LS & Gen V LT computers & tuning tools

• The GM computer handles most swaps. The factory PCM works well with either the 24x or 58x reluctor and is the path with a real chance at emissions compliance. We save aftermarket ECUs for serious high-hp builds that need things like traction control.
  How to do it

  Try this, then this

  1. Get VATS deleted from your PCM by a tuner or mail-order flash, add constant 12V and switched ignition power and fuel at ~58 psi — an unmodified truck harness will run the engine — Cheapest path; stock unmodified harness works with power in two spots once VATS is gone. Worked if: engine fires and idles with no theft-deterrent lockout
  2. For emissions-legal swaps keep the stock computer with all donor emissions equipment installed — Factory PCM is what passes; standalone loses emissions functions. Worked if: vehicle passes inspection as donor-year vehicle

  Key numbers

  What	Value
  Factory PCM handles both 24x and 58x crank signals on LS truck engines	24x and 58x supported
  Turning off VATS is a basic requirement of running a stock PCM in a swap	VATS must be deleted by tuner
  Howell ships tested factory ECUs with emissions and downstream O2 functions disabled, no tuner needed for a stock engine	emissions disabled in tune

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  Howell EFI LS swap harness + tested factory ECU package		Gen III/IV LS engines		Howell EFI	new
  Current Performance Wiring Stock harness modification service + programming		LT1, LSx Gen 3/4, Gen V	Stage 1 $325 (Gen V $425); LS1 programs from $175	currentperformance.com	service harness mod does not eliminate VATS — programming required
  Edelbrock Pro-Flo 4 LS ECU and Harness Kit	357110 (24x), 357120 (58x)	Gen III/IV LS	just under $900	retailers	new engine only — does not control electronic transmissions
  Holley Terminator X / Terminator X Max	550-910 · CARiD, 550-927 (LS2/3/7); 550-926T (LS3 24x)	Gen III/IV LS	about $1,150-1,300	Holley, retailers	new switched +12V must be hot in Run AND Crank
  DIY Trim stock harness + mail-order VATS delete reflash		1999-2007 truck harnesses	free to ~$100	eBay tuners (e.g. CoolMike222), LT1swap.com pinouts	DIY labor-intensive; emissions equipment removal needs tune

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: $0 (stock harness + tuner VATS delete) to ~$325-500 (harness mod service) to ~$900+ (Edelbrock/Holley package)

• Get the ECM and harness with the donor. A complete donor with harness and ECM saves money. You can trim the stock harness yourself: reprogram the ECM, push out unused pins, pull the unneeded wires (about half) and rewrap, leaving only a handful of wires to tie into the vehicle.
  How to do it

  Check this first

  1. Label every connector and photograph the harness before unwrapping — you can't rebuild what you can't identify
  2. Decide what stays before cutting: needed circuits are PCM power/ground, injector/coil/sensor feeds, fuel pump, fan control, VSS, and check-engine light

  Try this, then this

  1. Get the ECM programmed first (VATS deleted, emissions off, fans/speedo set) — The tune must match what the harness keeps. Worked if: PCM matches retained circuits
  2. Unwrap, remove unneeded wires and push out unused pins, re-loom and shorten as needed — One day of work saves $300-800. Worked if: harness lays clean and all connectors reach

  Still not fixed?

  • If the harness is brittle or hacked, buy a pre-made standalone harness (Howell, Painless, BP Automotive) instead of fighting it.

  Key numbers

  What	Value
  Current Performance Stage 1 harness modification: $325 (Gen III/IV), $425 (Gen V)	$325 / $425
  Harness modification does not remove VATS — that must be done in the programming	programming required for VATS

