LS1 Firing Order and Wiring: Coil, Injector, and Swap Harness Basics
LS1 Firing Order and Wiring: Coil, Injector, and Swap Harness Basics
An LS engine will tolerate a lot.
Bad exhaust.
Ugly swaps.
Questionable motor mounts.
A wiring harness that looks like it was built on a tailgate at midnight.
But it will not run right if the firing order, coil wiring, injector wiring, or plug wires are wrong.
If your fresh LS swap cranks, pops, backfires, runs on four cylinders, has random misfires, or sounds like the timing is way off, do not start tuning yet.
Check the basics first.
The tune cannot fix crossed plug wires, wrong injector connectors, bad grounds, or coils firing the wrong cylinders.
Here is the simple LS1 firing order and wiring breakdown.
LS1 Firing Order
The standard LS firing order is:
1-8-7-2-6-5-4-3
This applies to most Gen 3 and Gen 4 LS-based V8 engines, including common 4.8, 5.3, 5.7, 6.0, and 6.2 setups.
The firing order is not the same as old-school small-block Chevy.
Traditional small-block Chevy firing order:
1-8-4-3-6-5-7-2
LS firing order:
1-8-7-2-6-5-4-3
Do not mix them up.
LS Cylinder Numbering
GM LS engines use this cylinder numbering:
Driver side bank:
1-3-5-7
Passenger side bank:
2-4-6-8
Front of engine to rear.
So looking from the front of the engine:
Driver side front cylinder is 1.
Driver side rear cylinder is 7.
Passenger side front cylinder is 2.
Passenger side rear cylinder is 8.
This matters when checking plug wires, coil connectors, injector connectors, misfire codes, and header temps.
LS Coil Wiring Basics
LS engines use coil-near-plug ignition.
That means each cylinder has its own coil, and each coil fires one spark plug.
The PCM commands each coil based on crank and cam position.
If the coil connector or plug wire is on the wrong cylinder, the spark happens at the wrong time.
Common symptoms of coil wiring problems:
Backfire through intake
Backfire through exhaust
Cranks but will not start
Starts and dies
Runs on only some cylinders
Random misfires
Cylinder-specific misfire codes
Glowing headers
Raw fuel smell
Popping under throttle
Timing looks wrong even though tune is unchanged
Do not chase the spark table if the coil wiring is wrong.
LS Plug Wire Routing
LS plug wires are short, but they can still get mixed up.
Each coil should connect to the spark plug for the same cylinder.
Do not assume the coil closest to a plug is always wired correctly, especially on swaps, aftermarket coil brackets, relocated coils, or used harnesses.
Verify each one:
Coil 1 goes to cylinder 1.
Coil 2 goes to cylinder 2.
Coil 3 goes to cylinder 3.
Coil 4 goes to cylinder 4.
Coil 5 goes to cylinder 5.
Coil 6 goes to cylinder 6.
Coil 7 goes to cylinder 7.
Coil 8 goes to cylinder 8.
If the engine has relocated coils, label everything.
Do not rely on memory.
LS Injector Wiring Basics
Injector wiring matters just as much as coil wiring.
If the injector connectors are crossed, the PCM may fire fuel into the wrong cylinder at the wrong time.
That can cause:
Hard start
Long crank
Rich smell
Backfiring
Misfires
Poor idle
Fuel trims all over the place
Header temps uneven
Cylinder wash
No-start condition
On a batch-fire engine this might be less obvious, but LS PCMs use sequential fuel control when operating normally.
That means injector timing matters.
Make sure each injector connector goes to the correct cylinder.
Common Swap Harness Mistakes
LS swaps often involve mixed parts.
Truck engine.
Car intake.
Aftermarket harness.
Different MAF.
Different injectors.
Relocated coils.
Standalone fuse block.
Modified DBW or DBC setup.
That creates plenty of places to mess up wiring.
Common mistakes include:
Injector connectors swapped bank to bank
Coil sub-harness installed wrong
Plug wires crossed after header install
Bad engine grounds
Wrong crank sensor connector
Wrong cam sensor connector
MAF and IAT wiring wrong
EVAP or purge connector confused with another plug
O2 sensor connectors swapped left to right
Bank 1 and Bank 2 O2 sensors reversed
DBW pedal or TAC wiring issues
Loose PCM pins
Poor fuse or relay power feed
A tune will not fix any of that.
Bank 1 vs Bank 2 on LS Engines
Bank 1 is the side with cylinder 1.
On an LS, Bank 1 is the driver side.
Bank 2 is the passenger side.
This matters for oxygen sensors, fuel trims, misfire data, and diagnostics.
If Bank 1 and Bank 2 O2 sensors are swapped, the PCM will correct the wrong side.
That can make one bank go rich while the other goes lean.
Symptoms of swapped O2 sensors:
One bank fuel trim very positive
Other bank fuel trim very negative
Fuel trims fight each other
Idle smells rich
Cruise feels unstable
No obvious vacuum leak
Corrections get worse after closed loop starts
Before tuning bank-specific trims, verify O2 sensor wiring and placement.
