How to Check if Your GM Truck's AFM Is Active

If you drive a GM V8 truck or SUV, there's a good chance Active Fuel Management (AFM) is quietly switching your engine from V8 to V4 every time you cruise on light throttle. Most owners never notice it happening. But if you're chasing a lifter tick, a faint shudder at steady speed, or you just want to know whether your engine is deactivating cylinders, the good news is you can check for yourself in a few minutes. Here's exactly how to tell if your AFM is active, using both the dash display and an OBD-II app.

What "AFM active" actually means

Active Fuel Management is GM's cylinder-deactivation system (originally called Displacement on Demand). On roughly 2007-2018 full-size trucks and SUVs, it shuts down four of the eight cylinders under light load to save fuel, running the engine as a V4. When load picks up, the system wakes those cylinders back up and you're on all eight again. This cycling happens constantly during normal driving, and it's the deactivation stress that's tied to AFM lifter failure on some engines.

One important note before you start: the 2019-and-newer successor is called Dynamic Fuel Management (DFM), and it works differently, running many cylinder patterns down to a single cylinder. The checking methods below still apply, but keep in mind that a 2019+ truck is a DFM truck, not an AFM one. If you're on a newer platform, verify your exact system before assuming an AFM-specific fix applies.

Method 1: Watch the V4/V8 dash indicator (2014+ trucks)

If your truck is a 2014 or newer Silverado, Sierra, Tahoe, Suburban, Yukon, or Escalade, GM built the easiest check right into the dash. The Driver Information Center (DIC) between your gauges can show a live "V4" / "V8" (or "4" / "8") readout that tells you in real time whether cylinders are deactivating.

1. Wake up the DIC. Use the steering-wheel arrow or info buttons to scroll through the driver information menu.
2. Find the fuel or engine screen. Look for a page that shows an "Active Fuel Management" or engine-mode readout. On many trucks it appears near the instant fuel-economy display.
3. Get the truck up to a steady cruise. Find a flat road, get to about 40-70 mph, and hold a light, steady throttle. AFM only deactivates under low load, so you won't see V4 while accelerating or climbing a hill.
4. Watch the indicator. When conditions are right, you should see it drop from V8 to V4. Ease off and cruise gently and it will hold in V4; add throttle and it snaps back to V8.

If you can make it toggle between V4 and V8 with your right foot, your AFM is active and doing exactly what GM designed it to do. If it never drops to V4, either the conditions weren't light enough or something is already keeping all eight cylinders on (more on that below).

Method 2: Use an OBD-II app or scanner (older or non-display trucks)

Trucks from roughly 2007-2013, and any model without the V4/V8 DIC page, don't give you a dash readout. That's where the OBD-II port comes in. Every AFM-era truck has one under the dash on the driver's side, and a cheap Bluetooth or Wi-Fi adapter plus a phone app lets you watch live engine data.

1. Plug in an OBD-II adapter. A common ELM327-style Bluetooth dongle plugs into the port under the dash. If your port is in an awkward spot, an OBD-II extension cable makes it easier to reach and keeps the adapter out of your knee.
2. Pair it with a live-data app. Open an OBD-II app that supports live PIDs (parameter IDs). Confirm it connects and starts streaming engine data.
3. Find the cylinder-deactivation reading. Generic OBD-II data doesn't always expose AFM cleanly, so look for an app or scan tool that reads GM-enhanced PIDs. Those can show an AFM/cylinder-deactivation status directly, or which cylinders are commanded on.
4. If you can't see AFM status directly, watch it indirectly. Set up gauges for calculated engine load and short-term fuel trim, then drive at a steady light cruise. You'll often see load and trim shift the moment the engine transitions between eight-cylinder and four-cylinder mode.

A dedicated GM-capable scan tool (the kind a shop uses) reads this data most reliably, but a decent app with the right PID support is enough for most owners to confirm what's happening. If you want the deeper background before you start poking at PIDs, it pays to understand how AFM behaves under load in the first place.

How to read what you're seeing

Whichever method you use, you're looking for one thing: does the engine drop cylinders under light load? If the dash shows V4 while cruising, or the scan data shows four cylinders deactivated, AFM is active. If you also hear a tick, feel a faint vibration, or catch a misfire code around the transitions, that's worth noting because those are the classic symptoms owners report with worn AFM lifters. Checking whether AFM is active won't diagnose a failing lifter by itself, but it confirms the system that stresses those parts is engaging.

What it looks like with a disabler in place

Once you know how to read the indicator, you can also verify a fix. The EirCore AFM/DFM Disabler plugs into the OBD-II port in about 60 seconds and keeps all eight cylinders active, stopping the deactivation cycling. It's a plug-in device: it does not reflash the ECU and leaves the vehicle physically unmodified, so unplugging it returns the truck to 100% stock.

Here's the useful part for this guide: on a 2014+ truck, the dash gives you instant confirmation it's working. With the disabler installed, run the same steady-cruise test from Method 1. Instead of dropping to V4, the DIC should hold on V8 no matter how light your throttle gets. On an older truck, your OBD-II app should stop showing the four-cylinder mode you saw before. That's the whole point of checking first, then re-checking: you can actually see the behavior change.

Be clear-eyed about what this does and doesn't do. Keeping all eight cylinders active prevents the deactivation stress going forward; it does not repair a lifter or cam lobe that's already damaged. If you're already hearing a hard tick or throwing misfire codes, get the engine inspected. Want the technical detail on the device itself? Our explainer on how an AFM disabler works covers it.

Fitment matters here too. The EirCore disabler is built for 2005-2018 GM V8 (AFM) engines. If you're on a 2019 or newer DFM truck, this AFM device is not the right part for your DFM system, so verify your VIN and RPO codes and confirm fitment before you buy.

Check it, then decide

Checking whether your AFM is active is genuinely a five-minute job: scroll to the V4/V8 page on a 2014+ dash, or plug an adapter into the OBD-II port on an older truck and watch the live data. If you confirm your engine is dropping to four cylinders and you'd rather it didn't, keeping all eight active is a reversible, plug-in change. Take a look at the EirCore AFM/DFM Disabler for 2005-2018 GM V8 trucks, or browse the full engine performance collection to see what fits your build. Every order ships with free worldwide shipping, prices include tax, delivery lands in 7-15 days, and you're covered by our 90-day money-back guarantee. Verify your fitment first, then decide what's right for your truck.