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  DIY Trim stock harness + mail-order VATS delete		1999-2007 truck harnesses	free to ~$100	LT1swap.com pinouts, eBay tuners	DIY time-intensive; bad for brittle/hacked harnesses
  Current Performance Wiring Stage 1 / Stage 2 harness modification		LT1, LSx Gen 3/4/5	Stage 1 $325 / Stage 2 $400 (Gen V $425/$500)	currentperformance.com	service does not remove VATS — programming extra
  Howell EFI Standalone LS swap harness + tested ECU		LS/Vortec family		Howell EFI	new cleaner but pricier than DIY
  Painless Performance LS standalone harness		LS Gen III/IV		retailers	new
  BP Automotive Standalone LS harness (ex-Speartech builder)		LS Gen III/IV		bp-automotive.com	new
  DIY Free pinout diagrams to identify and remove circuits	LT1swap.com pinouts

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: $0-100 DIY (a day of labor) / $325-500 for a harness-mod service

• Match the reluctor to the ECM. 24x and 58x use different ECM strategies and don't interchange. A converter box costs almost $300, so it's often cheaper to buy an engine with the right reluctor; confirm the reluctor and sensors before you buy.
  How to do it

  Check this first

  1. Confirm your engine is 58x (Gen IV, 2006+) and your PCM is a 24x Gen III unit (P01 0411 or P59) — the TRG-002 only converts 58x crank to 24x
  2. Verify cam sensor compatibility: the crank converter does not fix cam sensor mismatch — match sensor type to PCM

  Try this, then this

  1. Use the TRG-002 to feed the Gen III PCM a 24x crank signal from the 58x engine — Lets you keep your existing tuned 24x PCM and harness. Worked if: PCM sees clean crank signal, engine starts and runs

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  Lingenfelter TRG-002 trigger converter	L460065397	58x engine + 24x Gen III PCM	MSRP $254.95, street $289-330	Lingenfelter, Summit, retailers	new crank signal only; cam sensor must be matched separately
  DIY alternative Swap to a 58x-capable PCM (E38) instead		Gen IV engines	about $150-250 PCM + tune	junkyard + tuner	used/new needs 58x harness changes
  DIY alternative Swap a 24x reluctor wheel onto the Gen IV crank		Gen IV blocks	reluctor + labor	machine work	DIY engine teardown required
  Lingenfelter 58x to 24x crank trigger signal converter	TRG-002			Buy

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: ~$255-330 for the module, about an hour to wire in

• LT engines mean a standalone. An LT is direct injection and an LS is port injection, so putting an LS into an LT truck requires a standalone ECU as a separate project. Building the existing engine is often cheaper and simpler.
  How to do it

  Try this, then this

  1. Keep the factory DI computer with a purpose-built DI swap harness (Howell/PSI) — Runs the high-pressure DI system natively; no aftermarket DI support needed. Worked if: engine runs on factory ECM with harness, needs only a tune

  Key numbers

  What	Value
  GM launched direct-injected LT1 Corvette engine and EcoTec3 truck engines for the 2014 model year	2014+ direct injection
  Howell offers LT/EcoTec3 conversion harnesses for L83, L86, LV3, LT1, LT4 DI engines as standalone swap harnesses	DI swap harnesses available
  Howell LT/EcoTec3 harnesses are stand-alone designs needing only battery, ignition, and fuel pump power	3-wire hookup

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  Howell EFI LT/EcoTec3 conversion swap harness (standalone, factory DI ECM)		LT1/LT4/L83/L86/LV3 DI engines		Howell EFI	new requires factory DI ECM + custom tune
  Holley Terminator X Max (with DI? — LS only; LT DI not supported by Terminator X)		LS Gen III/IV only			n/a Terminator X does NOT support LT direct injection — verify before buying
  PSI Conversion LT swap harness + ECM package		LT DI engines		PSI Conversion	new needs unlocked DI ECM
  BP Automotive LT standalone harness		LT DI engines		bp-automotive.com	new

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: Harness ~$700-1,200 plus custom tune

• Budget tuning can be done right. DIY ECU tuning is possible for under $100 with an OBDxPro VT interface and free software like PCM Hammer or LS Droid. Whatever you use, log what the engine is doing and verify changes rather than tuning blind.
  How to do it

  Check this first

  1. Identify whether you have a P01 (0411) or P59 PCM before buying — both work with PCM Hammer/LS Droid + OBDX Pro

  Try this, then this

  1. Use the free LS Droid phone app + cheap Bluetooth dongle for a simple VATS delete — Cheapest possible path for basic flashes. Worked if: flash completes and VATS is gone