Misfire Codes and Cylinder Numbers
If you get a misfire code, use the cylinder number to diagnose the correct hole.
Common examples:
P0300 = random/multiple misfire
P0301 = cylinder 1 misfire
P0302 = cylinder 2 misfire
P0303 = cylinder 3 misfire
P0304 = cylinder 4 misfire
P0305 = cylinder 5 misfire
P0306 = cylinder 6 misfire
P0307 = cylinder 7 misfire
P0308 = cylinder 8 misfire
Do not replace random parts.
Use the code, cylinder numbering, and log data to narrow it down.
Quick LS No-Start Wiring Checklist
If the engine cranks but will not start, check these before touching the tune:
Battery voltage
Engine grounds
PCM grounds
Ignition power
Injector power
Coil power
Fuel pump power
Fuel pressure
Crank sensor connected
Cam sensor connected
MAF/IAT connected if required
MAP sensor connected
Coil connectors correct
Injector connectors correct
Plug wires correct
Security/VATS status
RPM signal while cranking
Injector pulse while cranking
Spark while cranking
If the scanner shows no RPM while cranking, the PCM may not be seeing crank signal.
If there is RPM but no injector pulse, look at VATS, injector power, PCM control, or wiring.
If there is injector pulse but no spark, check coil power, grounds, crank/cam signal, and coil control.
Header Temperature Trick
A quick way to find a dead cylinder is checking header tube temperature after startup.
Use an infrared temp gun carefully.
A cold tube usually means that cylinder is not firing.
Possible causes:
No spark
No fuel
No compression
Wrong plug wire
Wrong injector connector
Bad coil
Bad plug
Bad injector
Mechanical problem
Do not burn yourself, and do not run the engine long if it is popping, dumping fuel, or misfiring badly.
Wiring Checks Before Tuning
Before trying to tune a rough-running LS swap, verify:
Correct firing order
Correct cylinder numbering
Plug wires correct
Coil connectors correct
Injector connectors correct
O2 sensors not swapped
MAF/IAT wiring correct
MAP sensor reading believable
Grounds clean and tight
Battery voltage stable
Fuel pressure correct
No vacuum leaks
No exhaust leaks before O2 sensors
If any of that is wrong, the log will lie to you.
The PCM can only report what the sensors and wiring give it.
What to Log for Wiring and Misfire Diagnosis
Useful HP Tuners channels include:
Engine RPM
MAP
MAF frequency
MAF airflow
IAT
ECT
TPS
Battery voltage
STFT Bank 1
STFT Bank 2
LTFT Bank 1
LTFT Bank 2
Commanded AFR, lambda, or EQ ratio
Wideband AFR or lambda if installed
Spark advance
Knock retard
Misfire current cylinder data if available
Injector pulse width
O2 sensor voltage or lambda if supported
Fuel pressure if available
VATS or starter/fuel enable status if available
For a startup issue, log cranking data too.
A lot of people only log after the engine starts. That misses the actual problem.
Common LS Firing Order Questions
Is LS firing order the same as old small-block Chevy?
No.
Old small-block Chevy firing order is:
1-8-4-3-6-5-7-2
LS firing order is:
1-8-7-2-6-5-4-3
Do not use old SBC plug wire logic on an LS.
Which side is cylinder 1 on an LS?
Cylinder 1 is the front cylinder on the driver side.
Driver side is 1-3-5-7.
Passenger side is 2-4-6-8.
What happens if plug wires are crossed?
The engine may crank, pop, backfire, misfire, run rough, or not start.
If it does run, it can dump raw fuel into the exhaust and damage catalytic converters or oxygen sensors.
Can wrong injector wiring cause a misfire?
Yes.
If the injector fires the wrong cylinder, fuel delivery happens at the wrong time. The engine may run poorly, smell rich, misfire, or stall.
Can swapped O2 sensors make fuel trims wrong?
Yes.
If Bank 1 and Bank 2 oxygen sensors are swapped, the PCM corrects the wrong bank.
That can make fuel trims look crazy and make the tune worse if you try to correct it.
Do truck and car LS engines use the same firing order?
Most LS-based Gen 3 and Gen 4 V8 engines use the same LS firing order:
1-8-7-2-6-5-4-3
Still verify your engine and ignition setup if anything is aftermarket or unusual.
Final Thoughts
Do not tune around wiring problems.
If the firing order is wrong, plug wires are crossed, injectors are swapped, or O2 sensors are on the wrong banks, HP Tuners will not save it.
Start with the basics:
LS firing order: 1-8-7-2-6-5-4-3
Driver side: 1-3-5-7
Passenger side: 2-4-6-8
Verify coil wiring.
Verify injector wiring.
Verify plug wires.
Verify O2 sensor banks.
Verify grounds.
Verify crank and cam signals.
Then start tuning.
If you want help reviewing your HP Tuners CSV after fixing the wiring basics, the StreetTunedAI LS/LT Assistant can look through the log and point out likely misfire, fueling, airflow, spark, and drivability issues.
If you want the calibration handled for you, check out our Remote Tuning Service.