P0304 on 2021 GMC Sierra 1500 3.0L Duramax: Cylinder 4 Misfire Causes and Fixes

What's Unique About the 2021-2021 GMC Sierra 1500

Unlike a gasoline engine where a misfire often points to a spark plug or ignition coil, on the 3.0L LM2 Duramax diesel, the cause is fundamentally different. Misfires are almost always related to the fuel delivery or its electronic control. For this specific engine, owner experiences and GM Technical Service Bulletins (TSBs) point heavily towards electrical connection issues at the injector, particularly a faulty injector wiring harness connector that becomes loose or damaged from heat, especially on cylinder 4. This is often due to fretting corrosion from vibration or heat degradation from its proximity to the EGR pipe.

Symptoms You May Notice

• Check engine light is on or flashing
• Engine running rough, especially under load
• Noticeable shaking or vibration at idle or while driving
• Reduced engine power and acceleration, sometimes with a 'Reduced Acceleration' message
• Vehicle may enter limp mode, with messages like '165 miles until 65mph max speed'
• Multiple warning messages on the dash, such as "Service ESC", "Service Emission System", or "Service Parking Brake"

Most Likely Causes

1. Faulty Fuel Injector Wiring Harness or Connector 🔴 High Probability → Shop Fuel Injector The connector for injector #4 is located directly under or near the EGR pipe, where high heat causes the plastic to become brittle and the connection to loosen over time. Vibration can also cause fretting corrosion (visible as black dust) inside the connector 🎬 Watch: Identifying terminal fretting and connector corrosion issues., leading to a poor electrical signal. This is a widely documented failure.
   How to confirm: Visually inspect the wiring connector at the cylinder 4 fuel injector for looseness, damage, or corrosion. A scan tool will often show a companion code, P0204 (Injector Circuit/Open - Cylinder 4), which strongly points to a wiring issue. Wiggling the connector with the engine running may induce the misfire. A technician can check pin tension on the connector terminals.
   Typical fix: Replace the entire fuel injector wiring harness. 🎬 See this step-by-step guide for installing a new engine harness. A temporary fix of applying dielectric grease may work for a short time, but replacement is the permanent solution. Some owners opt to replace just the pigtail connector.
   Est. part cost: $40-$80 for the full harness, ~$20 for a pigtail connector.
2. Chafed Engine Wiring Harness 🟡 Medium Probability GM issued TSB #21-NA-149 which details several locations where the main engine harness can rub against sharp metal edges, such as on top of the generator, the ECM/TCM bracket, or the driver-side upper control arm. This can cause wires for any cylinder's injector to short out.
   How to confirm: Perform a thorough visual inspection of the entire engine wiring harness, focusing on the specific chafe points identified in TSB #21-NA-149. This may require removing components like the air cleaner housing or the driver-side wheel and liner to gain access. The bulletin warns that the ECM is often unnecessarily replaced for this issue.
   Typical fix: Repair the damaged wires using approved wire-to-wire repair procedures and protect the harness with anti-abrasion tape or loom. Re-route the harness to prevent future contact.
   Est. part cost: $5-$25
3. Failed Fuel Injector ⚪ Low Probability → Shop Fuel Injector While less common than wiring issues, the injector itself can fail. TSB #22-NA-087 notes that a broken terminal pin *inside* the injector's connector socket can cause codes like P0204 and P0304. This specific failure requires replacing both the injector and the harness together.
   How to confirm: After ruling out external wiring issues, a technician can perform an injector balance rate test with a scan tool. The definitive test is to swap the cylinder 4 injector with another cylinder (e.g., cylinder 3). If the misfire code changes to P0303, the injector is faulty. A resistance test can also be performed; the piezo stack resistance should be between 160k-190k Ohms.
   Typical fix: Replace the cylinder 4 fuel injector and its corresponding high-pressure fuel line. Per TSB #22-NA-087, if a broken internal pin is found, the injector and harness must be replaced as a pair. The new injector's flow rate (IQA) number must be programmed into the ECM using a scan tool like GDS2.
   Est. part cost: $300-$500

Rare But Worth Checking

• Low Cylinder Compression: A mechanical issue like a bad valve, piston, or head gasket could cause a misfire, but this is much less likely than a fuel or electrical problem on the LM2 and would likely present with other symptoms like smoke or coolant loss.
• Contaminated Diesel Fuel: A recent fill-up before the issue started could point to bad fuel. This would typically cause misfires on multiple cylinders (P0300) rather than an isolated P0304.

