Air con blows cold only on freeway.

[Bin_jammin]

I'd gamble on an issue with the aux fan, either the fan motor or the resistor. At idle it's not functioning properly and the high pressure switch is turning the system off. Cruising on the highway will aid the operation of the aux fan, making you cold.

[DanDombrowski]

In theory, yes, without the aux fan you will have poor cooling at low speeds and adequate cooling at high speeds. However, in this case, we tested two things at once by running a garden hose over the condenser - blockage and airflow.

Because running water will extract heat far better than any moving air, I told him to run the garden hose over the condenser and see if anything changed. With the condenser presumably much cooler than normal, the AC still did not function properly, ruling out both debris blockage and a bad aux fan as the main culprit.