible leaks, you can usually drive short trips while you book a shop appointment. If the gauge enters the red zone, stop immediately. Driving an overheating engine for even five minutes can warp the head and turn a $250 flush into a $3,000 head gasket repair.
Healthy coolant is bright and translucent in its original colour: green, orange, pink, blue or yellow depending on chemistry. Murky brown, milky or gritty is not normal and indicates corrosion or oil contamination.
No. Coolant loss means the system loses pressure, which lowers the boiling point and reduces cooling capacity. Continued topping off masks the underlying problem and can lead to sudden overheating. Find and fix the leak.
Continue the diagnosis
Flush, repair or replace?
The decision flowchart for what to do next.
Leak repair cost
Detailed pricing for every type of radiator leak.
Replacement by vehicle
Cost ranges by vehicle class and 10 popular US models.
Heater core deep-dive
Why the heater is often the first thing the radiator takes down.
Engine overheating cost ladder
Every overheating cause from $50 top-off to $5,000 engine.
Back to flush cost overview
Service tier table and the diagnostic estimator.