Diagnosis Steps

1. Scan for all stored diagnostic trouble codes. The presence of P0204 with P0304 is a very strong indicator of a wiring or connector issue at cylinder 4.
2. Perform a careful visual inspection of the fuel injector connector on cylinder 4. It is located under the EGR pipe. Check if it is loose, cracked, or shows signs of heat damage (brittle plastic). Disconnect it and look for black dust (fretting corrosion).
3. If the connector seems loose, attempt to secure it, clear codes, and test drive. Some owners report a temporary fix by applying dielectric grease, but this is not a permanent solution.
4. If the connector is intact, inspect the engine wiring harness for signs of chafing per TSB #21-NA-149. Key locations are on top of the generator, near the ECM/TCM bracket, and around the driver's side shock tower/upper control arm.
5. A 'wiggle test' can be performed. With the engine idling, carefully wiggle the harness at the injector and along its path to see if it induces a misfire or causes the engine to stumble.
6. If wiring is confirmed good, swap the cylinder 4 fuel injector with an adjacent one (e.g., cylinder 3). Clear the codes and drive the vehicle. If the code changes to P0303, the injector is faulty. If the code remains P0304, the problem is in the wiring or the cylinder itself.
7. If the injector is suspected, inspect the injector's male pins for breakage, as noted in TSB #22-NA-087. If a pin is broken, both the injector and harness must be replaced.
8. As a final step if all else fails, perform a cylinder compression test to rule out mechanical engine problems.

Parts You'll Likely Need

• Fuel Injector Wiring Harness (OEM #40009605 (supersedes 55513485, 55512145, 55510039, 55508024)) — This is the most common failure point. The connector at the injector becomes loose or damaged from heat and vibration, causing an open circuit.
  Trusted brands: GM Genuine Parts, ACDelco
  OEM price range: $70-$90
  Aftermarket price range: $50-$70
• Fuel Injector Connector Pigtail (OEM #19368140 (for individual connector)) — If only the connector at cylinder 4 is damaged and the rest of the harness is intact, splicing in a new pigtail is a cheaper alternative to replacing the entire harness. Aftermarket options from brands like Fleece Performance are also available.
  Trusted brands: ACDelco, Fleece Performance
  OEM price range: $20-$30
  Aftermarket price range: $15-$25
• Fuel Injector (OEM #55506244) — If the injector itself has failed electronically or has a broken internal pin, it will need to be replaced. This should only be done after confirming the wiring is not at fault.
  Trusted brands: ACDelco, Bosch
  OEM price range: $350-$500
  Aftermarket price range: $250-$400

Related Codes That Often Appear With This One

• P0204 — This code means 'Injector Circuit/Open - Cylinder 4'. It is the most common companion code to P0304 because a faulty electrical connection (open circuit) to the injector is what causes the misfire.
• P0216 — This code for 'Injection Timing Control Circuit Malfunction' can also appear alongside P0304 and P0204, as seen in owner reports. TSB 22-NA-087 also groups this code with injector circuit faults, indicating it can be triggered by the same underlying wiring harness issues.
• P0300 — This code for 'Random/Multiple Cylinder Misfire' might appear if a wiring harness chafe is intermittent or affecting multiple injector circuits, or if the initial misfire on cylinder 4 is severe enough to disrupt overall engine stability.

Technical Service Bulletins (TSBs) & Recalls

• TSB #21-NA-149: Addresses engine wiring harness chafing against various underhood components, including the generator and ECM bracket. This can cause a wide variety of DTCs, including injector circuit faults P0201-P0206 and misfires P0301-P0306.
• TSB #22-NA-087: Specifically mentions a broken terminal pin inside an injector harness connector as a cause for P020x codes on the 3.0L LM2. It directs technicians to replace both the fuel injector and the injector wiring harness if this internal pin is found to be broken.
• TSB Bulletin #23-NA-043: This bulletin was released to correct customer concerns regarding poor engine performance, engine misfires, and the Malfunction Indicator Lamp (MIL) being illuminated due to DTCs P0300 through P0308.

