What would cause truck to overheat when air conditioner is on?
on 2005 GMC Canyon

Shop says it is a plugged radiator and that it cannot be fixed but must be replaced at a cost of $800.

Asked by for the 2005 GMC Canyon
2 answers
Possible Causes in overheating :
1. Engine coolant level may be very low.
The Fix: Refill coolant to proper level.
2. Engine's drive belts may be broken or slipping.
The Fix: Tighten or replace belts.
3. The electric cooling fan may not be coming on.
The Fix: Repair or replace cooling fan. Repair wiring. Replace cooling fan temp sensor.
4. The ignition timing may be set wrong.
The Fix: Adjust ignition timing.
5. There may be a vacuum leak.
The Fix: Check and replace vacuum lines as required.
6. The engine may have mechanical problems.
The Fix: Check compression to determine engine condition.
7. The engine's thermostat may be stuck closed.
The Fix: Replace thermostat.
8. There may be a leak in the cooling system.
The Fix: Repair leak and refill coolant. Cylinder head gasket(s) may be bad.
The Fix: Replace cylinder head gasket(s).
china is correct all these can be an issue. fan clutch may be bad but its like 400.00. seek a 2nd opinion that would help