P0301 on 2006-2010 Chevrolet HHR: Cylinder 1 Misfire Causes and Fixes

What's Unique About the 2006-2010 Chevrolet HHR

The Chevrolet HHR uses the Ecotec family of engines. The 2006 models with the 2.2L and 2.4L engines feature a unique, one-piece ignition coil/module cassette that covers all four cylinders. This entire unit can fail, causing misfires. From 2007-onwards, GM switched to a more common individual coil-on-plug (COP) design, where coils can be replaced one at a time. Additionally, GM issued a Technical Service Bulletin (TSB #PIP3333C) that mentions intermittent P0301 misfires on these vehicles, particularly during turns, which can be caused by fuel contamination in the fuel rail.

Diagnostic Flowchart

Tap your situation to follow the diagnostic path that matches what you're seeing on this vehicle.

Symptoms You May Notice

• Rough or shaking idle
• Engine stumbling or hesitating during acceleration
• Noticeable loss of engine power
• Flashing or solid Check Engine Light
• Smell of unburned gasoline from the exhaust
• Decreased fuel economy
• The "Service StabiliTrak" or "ESC Off" message may appear on the driver information center; this is a default response to many engine performance faults.

Most Likely Causes

1. Worn or Fouled Spark Plug 🔴 High Probability Spark plugs are a routine maintenance item. The Ecotec engine is known to be sensitive to the correct plug type and gap. Using non-OEM plugs or extending service intervals can cause misfires. The recommended plug is the ACDelco Iridium 41-103, gapped to 0.042-0.043 inches.
   How to confirm: Remove the spark plug from cylinder 1 and inspect it. Look for a worn electrode, heavy carbon fouling, oil contamination (indicating a valve cover gasket leak), or cracks in the porcelain. Compare it to a new plug.
   Typical fix: Replace the spark plug in cylinder 1. It is best practice to replace all four spark plugs at the same time with the recommended OEM-spec plugs (ACDelco 41-103) to ensure consistent performance.
   Est. part cost: $12-$20 per plug
2. Failed Ignition Coil or Coil Pack Cassette 🔴 High Probability On 2007-2010 models, the individual coil-on-plug units are a common failure point due to heat and vibration. On 2006 models, the entire integrated coil pack cassette is a unique failure point that must be replaced as a single unit. 🎬 Watch: How to replace the 2006 ignition coil cassette and plugs.
   How to confirm: For 2007+ models, swap the ignition coil from cylinder 1 with the coil from another cylinder (e.g., cylinder 2). Clear the codes and drive. If the code changes to P0302 (Cylinder 2 Misfire), the coil is faulty. For 2006 models, this test is not possible; the entire cassette is typically replaced if spark is weak or absent on cylinder 1 and the plugs are good.
   Typical fix: Replace the failed ignition coil (2007-2010 models) or the entire ignition coil cassette (2006 models). It is a single component held in by one 10mm bolt per coil, 🎬 See this walkthrough for replacing coils and boots on 2007-2010 models. or four 10mm bolts for the cassette.
   Est. part cost: $40-$75 per coil, $100-$180 for 2006 cassette
3. Clogged or Faulty Fuel Injector 🟡 Medium Probability Fuel injectors can become clogged with carbon deposits over time, especially with lower-quality fuel, preventing the correct amount of fuel from being delivered to the cylinder and causing a lean misfire.
   How to confirm: After ruling out spark plugs and coils, the injector is a likely suspect. A mechanic can perform a fuel injector balance test using a tool like the AFIT CH-47976. A DIY method is to swap the injector from cylinder 1 with another cylinder. This is more labor-intensive but will cause the misfire code to follow the bad injector. You can also test the injector's resistance with a multimeter; it should be between 11 and 14 Ohms.
   Typical fix: Replace the faulty fuel injector. It's recommended to replace the O-rings and seals at the same time. 🎬 Watch: Step-by-step guide to replacing a fuel injector on an HHR. Some owners have had success with running a high-quality fuel system cleaner like Techron or Seafoam as a first step.
   Est. part cost: $50-$120

Rare But Worth Checking

• Low Engine Compression (Burnt Valve): If ignition and fuel system checks don't reveal the problem, a mechanical issue is likely. Ecotec engines can be susceptible to burnt exhaust valves, which prevents the cylinder from sealing properly. A forum member on chevyhhr.net confirmed this was the cause of their P0301 after extensive diagnosis. A compression test is required to confirm this; a healthy Ecotec should have around 180-200 PSI per cylinder, with minimal variation.
• Vacuum Leak: A leak in a vacuum hose or the intake manifold gasket can introduce unmetered air, throwing off the air/fuel ratio and causing misfires. This would usually be accompanied by a lean code (P0171) and would often cause a random misfire code (P0300) rather than just a single cylinder.
• Fuel Contamination: TSB #PIP3333C notes that on vehicles with returnless fuel systems like the HHR, contaminants (like water or debris) can get trapped in the fuel rail. This can cause a misfire on cylinder 1 during a hard left turn or cylinder 4 on a hard right turn as fuel sloshes and pushes contaminants into the end injectors.