Platform-Specific Known Issues

• The 3.0L LM2 engine platform is known for wiring harness issues, as documented in GM TSB #21-NA-149 (harness chafing) and TSB #22-NA-087 (injector connector pin failure). These make electrical faults a much higher probability for misfire codes than on other diesel engines.

Mechanic-Grade Diagnostic Values

• Fuel Injector Piezo Stack Resistance — expected: 160,000 - 190,000 Ohms (160k - 190k Ω). Failure: A reading significantly outside this range, OL (open circuit), or near zero (shorted) indicates internal injector failure.
• Injector Balance Rates (at idle, via GDS2 scan tool) — expected: Typically +/- 4 mm³. Failure: A value consistently outside this range for a specific cylinder points to a fueling problem with that injector or cylinder.
• High-Pressure Common Rail Fuel Pressure (Max) — expected: Up to 36,250 psi (2,500 bar). Failure: Inability to reach target pressure under load can indicate a weak high-pressure pump, but is not specific to a single cylinder misfire.
• Injector Circuit Wire Continuity (from ECM to injector connector) — expected: < 0.5 Ω. Failure: Resistance higher than 0.5 Ω indicates corrosion or a partial break in the wire.
• Injector Circuit Voltage (Key On, Engine Off) — expected: 10 - 14 Volts (Battery Voltage). Failure: Low or no voltage points to a problem in the power supply circuit, not the injector itself.

Scan Tool Commands That Help

• GDS2 (GM Global Diagnostic System 2): Cylinder Power Balance — Used to confirm which cylinder is misfiring. The tool disables injectors one by one and measures the RPM drop. If disabling cylinder 4 causes little or no change in RPM, it confirms that cylinder was not contributing power.
• GDS2 (GM Global Diagnostic System 2): Fuel Injector Balance Test — This is a critical test after confirming the wiring is good. The tool commands each injector to fire and measures the corresponding drop in fuel rail pressure. An inconsistent or smaller pressure drop for cylinder 4 points to a clogged or mechanically failed injector.
• GDS2 (GM Global Diagnostic System 2): Injector Flow Rate Programming (IQA) — This is a mandatory service function that must be performed after replacing any fuel injector. Each new injector has a unique flow rate code printed on it that must be programmed into the ECM for proper fuel delivery.

Wiring & Ground Locations

• Harness Chafe Point 1 (TSB 21-NA-149) — On top of the generator (alternator), approximately 18 inches from the ECM connectors.. TSB 21-NA-149 specifically identifies this as a chafe point for the 3.0L LM2 engine harness that can cause injector circuit DTCs like P0204 and subsequent misfires like P0304.
• Harness Chafe Point 2 (TSB 21-NA-149) — At the PVC hose clamp at the crankcase pressure regulator, about 4 inches from the sensor connector.. This is another specific chafe point listed in the TSB for injector circuit faults. The metal clamp can wear through the harness loom and short a wire.
• Harness Chafe Point 3 (TSB 21-NA-149) — Near the black ECM/TCM bracket, located behind the felt liner in the front driver's side wheel well.. This is a general chafe point for all engines on this platform. The metal bracket has sharp edges that can damage the main harness. Access requires removing the driver's front wheel and liner.
• Harness Chafe Point 4 (TSB 21-NA-149) — At the driver-side upper control arm and the nearby shock tower bolt.. As the suspension moves, the harness can rub against the control arm or shock tower bolt, causing abrasion and short circuits.

Real Owner Repair Stories

• Reddit user on r/gmcsierra (2021 GMC Sierra 3.0L Duramax with 113,000 miles) — Started running terribly, multiple dash warnings ('Service ESC', 'Service Emission System'), and codes P0204, P0304, and P0216. The misfire was only present when driving, not at idle.
  ❌ Tried (didn't work) The owner suspected bad fuel from a recent fill-up, but the issue was isolated to cylinder 4.
  ✅ What actually fixed it The owner replaced the fuel injector wiring harness. The root cause was confirmed to be a loose connection at the #4 injector, which had been affected by heat from the nearby EGR pipe.
• Reddit user on r/Duramax (2021 Chevrolet Silverado 3.0L Duramax with 58,000 miles) — Check engine light with code P0305 (Cylinder 5 Misfire).
  ❌ Tried (didn't work) The first dealership drove the truck for three days and could not diagnose or fix the issue.
  ✅ What actually fixed it A second dealership's diesel technician diagnosed and solved the problem in 24 minutes, identifying it as bad connectors in the injector harness, a known issue covered by a GM memo.

