Which Cylinders Does AFM Deactivate on a GM V8?

If you own a GM V8 truck or SUV, you have probably heard that Active Fuel Management (AFM) shuts down half the engine to save fuel. The question owners ask when they are chasing a rough idle or a misfire code is simple: which cylinders does AFM actually deactivate? On GM's small-block V8s, AFM turns off four of the eight cylinders under light load - most commonly documented as cylinders 1, 4, 6, and 7 - while the other four keep firing. That is not just trivia. When a misfire shows up on one of those four cylinders, it is a strong clue you may be dealing with an AFM lifter problem rather than a random coil or spark plug.

What AFM actually does

Active Fuel Management is GM's cylinder deactivation system. Under light load - cruising on flat ground, coasting, idling in traffic - the engine does not need all eight cylinders, so the computer drops it from a V8 to a V4 to burn less fuel. When you ask for power again, it wakes those cylinders back up. Most drivers never feel the switch. It happens through a set of special collapsing AFM lifters that stop opening the valves on the deactivated cylinders, with oil pressure routed to trigger the change.

The idea is sound on paper. The trouble is that the same collapsing lifters that make AFM work are also the parts that fail, and when they do, they can take expensive hardware with them.

Which cylinders does AFM deactivate?

On a GM V8, AFM deactivates four cylinders and leaves four running. The set that is commonly documented is cylinders 1, 4, 6, and 7. Those four get the special collapsing lifters; the remaining four (2, 3, 5, and 8) use standard lifters and fire the whole time. When the system goes into four-cylinder mode, it is those AFM cylinders that shut off, and when it returns to V8 mode they all come back online.

Because only those four cylinders carry the deactivation hardware, they are the four that can suffer an AFM-specific lifter failure. A cylinder that never deactivates cannot have a collapsed AFM lifter, because it was never given one. That single fact is what makes the cylinder numbers so useful when you are diagnosing.

GM V8 cylinder numbering, so the numbers make sense

Cylinder numbers only help if you know where they sit. On GM's V8, the driver side bank holds the odd cylinders - 1, 3, 5, 7 - and the passenger side bank holds the even cylinders - 2, 4, 6, 8. Cylinder 1 is the first on the driver side; cylinder 2 is the first on the passenger side, and so on down each bank.

Line that up with the AFM set and you get a clear picture: the deactivated cylinders are 1 and 7 on the driver side and 4 and 6 on the passenger side - two per bank, roughly balanced side to side so the engine stays smooth in V4 mode.

One honest caveat: the numbering and the AFM cylinder set are near-universal across GM's V8 AFM engines, but you should still verify against your specific engine. Decode your VIN and check the engine RPO code before you commit to a repair. If you are ever unsure which cylinder is which, scan the engine and read the misfire counters directly rather than guessing from a diagram.

Why this matters when you are diagnosing a misfire

Here is where the cylinder numbers pay off. A random misfire code like P0300, or a cylinder-specific code like P0301 (cylinder 1) or P0306 (cylinder 6), tells you where the engine is stumbling. If that misfire lands on cylinder 1, 4, 6, or 7 - one of the AFM cylinders - and it comes with a tick or tap from the top of the engine, a collapsed AFM lifter jumps to the top of the suspect list.

The failure pattern is consistent. An AFM lifter can collapse or fail to reset, which stops the valve from opening correctly. That gives you the tick and the misfire. Worse, a collapsed lifter can lose its shape and start chewing on the camshaft lobe it rides on. Once a lobe is wiped, you are no longer replacing a lifter - you are replacing a camshaft, lifters, and often more. Catching an AFM cylinder misfire early is the difference between a manageable job and a big one.

Two things to keep straight before you condemn a lifter:

• A DI injector tick is not a lifter tick. The L83 and L86 are direct-injection engines, and a normal DI injector makes a light, steady ticking sound. That is different from the deeper, load-related knock of a failing lifter. Do not confuse the two.
• An active misfire is a stop sign. If your check-engine light is flashing or blinking, you have a live misfire that dumps raw fuel into the exhaust and can damage or destroy your catalytic converter. Ease off the throttle, keep speeds and loads low, and get it checked right away rather than driving on it.

A quick AFM misfire checklist

1. Pull the codes and note the exact cylinder. Is it 1, 4, 6, or 7? That points toward AFM; 2, 3, 5, or 8 points elsewhere.
2. Listen at idle and under light load for a tick that tracks with the misfire, especially from the affected bank.
3. Rule out the cheap stuff first - coil, plug, and connector on the affected cylinder. Swap a coil to a known-good cylinder and see if the misfire follows it.
4. Watch live misfire counters with a scan tool while the engine transitions in and out of AFM mode.
5. Confirm your engine and cylinder layout against the VIN and RPO code before pulling anything apart.
6. If the check-engine light is blinking, stop driving hard and get it inspected before you damage the converter.

For a deeper walk-through of how these lifters let go, see our guide on GM AFM lifter failure explained, and if you are chasing a tick on a 5.3 or 6.2, the Chevy 5.3 and 6.2 lifter tick breakdown covers the exact symptoms.

Stopping the problem before it starts

The catch with AFM is that the wear comes from the deactivation cycle itself. Every time the engine drops to V4 and back, those AFM lifters collapse and reset. The more cycles they see, the more chances one has to fail. If you want to take that stress off the table, the cleanest option is to stop the deactivation from happening in the first place.

That is what the EirCore AFM/DFM Disabler is built to do. It plugs into your OBD-II port in about 60 seconds and keeps all eight cylinders active, so the engine never cycles into four-cylinder mode and the AFM lifters stop collapsing and resetting. It is fully reversible, it does not reflash your ECU, and it fits 2005-2018 GM V8s. Be clear-eyed about what it does and does not do: it prevents the deactivation stress that wears lifters, but it will not repair a lifter or camshaft that is already damaged. If you already have a wiped lobe, that needs to be fixed first.

Want the full picture before you decide? Start with our GM AFM delete guide to see how disabling AFM works, then browse the engine performance collection for the disabler and related parts. We offer free worldwide shipping, prices include tax, delivery in 7-15 days, and a 90-day money-back guarantee - so if you decide it is not right for your truck, you are covered.