If you're putting a Gen 3 LS into something it didn't come in, the computer is one of the first decisions you'll make. Most Gen 3 swaps run one of two GM PCMs, the P01 or the P59. They look nearly identical, and a lot of swappers aren't sure which one they have or which one they need.


Here's how to tell them apart, which one fits your swap, and what to check before you buy or program one.


The quick answer


P01: Typically used on 1999 to 2002 Gen 3 LS engines. Usually drive-by-cable. Red and blue connectors.


P59: Typically used on 2003 to 2006 Gen 3 LS engines. Usually drive-by-wire. Green and blue connectors.


Most swaps come down to two questions: what harness are you running, and how is your throttle controlled?


How to tell them apart


The fastest way is the connector colors on the end of the PCM:


Red and blue connectors: P01

Green and blue connectors: P59


The service-number label on top of the case can also help identify the exact PCM and what it came out of. If you're not sure, a photo of the connectors and the label is usually enough to identify it.


Match the PCM to your harness


This is the rule that saves the most headaches: the PCM needs to match your wiring harness.


The P01 and P59 use different connectors and different pin layouts. A harness built for a P01 won't plug into a P59, and the reverse is also true. Whether you're running a modified factory harness or an aftermarket standalone, look at the PCM connectors on the harness. That tells you which computer you need.


If you haven't bought a harness yet, decide which PCM you want first, then buy the harness to match.


Drive-by-cable vs. drive-by-wire


The second big factor is throttle control.


Drive-by-cable uses a traditional throttle cable from the pedal to the throttle body. It's simple, reliable and easy to adapt to older vehicles. Most P01 swaps are drive-by-cable.


Drive-by-wire uses an electronic throttle body and pedal, with the PCM controlling the throttle opening. It gives the computer more control over idle and throttle behavior, but it requires the matching throttle body, pedal and wiring. Most P59 swaps are drive-by-wire.


There are exceptions depending on the donor vehicle, so it's worth verifying your specific setup. As a general guide, if your engine came with a cable throttle body, you're probably looking at a P01. If it came with an electronic throttle body and pedal, you're probably looking at a P59.


Is one better than the other?


For most street swaps, both work well. The P59 is the newer design, but a properly configured P01 runs a stock or cammed Gen 3 engine just fine. Neither one is a bad choice.


The better question is which one fits the parts you already have: your harness, your throttle body and your engine. Building around what you have is usually cheaper and simpler than converting to the other PCM.


Gen 3 only: what about Gen 4 engines?


The P01 and P59 are Gen 3 PCMs. They're designed for engines with a 24x crank reluctor, like the common 4.8, 5.3, 5.7 and 6.0 Gen 3 engines.


Gen 4 LS engines use a 58x reluctor and different computers, typically the E38 or E67. Before choosing a PCM, confirm which generation your engine is. It's one of the most common swap mix-ups.


Whichever you choose, it still needs to be programmed


Picking the right PCM is only half the job. Out of a donor vehicle, either one is still set up for that vehicle. It needs changes like:


VATS / security removal

Engine displacement

Automatic or manual transmission, and 4L60E or 4L80E setup

Tire size, rear gear and speedometer correction

Electric fan control

MAF and MAP setup

Injector data

Idle and startup settings for aftermarket cams

Swap-related trouble code changes


Skip those and you'll often be chasing no-starts, stalling, wrong shifts and check engine lights instead of driving.


Your options with Midgard Industries


Already have a P01 or P59?

Mail it in and it'll be programmed for your swap.


$100 for stock or near-stock engines

$150 for cammed or modified engines


Order your mail-in base tune here.


Need a PCM?

Get a genuine GM P01 or P59 programmed for your engine and swap, with the PCM included.


$200 for stock or near-stock engines

$250 for cammed or modified engines


Shipping is available, and local pickup or drop-off is available in Longview, Texas.


Not sure which one you need? Message me with your engine, cam, transmission, harness and throttle setup, or send a photo of your PCM connectors. I'll confirm which PCM fits your swap before you order.