Radiator or auxiliary radiator? Sub-groups 251, 253 and 212

Search for an Audi radiator and you will get three different components back, all labelled "radiator", often on the same page. The part numbers tell you which is which, and the difference is not cosmetic.

The three sub-groups

Within main group 121 — cooling — the sub-group is the component:

• `251` and `253` — the main radiator. The large core behind the grille that the engine coolant circuits through. This is what people mean when they say radiator.
• `212` — an auxiliary radiator. A smaller, secondary core. Fitted where the main radiator alone cannot shed enough heat, or where a separate circuit needs its own cooling.

There is a fourth component that gets mislabelled constantly. Main group `145`, sub-group `804`, is a charge air cooler — an intercooler. It cools intake air, not coolant. It is not a radiator in any sense, yet source listings routinely file it as one. Our charge air cooler for the C6 A6 is catalogued as what it actually is.

Why the wrong one gets ordered

Three reasons, in the order we see them:

1. The listing says "radiator" for all of them. Auxiliary radiators are usually described identically to main ones.
2. The car has more than one. Larger and higher-output cars often carry a main radiator plus one or two auxiliaries. Pulling a part number off a diagram without checking which core you are looking at is easy to do.
3. The old part is already out. Once the front end is stripped, a small core and a large core both just look like "the radiator that was in there".

How to tell before you order

Read the sub-group. It is three digits and it is unambiguous:

4M0 121 251 M   ->  251  ->  main radiator
4E0 121 212 B   ->  212  ->  auxiliary radiator
4F0 145 804 K   ->  145 804  ->  charge air cooler

If you are working from the old part, the number is cast or printed on the end tank or the frame. If you are working from a diagram, check the callout position against the car rather than the description.

If your cooling problem is the engine running hot under sustained load, it is very often the main radiator. If it is a specific circuit — transmission, or a secondary loop on a W12 or a high-output V8 — the auxiliary is the more likely culprit.

Which cars carry auxiliaries

Broadly: the more power and the more thermal load, the more cores. The D3 A8 in W12 form is a well-known example, and the 4E0 121 212 B is its auxiliary. Mid-range platforms such as the PQ35 cars also carry one on some engine variants — the 1K0 121 212 C is that part.

Note that an auxiliary radiator is often not cheaper than a main one. A low-volume auxiliary for a W12 can cost more than a mainstream main radiator, because volume drives price more than size does.

Before you buy

• Confirm the sub-group on the number, not the description.
• Confirm which core you are replacing if the car has more than one.
• Confirm the engine variant. Cooling packs differ between engines on the same platform more than almost any other system.

Our full range is under radiators and cooling. If you have the number off the old core and want it checked before you commit, send it over — this is exactly the sort of thing worth thirty seconds of a technician's time.