Documented NHTSA Reports

• An owner reported in NHTSA ODI #11715507 that their vehicle experienced a check engine light and traction control light, becoming undriveable; the dealer identified code P0304 and stated the lifters needed to be replaced.
• NHTSA ODI #10971531 describes an Active Fuel Management failure on the number 4 cylinder with a P0304 DTC, noting a loud ticking noise and a miss on that cylinder.

OEM Part Supersession History

• 55508024, 55510039, 55512145, 55513485 → 40009605 — GM has released multiple revisions of the fuel injector wiring harness for the LM2 engine, likely to improve durability and prevent the common connector and chafing issues.

Diagnostic Flowchart

Other Known Issues on This Vehicle

Issues unrelated to this code that are worth knowing about as an owner of this generation:

• Long Crank or No-Start Condition 🔴 High — Common on early LM2 engines. Can be intermittent. (Ref: Multiple TSBs exist, often pointing to a faulty camshaft position exciter wheel that may be bent or warped from the factory.)
• Rear Main Seal Oil Leak 🔴 High — Can occur at low mileage, sometimes leading to rapid and catastrophic oil loss. (Ref: TSB #22-NA-022 (supersedes PIP5795) addresses the rear main seal shifting out of position, requiring replacement of the timing chain lower cover which contains the seal.)
• Faulty Engine Coolant Control Valve 🟠 Medium — Prevalent on 2021-2023 models, causing Check Engine Lights and cooling system warnings. (Ref: TSB #22-NA-082 and Special Coverage Program N252508340 (15yr/150k miles) exist for this issue, which can cause codes like P1098 and P2B60.)
• Internally Collapsing Charge Air Cooler (CAC) Hose 🟠 Medium — Affects 2020-2026 models, causing a significant lack of power or stalling during acceleration while idling normally. (Ref: A GM service bulletin was released to address internal restriction of the CAC inlet/outlet hose, requiring hose replacement.)
• DEF System and Emissions Sensor Failures 🟠 Medium — A frequent complaint leading to 'Service Emission System' warnings and potential limp mode. (Ref: TSB #21-NA-07 addresses issues with faulty DEF sensors.)

Used vs. New Parts: Buying Guide for This Vehicle

When a used part is the smart pick: For this repair, using a used part is generally not recommended. The primary failure points—the injector wiring harness and the fuel injector—are wear items and are subject to heat and vibration damage. A used harness carries the same risk of failure as the part being replaced.

Donor-vehicle mileage cap: roughly under 50000 miles for the part to have meaningful remaining life.

What to inspect on the donor part:

• If considering a used harness, inspect the connectors for any signs of cracking, brittleness, or heat damage.
• Check the connector terminals for any black dust (fretting corrosion) or signs of spreading.
• Ensure the loom is intact with no signs of abrasion, melting, or previous repairs.

OEM-only on this vehicle (don't cheap out):

• Fuel Injector Wiring Harness
• Fuel Injector

Aftermarket brands forum-validated for this vehicle:

• Fleece Performance (for individual injector connector pigtails)

Brands owners have reported issues with on this vehicle:

• Unbranded, no-name wiring harnesses or injectors from online marketplaces should be avoided due to the precision required for the common rail fuel system.

Real Owner Stories

Aggregated from forums and TSBs cited above. Mileages and costs reflect what owners reported in those sources.

Related OBD-II Codes

• P0204 — Injector Circuit/Open - Cylinder 4; this is the most common companion code for P0304 on this vehicle, indicating a wiring or connector failure at the specific injector.
• P0300 — Random or Multiple Cylinder Misfire Detected; often triggered alongside P0304 when harness chafing affects multiple circuits or causes significant engine instability.
• P0201-P0206 — Injector circuit faults for other cylinders that may appear if the main engine wiring harness is chafing against the generator or ECM bracket per TSB #21-NA-149.
• P0216 — Injection Timing Control Circuit; reported by owners in conjunction with P0304 during injector circuit wiring failures.
• P0303 — Cylinder 3 Misfire; used during diagnosis to confirm a failed injector if the code moves from P0304 to P0303 after swapping injectors between cylinders.

Frequently Asked Questions