Manufacturer interval reference
When to flush your coolant: intervals by manufacturer
The "every 30,000 miles" advice everyone repeats online comes from the green IAT coolant era. Modern OAT, HOAT, and PHOAT coolants last 5 to 10 years. Here is what each manufacturer actually specifies.
The honest answer
It depends on the coolant in your car, not on a single number
"Every 30,000 miles" is correct for one specific case: green IAT coolant in vehicles from before 2000. That advice has been copied across the internet for 25 years and applied to vehicles that never used IAT in the first place.
Modern coolants use organic acid chemistry that lasts 5 times longer. Toyota SLLC, GM Dex-Cool, Honda Type 2, and the European Si-OAT formulations all push 100,000 miles or more on the first interval. Following the 30k advice on a Toyota means paying for a flush 5 times when 1 would do.
Reference table
Flush interval by manufacturer
| Manufacturer | Coolant | First Flush | Subsequent | Notes |
|---|---|---|---|---|
| Toyota | SLLC (pink) | 100,000 mi / 10 yr | 50,000 mi / 5 yr | Pre-2004 models on green IAT use 30k / 2 yr. |
| Honda | Type 2 (blue) | 60,000 mi / 5 yr | 60,000 mi / 5 yr | Older models on green IAT use 45k / 3 yr. |
| GM (Dex-Cool) | OAT (orange) | 150,000 mi / 5 yr | 150,000 mi / 5 yr | Long interval, but check the reservoir often. |
| Ford | HOAT (yellow) | 100,000 mi / 6 yr | 50,000 mi / 3 yr | Some newer models specify OAT instead. |
| Chrysler / Stellantis | HOAT (purple/orange) | 150,000 mi / 10 yr | 150,000 mi / 10 yr | Verify the spec in the owner manual. |
| BMW | HOAT (blue) | 100,000 mi / 4 yr | 50,000 mi / 4 yr | Use BMW-approved G48-spec coolant only. |
| Mercedes-Benz | Si-OAT (purple) | 100,000 mi / 5 yr | 75,000 mi / 5 yr | Use coolant meeting MB 325.0 or 325.5 spec. |
| Subaru | SLLC (blue) | 135,000 mi / 11 yr | 75,000 mi / 5 yr | Older models on green IAT use 30k / 2 yr. |
| Nissan | SLLC (blue/green) | 105,000 mi / 7 yr | 75,000 mi / 5 yr | Verify the specific model interval. |
| Hyundai / Kia | PHOAT (pink) | 120,000 mi / 10 yr | 30,000 mi / 2 yr | Big drop after the first flush. |
| Volkswagen / Audi | G12++ / G13 (pink/violet) | 75,000 mi / 5 yr | 75,000 mi / 5 yr | Use G12++ for compatibility with G11/G12. |
| Pre-2000 vehicles | IAT (green) | 30,000 mi / 2 yr | 30,000 mi / 2 yr | The classic short-interval coolant. Most online advice still assumes this. |
Intervals sourced from current owner manuals and manufacturer service bulletins as of April 2026. Always defer to the specific owner manual for your year, model, and engine combination.
Why the internet still says every 30,000 miles
Because that was right for green IAT coolant used in nearly every US vehicle before 2000. Mechanics learned it, blog posts repeated it, and Google Featured Snippets froze it in place. Modern coolant chemistry is fundamentally different. Organic acids target corrosion sites specifically rather than coating every surface, so the inhibitors last longer.
If you drive an OAT or HOAT vehicle and follow 30k advice, you are not protecting the engine more, you are wasting money and disposing of perfectly good coolant. The exception: if the coolant tests acidic or looks bad regardless of mileage, the chemistry has failed early. Flush it.
Override the schedule
6 warning signs that mean flush now, regardless of mileage
Discoloured or murky coolant
Brown, rust-coloured, or opaque coolant in the overflow tank means corrosion products are suspended in the fluid. Flush regardless of mileage.
Temperature gauge running hot
If the gauge climbs higher than usual on a route the vehicle handled fine before, blocked passages and depleted inhibitors are reducing flow.
Heater blowing cooler air
Reduced heat output points to deposits in the heater core restricting flow. Flush first. Heater core replacement runs $400 to $1,000 if you wait too long.
White deposits in the reservoir
Crusty buildup around the cap or inside the overflow tank indicates the coolant has overheated and left mineral scale. Flush with a chemical descaler.
Sweet smell under the hood
Coolant has a distinctive sweet odour. Smelling it means a leak is feeding fumes into the engine bay. Find the leak first, then flush and refill.
Coolant pH below 7.0
Test strips ($8 to $12 a pack) read pH and inhibitor level. Below 7.0, the coolant is acidic and actively corroding the system. Replace immediately.
Test your coolant before paying for a flush
A pack of test strips and 30 seconds at the reservoir tells you whether the coolant still has protection.
Tool
Coolant test strips
$8 - $12
Dip in the reservoir for 2 seconds, compare to the colour chart on the bottle. Reads pH and inhibitor reserve. Replace coolant when both fall outside the safe range.
Tool
Refractometer
$15 - $30
Measures freeze point and concentration. Drop a sample on the prism, hold to light, read the line. Catches dilution from water top-offs.
Reading the result
What the numbers mean
- pH 8 to 11: healthy, alkaline
- pH 7 to 8: marginal, plan service
- pH below 7: acidic, replace now
Skip the flush, pay for it later
1 to 2 years past due
Inhibitors depleted, mild corrosion begins on aluminum surfaces
Still cheap to fix
3 to 5 years past due
Deposits build up, flow drops, heater performance starts dropping
Flush now: $130-$250
5+ years past due
Heater core blocks, water pump seal fails, thermostat sticks
$400-$1,000 + flush
Severe neglect
Overheating cycles damage the head gasket, cylinder head warps
$1,500-$3,500
Climate and driving conditions shorten the interval
Hot climates
Phoenix or Miami summers run the cooling system harder. Inhibitor depletion accelerates. Knock 20 to 30 percent off the manufacturer interval.
Stop-and-go traffic
Heat cycles age coolant faster than steady highway driving. Urban delivery vehicles and rideshare cars see shorter intervals.
Towing and hauling
Towing pushes coolant temperatures into the upper end of the operating range. Trucks and SUVs used for towing should follow the severe-service interval, often half the standard.