Sign 01
Coolant looks brown, gritty or murky
Flush now, do not wait for the interval
now
Bench card / form 10
Radiator flush intervals by coolant chemistry
Modern long-life coolants stretch the interval to 100,000 miles or 5 years. Older green IAT coolants need flushing every 30,000 miles or 2 years. Whichever comes first.
Mileage interval ladder
Five coolant chemistries, five intervals
IAT (green, traditional)
Older domestics, pre-2000 vehicles
30k miles
30,000 miles or 2 years
OAT (orange, long-life)
GM, Saab, VW (post-2000)
100k miles
100,000 miles or 5 years
HOAT (yellow / pink hybrid)
Chrysler, Ford, BMW, Mercedes
50k miles
50,000 miles or 3 years
PHOAT (phosphated HOAT, pink)
Toyota, Honda, Nissan, Hyundai
120k miles
120,000 miles or 5+ years
SOAT (silicated OAT, blue / green)
Some VW, Audi, Porsche
100k miles
100,000 miles or 5 years
Flush sooner than the interval if
Six signs that beat the mileage clock
Sign 02
Heater is weaker than it used to be
Likely time to flush; mild deposits in the heater core
soon
Sign 03
Temperature gauge runs slightly higher
Flush now, check thermostat at the same time
soon
Sign 04
Coolant has not been changed in 5+ years
Flush regardless of mileage
soon
Sign 05
Reservoir level keeps dropping
Find the leak before flushing
now
Sign 06
You bought the car used and history is unknown
Flush as part of new-to-you baseline service
schedule
The cost of skipping the interval
What two extra years of old coolant actually costs
Year 0
$0
Fresh coolant. System is healthy.
Year 5
$150
On-time flush. System stays healthy.
Year 7
$800
Skipped flush. Radiator clogged or corroded, replacement needed.
Year 9
$2,500
Continued neglect. Head gasket failure from chronic overheating.
Frequently asked
Interval questions
Can I just go by mileage alone?+
No. Coolant degrades from time as well as use. A car driven 5,000 miles a year still needs a flush every 5 years even though it has not hit a mileage target. Whichever comes first is the rule.
What happens if I skip the flush interval?+
Old coolant turns acidic and corrodes the radiator, water pump and heater core. Deposits build up that restrict flow. The first symptom is usually a weak heater. The expensive symptom is overheating that ruins the head gasket. A $150 skipped flush often becomes a $2,500 head gasket repair within 5 years.
How can I tell what coolant chemistry my car uses?+
Three places to check: the owner manual (under cooling system), the radiator cap (often colour-coded or printed), and the original coolant colour. If you cannot tell, ask a dealer parts counter for the OEM chemistry for your year, make and model.
Is it bad to flush more often than the interval?+
No, it is harmless and slightly preventive. The cost-benefit is poor (you are paying $150 for marginal extra protection), but it does no damage. Some owners in hard-water areas or extreme climates flush more often by choice.
Continue the diagnosis
Related radiator topics
Flush cost overview
Service tier table and the diagnostic estimator.
DIY radiator flush
If you are doing it yourself, here is the procedure.
Save money
Six tactics to lower the cost of every flush.
Symptoms
How to tell whether you need a flush or something more.
Heater core
Why a missed flush hurts the heater core first.