Ihope this is in the right section. #5 or top speed on my heater/AC dosn't work. its the only one that dosn't to my knowledge. 2002 Duramax Chevy 2500HD. any help. Thanks in advance.

You have a wiring harness that has different resistors wired in parallel to slow the motor down depending which fan speed you choose. If it is working on all speeds except one, your motor is OK, the problem lies in the wiring harness, or the switch itself. Find the harness(sorry can't tell you where it is, but it is probably on the motor housing or close to it. Or it may be on the switch box itself) and check continuity on the harness' terminal for the high speed and also check continuity on the speed selector switch itself. The ternimal for high speed will be the one with no reisistor on it.

Most likely you have a fuse blown, (FRT HVAC Fuse 30A, located in Fuse Block-Underhood), or you have a bad Blower Motor Relay. Ground G200 works for the slow speeds, so this should not be in question.

The Blower Motor Switch has six positions: Off, Low, M1, M2, M3, and High.
"Off" is off
Speeds Low, M1, M2, and M3 are powered through the Blower Motor Resistor Assembly. This BMRA has four resistor/fuses in series with each other, which are powered from HTR-A/C Fuse 25A, located in Fuse Block-Left I/P.
Speed Low uses all four resistor/fuses in series, speed M1 bypasses one resistor/fuse to include three in the series circuit, speed M2 uses two, and speed M3 only includes one resistor/fuse in the circuit.
The High Speed for your Blower Motor is NOT powered through these current limiting resistors. The power from fuse HTR-A/C Fuse 25A flows through the "High Speed" contact on your Blower Motor Switch, to the coil of the Blower Motor Relay, (which looks to be part of the Blower Motor Resistor Assembly). I don't know if this relay is replaceable itself, or if you have to replace the complete Blower Motor Resistor Assembly. This BMRA should be located under the passenger side dash.
The power for the Blower Motor for High Speed, (which flows through the Blower Motor Relay contacts), is supplied from a second fuse, FRT HVAC Fuse 30A, located in Fuse Block-Underhood.
Hope this helps, sorry for the long, (and late), story.
Reference Helms 2002, page 1-123.

same thing happened to me had to replace the Resistor and connector was burned out all set now

I want to thank everyone who posted with help, It was the resistor board under the glove compartment. Easy fix. Thanks!!!!!!!!!

In my 02 the low (1) position doesn't work. All others work fine. Suggestions on how to fix? Replace resistor or board or what?

Most likely a defective Blower Motor Resistor Assembly. The other causes could be a defective Blower Motor Switch, or a bad electrical connection from the BMS to the BMRA.

I'm with Number 9....

I was leaning towards taking the control out of the dash.....

Where should I be starting to look?

Most likely a defective Blower Motor Resistor Assembly. The other causes could be a defective Blower Motor Switch, or a bad electrical connection from the BMS to the BMRA.

I have recently had my Blower Motor Switch (bank of 3 switches for HVAC on dash) out when I installed my Aux. Lamps switch for my backup lights, so I think that may rule out a bad connection as the condition didn't change after I put the BMS back in.

When you turn switch to position 1 you can't feel anything AT ALL coming from the vents. Even in OFF position it *seems* you can feel a very small amount of flow through the vents. It has done this since I bought it the first of this year.