  Key numbers

  What	Value
  Maker's own site now advertises diagnostic kits from $80 — the $60 figure is outdated	from $80
  OBDX Pro reads P01 and P59 PCMs with PCM Hammer or LS Droid; Universal Patcher handles most OS numbers	P01/P59 support

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  OBDX Pro OBDX Pro VT (v3)	XProVT	GM VPW 1996-2006 (P01/P59 PCMs)	kits from $80	obdxpro.com	new (batch drops) supply comes in batches that sell out; Bluetooth Android only
  OBDX Pro OBDX Pro VX		GM CAN + VPW 1996-2021+		obdxpro.com	new
  DIY cheapest Generic Bluetooth ELM327 dongle + LS Droid app (free)		VPW PCMs, basic flashes	about $15-25	Amazon	new slow reads; no 4x VPW
  AVT AVT-852	852	PCM Hammer	about $275	manufacturer direct (email order)	new price + ordering hassle
  VXDIAG VCX Nano GM (J2534)		PCM Hammer via zip install	about $110	vxdiagshop.com	new get the GM-compatible version

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: OBDX Pro VT ~$80+ (update the page's $60 figure); software free; an afternoon to learn the tools

• Standalone power must be hot cranking. If you move to a Holley Terminator Max, the main switched-power wire must be hot while cranking. Confirm that before chasing any other no-start cause.
  How to do it

  Check this first

  1. With a meter on the red/white switched wire, watch voltage while cranking — if it drops out or sags hard in Crank, that's your no-start
  2. Check Holley's no-start list: TPS Autoset done (ECU won't fire injectors/coils without it), fuel pump primes 5 sec at key-on, 100+ RPM cranking signal

  Try this, then this

  1. Meter the red/white switched +12V wire in Run and Crank — Most common Terminator no-start cause; 30-second test. Worked if: 12V present in both positions
  2. Rewire switched power through a relay fed directly from the battery — Old ignition switches sag or drop out under crank load. Worked if: engine fires instantly
  3. Run Holley's no-start checklist (TPS Autoset, cam/crank sync LEDs, fuel pressure) — Covers the remaining common causes. Worked if: all checks pass and engine starts

  How you know it's fixed

  • Red/white wire shows 12V in Run and stays at 12V through the whole crank, engine fires immediately

  Still not fixed?

  • If the switched wire is good, run Holley's full no-start checklist: TPS Autoset, crank/cam sensor sync (watch ECU LEDs), fuel pressure, injector impedance selection.

  Key numbers

  What	Value
  Holley switched +12V (red/white wire) must be powered in Run AND Crank positions	12V in Run + Crank
  Owner fixed no-start by correcting the 'hot when cranking' wiring — fired up instantly after	wiring fix = instant start
  Voltage dropping below ~2V on the keyed wire during crank is not enough for the Holley ECU — use a relay off the battery	relay fix for voltage drop

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  Holley Terminator X (LS)	550-910 · CARiD	Gen III/IV LS	about $1,150	Holley, retailers	new switched 12V must be hot in Run AND Crank
  Holley Terminator X Max (LS + trans control)	550-927 / 550-926T	Gen III/IV LS + 4L60E/4L80E	about $1,300	Holley, retailers	new same switched-power requirement
  Generic Relay to feed ECU switched power straight from battery, triggered by key-on	40A relay + fuse holder

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: $0 to diagnose; ~$10-20 in relay parts to fix

• Let the PCM run the fans. A Gen 3 PCM grounds the low-speed fan wire (C1 pin 42) around 190 F and the high-speed wire (C2 pin 33) around 205 F. If an A/C trinary switch also triggers the fans, isolate it with diodes and resistors so it doesn't back-feed ground into the PCM and set codes.
  How to do it

  Check this first

  1. Command the fans on with a scan tool (VCM Scanner output controls) — if the PCM grounds pins 42/33, wiring is good and it's a tune problem
  2. Verify fan control is enabled in the tune (truck OSes ship with it off) and fan type/mode matches your relay wiring