Diagnosis Steps

1. Read the code with an OBD-II scanner to confirm P0301 and check for any other stored codes.
2. Visually inspect the wiring and connector for the cylinder 1 ignition coil and fuel injector for any damage or loose connections.
3. For 2007-2010 models, perform an 'ignition coil swap'. Move the ignition coil from cylinder 1 to cylinder 2. Clear the codes and run the engine. If the code returns as P0302, the ignition coil is the faulty part.
4. If the code remains P0301 (or for all 2006 models), inspect and/or replace the spark plugs. Check the spark plug from cylinder 1 for wear, fouling, or damage. Ensure new plugs are gapped correctly (0.042").
5. While the spark plug is out, inspect the spark plug tube for oil. Oil presence indicates a leaking valve cover or spark plug tube seals, which can also cause misfires.
6. If the issue persists, the problem is likely with the fuel injector or the cylinder's mechanical health.
7. Listen to the fuel injector with a mechanic's stethoscope. It should have a consistent, rhythmic clicking sound. Compare it to the other injectors.
8. As a next step, consider running a bottle of high-quality fuel injector cleaner (like Techron Concentrate Plus) through a tank of gas.
9. If the misfire continues, perform a compression test on all cylinders. A healthy reading should be around 180-200 PSI. A significantly lower reading on cylinder 1 (e.g., below 120 PSI) points to a mechanical problem like a burnt valve or bad piston rings.
10. If compression is good, a fuel injector swap is the final step to confirm a bad injector before replacing it.

Parts You'll Likely Need

• Spark Plug (OEM #41-103) — This is the most common maintenance part to fail and cause a single-cylinder misfire. The Ecotec engine performs best with the specified ACDelco Iridium plug.
  Trusted brands: ACDelco
  OEM price range: $12-$20
  Aftermarket price range: $8-$15
• Ignition Coil (2007-2010) (OEM #D522C (Also UF491)) — This is the second most common failure item for a P0301 code on later models. It is responsible for creating the high voltage needed to fire the spark plug.
  Trusted brands: ACDelco, Delphi, NGK
  OEM price range: $50-$75
  Aftermarket price range: $35-$60
• Ignition Coil Cassette (2006 only) (OEM #12590954) — Unique to the 2006 model year, this entire assembly fails as a unit and is a primary suspect for misfires on that specific year.
  Trusted brands: ACDelco, Delphi
  OEM price range: $150-$220
  Aftermarket price range: $90-$150
• Fuel Injector (OEM #12582704 (2006-2007), 12613163 (2009-2011)) — If spark components are good, a clogged or electrically failed fuel injector is the next logical cause, preventing fuel from entering the cylinder.
  Trusted brands: ACDelco, Bosch, Denso
  OEM price range: $70

Related Codes That Often Appear With This One

• P0300 — If the misfire is intermittent or begins to affect other cylinders due to a shared cause like a vacuum leak or fuel pressure issue, a P0300 (Random/Multiple Cylinder Misfire) code may appear alongside P0301.
• P0302, P0303, P0304 — If you swap a faulty component (like an ignition coil or fuel injector) from cylinder 1 to another cylinder during diagnosis, the corresponding code for that new cylinder will appear, confirming the part is bad.
• P0171 — If the misfire is caused by a vacuum leak or a clogged fuel injector, the engine may run lean, triggering a P0171 (System Too Lean Bank 1) code.

Technical Service Bulletins (TSBs) & Recalls

• PIP3333C: Notes intermittent engine misfire (P0300, P0301, P0304) that can occur during left or right turns. It suggests this may be caused by fuel contaminants getting trapped in the returnless fuel rail and temporarily blocking an injector. The fix involves cleaning the entire fuel system.

Platform-Specific Known Issues

• 2006 Model Ignition Cassette: The 2006 HHR (2.2L/2.4L) uses a one-piece ignition module/coil cassette that is a known failure point. Unlike later years, individual coils cannot be swapped for diagnosis.
• Burnt Valves: A number of owners on forums have traced a persistent P0301 code to a burnt exhaust valve, requiring a cylinder head repair. This should be suspected after ignition and fuel delivery issues have been ruled out.
• TSB on Fuel Contamination: GM Technical Service Bulletin #PIP3333C addresses intermittent misfires (P0301/P0304) that occur during sharp turns. It suggests fuel contamination may be sloshing to the ends of the fuel rail and temporarily clogging the injector for cylinder 1 (left turns) or 4 (right turns).

