Re: No heat on 2001 GMC Jimmy, please help!
As binge said, it's a clogged heater core. It also happened to me, no heat whatsoever, another victim of GM's pink antifreeze.
To avoid removing the core as I did: (1) Remove the heater hoses from the inlet and outlet ports of the heater core at the engine compartment firewall. Be careful removing the old hoses as excess force can damage the heater core ports. The smart way is to remove the original hose clamps, slice the now exposed hose ends lengthwise with a knife and then peel radially vs. pulling and twisting. It's a good time to replace the original hoses anyway. (2) Fashion two separate segments of heater hose about two or three feet long at attach to the ports. Dish detergent will help them slide on easily, especially if the rubber is cold. (3) Tie or affix the open ends of these hoses elevated such that they will hold fluid. (4) Purchase and pour in a caustic radiator flush into these hoses and leave overnight. (5) From a hardware store or Home Depot purchase a brass female garden hose adapter to male 3/4'" NPT, 3/4" NPT female-female ball valve, and 3/4" NPT male to male barbed connector, the latter which will connect to output/return flow side of the core via the segment of hose. If memory serves, it was 3/4" hose (vs. 5/8" with the ports being differently sized), but do verify this. It's very important that pressure be applied in the reverse direction of the heater core's normal flow to blow out the deposits. Also, don't forget hose clamps. (6) Attach garden hose, open spigot and bleed air to the ball valve, then tighten garden hose female fitting all the way. (7) Open ball valve. With luck, the deposits will blow out and water will flow through in reverse unobstructedly. Mine did, and I avoided having to disassemble the dashboard and purchase a new core. One year plus later, it still flows and heats fine.