  Try this, then this

  1. Enable fan control in the tune and set on/off temps (e.g. 190F on / 205F range) — Truck PCMs ship with fan control off. Worked if: scan tool can command fans on
  2. Wire PCM pins C1-42 (low) and C2-33 (high) to ground-triggered relays in the factory 3-relay pattern — Gives dual-speed operation like GM intended. Worked if: fans run low at temp 1, high at temp 2
  3. If combining with a trinary A/C switch, add blocking diodes between the two control sources — Prevents backfeeding between PCM and A/C switch. Worked if: fans respond to both temp and A/C with no interaction

  Key numbers

  What	Value
  F-body LS1 pinout: C1 pin 42 (DK GRN 335) = Low Speed Cool Fan Control; C2 pin 33 (DK BLU 473) = Hi Speed Cool Fan Control	C1-42 low, C2-33 high
  PCM grounds the fan pins to trigger relays; fan-on temps are set in the tune	ground-switched, tune-set temps
  Factory 3-relay wiring runs both fans at low speed on Fan 1 command, both at high speed on Fan 2	3-relay dual-speed setup
  Truck/van PCMs need electric fan control individually enabled in the tune	enable fans in tune

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  GM Factory 3-relay dual-fan wiring		dual electric fans	junkyard	donor vehicle / wiring diagrams	used
  Derale Dual fan controller with temp probe	16749 / 16759	dual fans	about $80-120	Buy retailers	new not PCM-integrated
  Flex-a-lite Variable speed fan controller	33054	single/dual fans	about $150	Buy retailers	new not PCM-integrated
  Generic 3-relay dual-speed fan setup like factory	40A automotive relays x3
  Generic Diodes to isolate PCM fan control from trinary A/C switch on the same fans	blocking diodes

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: $0 (tune change) + ~$20-40 in relays/diodes

• Keep emissions swaps by the book. For a California-legal swap, the donor must be the same year or newer and the same vehicle class, with all original emissions equipment working, a stock calibration printout and a BAR referee inspection.
  How to do it

  Check this first

  1. Call the BAR Referee before buying the donor — confirm your exact combo qualifies as a legal engine change

  Try this, then this

  1. Pre-approve the swap with the referee and keep every emissions component from the donor (ECM, cats, EVAP, AIR) — Missing one device fails the inspection. Worked if: referee signs off and affixes BAR label

  Key numbers

  What	Value
  CA BAR: all vehicles with engine changes must pass an initial Referee inspection and get a BAR Referee label; later checks can be at normal stations	referee + BAR label required
  Engine replacements (same make, cylinders, engine family/test group with original emission controls) are not engine changes and skip the referee	replacement vs change distinction
  Engine changes must come from the same year or newer donor, same weight class, with all stock emissions components	same-year-or-newer + full emissions
  Bring the donor VIN to the referee appointment to simplify the process	donor VIN

  Every option we know of

  Maker / product	Part #	Fits	Price	Where	Status / notes
  GM CARB-exempted crate engine package (E-ROD)	LS3 E-ROD 19257234	pre-OBD2 chassis swaps	about $9,000-11,000	GM Performance Parts dealers	new CARB EO makes referee visit straightforward
  DIY Junkyard donor engine + full emissions equipment + referee visit		same-year-or-newer donors	donor cost + referee fee	salvage yards	used must retain every emissions device from donor

  Prices are approximate as of September 2026. We list every option we know of, including small shops and used parts we can't link to. Confirm fitment with the seller.

  Time and cost: Donor engine cost + referee inspection fee; E-ROD route ~$9k-11k all-in

• Flex fuel on a P59. It needs a flex-capable operating system. Turn on Flex Fuel Enable (Engine > Fuel > Open & Closed Loop > Flex Fuel) and enable P0178 and P0179 so a sensor fault sets a code.

Based on what we've seen work across our own builds, our friends' builds and the wider LS community, reviewed by our lead shop tuner (over 20 years of LS and LT tuning experience). Every combination is different, so use at your own risk and check our safe tuning limits before any wide-open pass.