I have a 2003 Trailblazer, It keeps cutting off at low speeds or stops. I definitely cannot run the A/C or it stops immediately. I replaced the battery, fuel filter, oil change, air filter change and it just happens now when I turn on the A/C. There are no engine light codes but I had a mechanic to pull these codes from it: START CLNT (F)-40 and SPARK ADV-64. Can anyone help me with this?

have the alternator charge rate checked as sounds likely to be low and not enough electrical power , when lights on , at low speed to keep ignition running

It should have an idle air control valve which lets air into the intake to control the idle speed. The valve could be faulty, or dirty, or the throttle body could be dirty. The computer opens and closes the valve to regulate idle speed.

Voltage low usually alternator/regulator not working and battery is discharging. Put voltmeter on battery when running should be 13 to 14.8 volt if not replace alternator and 13 is low should be about 13.8. A fully charged battery is 12.6 not 12.5or lower

Check out the link below, your blower motor resistor pack is probably defective.

Blower Resistor Replace link
http://www.sparkys-answers.com/2009/01/2002-gmc-envoy-no-low-blower-speeds.html

replace blower motor resister

Check the relay in the relay/fuse box under the hood. There should be a diagram either on top of the cover or under the cover to tell which relay it is just like your internal fuse box.

Replace the crankshaft sensor. NO adjustment needed by software. Good luck and keep me posted, be glad to know your got your trailblazer running 100% again soon.

by cut off, do you mean it turns off, or stops pumping out cold air? If it stops pumping out cold air, definitely sounds like you have a freon leak. If it's turning off completely, it could be the switch is just acting up, or it could mean the condensor is going bad

Check your dimmer switch on your steering column. Your plug may be loose or not getting a good connection. Your dimmer switch may be going bad as well. The same thing happened on my 1994 Chevy, but the lights never came back on, on mine. I found that the dimmer switch had gone bad. If this is the case, I recommend buying one from your local chevy dealer and spending the extra money. I went through two that I bought through napa before buying a OEM replacement because the napa ones just didn't last.

Here is the common problem and fix.

Here are some of the causes of surges, stalls at stops, slow idle speed, erratic idle speed, rough idle and engine hesitation (and other problems), it is in most cases the idle speed control air-bypass valve and or throttle valve and upper intake, these area's get full of gunk and combustion residue over the miles and cause idle issues (stalls, low idle) like yours, Get a can of intake cleaner from any local parts store, not carb spray, intake cleaner, it is made by a company called CRC, remove the air intake hose to the engine, hold the idle high so the engine won't stall, then spray the can of cleaner into the intake while keeping the engine running, use at least 1/2 the can, shut down the engine and disconnect the battery for 5 minutes, then restart and complete a number of mixed driving cycles, town, freeway, stop and go etc., after a few days the problem will go away as the system will relearn to the clean intake.