Mechanic-Grade Diagnostic Values

• Fuel Pressure (Key On, Engine Off) — expected: 50-60 PSI. Failure: Pressure below 50 PSI, or pressure that drops to 0 immediately after the pump primes, can indicate a weak fuel pump or a leak in the system.
• Fuel Injector Resistance — expected: 11 - 14 Ohms. Failure: A reading significantly outside this range, or showing an open circuit (infinite resistance), indicates a failed injector coil.
• Ignition Coil Primary Resistance (2007+ models) — expected: ~0.3 - 2.0 Ohms. Failure: A reading outside the typical range for a coil-on-plug system suggests a faulty primary winding.
• Ignition Coil Secondary Resistance (2007+ models) — expected: ~5,000 - 15,000 Ohms (5k-15k Ω). Failure: A reading significantly outside this range indicates a failed secondary winding.

Hidden / Shadow Codes Worth Checking

• Mode $06, Test ID $A2-$A7: Advanced scan tools can access Mode $06 data, which shows misfire counts per cylinder before they are high enough to set a P0301-P0304 code. Test IDs $A2 through $A7 typically correspond to misfire counters for cylinders 1 through 6. This allows a technician to see which cylinder is misfiring, even if the Check Engine Light is not on. (see via An OBD-II scanner with Mode $06 functionality.)

Scan Tool Commands That Help

• Tech2 / GDS2 (GM Dealer Tools): Misfire Graphic / Cylinder Power Balance Test — This function provides a real-time display of engine power contribution from each cylinder. A cylinder that is misfiring will show a significant drop in its power contribution compared to the others, instantly confirming the location of the misfire without swapping parts.
• Tech2 / GDS2 (GM Dealer Tools): Fuel Injector Balance Test — This command allows the technician to individually fire each injector and measure the corresponding drop in fuel pressure. A smaller-than-specified pressure drop for cylinder 1 would indicate a clogged or failing injector. GM's specific tool for this is the AFIT CH-47976.

Wiring & Ground Locations

• G105 / G107 — G105 is on the left front of the engine block. G107 is on the front left corner of the transmission case, near the oil filter housing.. These are primary engine and transmission ground points. A loose or corroded ground at these locations can cause erratic behavior in sensors and actuators, including the ignition coils and fuel injectors, leading to misfires.
• Ignition Coil/Injector Ground — A common ground point for the ignition coils and injectors is located on a stud at the back of the cylinder head on the driver's side.. A specific failure of this ground can cause misfires on one or multiple cylinders. It is a critical point to check for looseness or corrosion if multiple ignition or fuel-related codes are present.
• Engine Harness Chafe Point (EVAP Purge Bracket) — The engine wiring harness can rub against the sharp edge of the canister purge solenoid bracket on the side of the cylinder head.. This chafing can short out various wires within the harness, including those for the ignition coils or fuel injectors, causing intermittent misfires and other electrical issues. A GM document notes this can affect any Ecotec engine.
• Engine Harness Chafe Point (ECM/TCM Bracket) — The engine harness can also chafe against the ECM/TCM bracket, the driver's side upper control arm, and the shock tower.. A GM service document identifies this as a potential area for wires to short to ground due to vibration and engine movement, which could interrupt signals to the cylinder 1 coil or injector.

Real Owner Repair Stories

• Chevy HHR Network Forum User (2011 HHR 2.2L) — P0301 misfire code, rough running.
  ❌ Tried (didn't work) Replaced catalytic converter, Replaced spark plugs, Replaced Mass Air Flow (MAF) sensor, Replaced all four fuel injectors
  ✅ What actually fixed it The user discovered the exhaust flex pipe, located before the catalytic converter, had broken open. This massive pre-O2 sensor exhaust leak was the root cause of the misfire and an associated P1174 lean code. Replacing the pipe assembly resolved the issue.
• Chevy HHR Network Forum User (2006 HHR 2.4L) — Intermittent electrical issues, Check Engine Light, various DTCs.
  ❌ Tried (didn't work) Initial diagnosis was inconclusive.
  ✅ What actually fixed it The problem was traced to the engine wiring harness rubbing through its insulation where it was anchored to the engine, specifically near the EVAP purge valve bracket. This caused intermittent shorting to ground. Repairing the worn wiring and adding protective conduit solved the problem.

OEM Part Supersession History

• 12629037 → 12638824 (Service PN), D522C (ACDelco PN) — Standard part evolution for improved durability and performance. The coil uses pure copper windings and coated laminations to resist corrosion and internal shorts.
  Heads up: While the D522C coil fits many GM Ecotec engines, it is only for 2007 and newer HHRs. It is not compatible with the 2006 model's one-piece ignition cassette system.

Model Year Variations Within This Range

• 2006 only: The 2006 2.2L and 2.4L models use a single, integrated ignition control module and coil assembly (cassette), part number 12590954. If one coil fails, the entire unit must be replaced. Diagnostic steps like 'coil swapping' are not possible.
• 2007-2011: These models use a modern coil-on-plug (COP) system with four individual ignition coils (ACDelco D522C). This allows for individual coil replacement and easier diagnosis by swapping coils between cylinders to isolate the fault.