I have a 2003 GMC envoy 4.2 l6. Changed a coil and a spark plug recently because it was giving me a code of a cylinder misfiring. At the time it also gave me code p0172. After replacing the coil and spark plug. I unplugged the cars battery completely . Removed the positive and negative wires and left it unplugged for a solid 20 mins. Plugged everything back up and everything was fine. No check engine light no nothing. The only problem that arose after a day or two is that when I place my car in park or change gears, at times the rpm would decrease and sometimes drop to 300 rpm. Normal readings while the engine is idle is about 600-700. This tends to happen more often when I turn the AC on. I've cleaned the throttle. But the problem still persist...

this shows info and repair for the p0526 its basically the fan clutch but on an envoy and trailblazers their electronic- prices vary but if this code popped up after changing plugs check the connections and harness for disconnect or damage ---recheck the plugs that the coil s are good and wires are on tight https://www.youtube.com/watch?v=UxwUvTiqtTA

When was the last time you changed spark plugs/wires? That's the first thing you should check

Coil packs was the problem in the first place. When a spark plug misfires, the high voltage goes no where and the coil starts to deteriorate. After prolonged misfire the coil will not make the hi voltage necessary to fire the plug. Replace the coil packs.

You are misfiring. If you have Autozone (or similar) research your check engine light, they will come up with one or more of the following codes P0300, P0301, P0302, P0303, P0304 etc. etc.P0308. The P0300 code refers to "Random/Multiple misfires" whereas the other codes refer to that particular cylinder (last digit) misfire. The P0300 code sets when 2 or more cylinders misfire (can be the same 2 over and over, but it takes at least 2 cylinders to misifre to set that code. The others only take one misfire to set the code. Now that you know what is happening, here is how to fix....
Takes all to fire a cylinder (Fuel, Spark, Air & Timing) properly. If you have random or multiple misfires, it is likely something that is COMMON to all cylinders - like the air intake/throttle body area - throttle body can get carbon buildup and not open/close properly. Check the fuel pressure to the fuel rail(s), fuel filter clogged? When you turn your key to the "ON" position, do you hear the fuel pump turn on (whine) and then shut off? No whine, means no fuel pump. Check all fuses and fuse relays related to the fuel delivery system. Check the distributor cap/rotor for any signs of carbon tracking/arcing. Do you have good spark from coil? Check wiring for the Camshaft Position Sensor/Crankshaft position Sensor for timing issues. Finally, check compression on all cylinders. Need good compression for combustion to take place.

For a single cylinder (example Cylinder #2) - Easiest to check is spark. Pull the plug and look at it. Is it wet and smell like fuel? If so, then swap it with cylinder next to it. If the problem moved, then its the plug. If the misfire is still on Cylinder #2, then swap the plug wire (both ends). If the misfire is still on Cylinder #2, then swap the coil. (see where this is going?) If the plug is dry, then check the fuel injector for operation. Noid light tester or you can get a long screwdriver and stick one end on your ear and the other on the injector - it should make a steady, rhythmic clicking sound if it is functional when car is running. No sound? Replace. Is the plug oily? Check cylinder #2 for compression. No compression or little compression means either head gasket blown or bad/sticking valve (intake or exhaust) = big money to get fixed. Timing on one cylinder is hard to test without correct test tools. Timing is likely not a problem on a single cylinder misfire. Good luck.

The P0305 code is indicating that there is a misfire of cylinder number 5. This could be caused by a bad spark plug, bad coil pack, injector, poor compression.

2002 Envoy sle 4.2 L straight 6. Random misfire(PO300). After erasing codes for several days, I finally took my car on the highway and ran it at high speed(70+) for about 4 to 5 miles. Check engine light came on as before but this time it gave me(PO305). Random misfire cylinder 5. So I switched coil 5 with coil 2. Went back out on the highway and repeated as before. This time it gave me(PO302). This was telling me I had a bad coil. Replacing one coil pack solved the problem. The professionals told me low fuel pressure would not give me random misfire, but rather a O2 code because the engine is running lean. Thank you------------------------------------ thank you very much.

You could have a bad spark plug or a bad coil pack for cylinder 1. Or it could be as simple as a poor connection at the coil pack.
Other possibilities are, bad injector or poor compression.

There can be other reasons for misfires beside spark plugs/coil, such as a bad injector, low compression in that cylinder, burnt valve seats.

Next step is to switch injectors between Cyl #1 and another cylinder. Another check to make is a compression test.