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.

by in Redding, CA on May 09, 2012
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2 answers
ANSWER by , May 10, 2012
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).
ANSWER by , May 10, 2012
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