Will a code reader detect a bad coil pack?
The short answer is yes—if the vehicle’s onboard computer has logged a fault related to the ignition system, a code reader can reveal it. A basic scanner may show generic misfire codes, while a more capable tool can display coil-specific codes and live data that point more directly to a faulty coil pack.
How a code reader relates to ignition coil faults
Code readers retrieve fault codes stored by the engine control unit (ECU). When a coil pack or its circuit fails in a way the ECU can detect, it will set a Diagnostic Trouble Code (DTC). The presence of a code can guide you toward a coil-related problem, but the exact diagnosis often depends on the type of tool you’re using and the behavior of the engine.
Key codes associated with coil pack faults
Common codes linked to coil-pack issues fall into two families: misfire codes and ignition coil circuit codes. The list below covers the most frequently seen codes, though exact codes may vary by vehicle make and model.
- P0300 — Random/Multiple Cylinder Misfire Detected
- P0301 to P0306 — Misfire Detected in Cylinder 1 through 6 (cylinder-specific misfires vary by engine)
- P0350 — Ignition Coil A Primary Circuit Malfunction
- P0351 — Ignition Coil A Primary Circuit Malfunction
- P0352 — Ignition Coil B Primary Circuit Malfunction
- P0353 — Ignition Coil C Primary Circuit Malfunction
- P0354 — Ignition Coil D Primary Circuit Malfunction
- P0355 — Ignition Coil E Primary Circuit Malfunction (where applicable)
- P0356 — Ignition Coil F Primary Circuit Malfunction (where applicable)
These codes indicate either a faulty coil, a bad coil wiring/connector, or a problem in the coil’s driver circuit in the ECU. In some engines, the codes will reference specific coils; in others, a misfire code (P0300/P0301 etc.) may be the first clue.
Limitations of a code reader in diagnosing coil problems
While a code reader is a valuable first step, it has limitations when it comes to coil packs. The following points explain why:
- Basic code readers may only show generic misfire codes (P0300) without identifying the exact coil or cause.
- Some coil pack faults are intermittent and may not trigger or store a code reliably.
- Code readers cannot measure coil resistance, current, or spark quality directly.
- Issues in wiring, connectors, or power supply to the coil can trigger P035x codes without a failing coil itself.
- Live data and misfire counters are often required to distinguish a failing coil from other problems; not all readers provide this capability.
Because of these limitations, a code reader is best used as a diagnostic starting point rather than a final, definitive test of a bad coil pack.
How to diagnose a suspected bad coil pack beyond the code reader
If you suspect a coil pack is faulty, use a systematic approach that combines software data with hands-on testing. The following steps help confirm the cause before you replace parts.
- Use a capable scan tool to read DTCs and view live data, including misfire counts per cylinder and ignition coil activity.
- Inspect the ignition system components physically: spark plugs, ignition wires or boots (if applicable), coil boots, and wiring harnesses for signs of damage, oil contamination, or corrosion.
- Perform a cylinder-by-cylinder swap test (where applicable): temporarily swap the suspect coil’s position with another cylinder’s coil and observe whether the misfire follows the coil. If the misfire code moves with the coil, the coil is likely failing.
- Check coil resistance: with the engine off and key out, measure the primary and secondary winding resistance of the coil using a multimeter, then compare to the manufacturer’s specification in the service manual. Significant deviation suggests a bad coil.
- Perform a spark test: use a spark tester to verify that each coil is producing a strong, consistent spark under compression. A weak or absent spark indicates a coil or its circuit problem.
- Examine power supply and grounding: ensure the coil’s power supply is stable and the grounding path is solid. A faulty ground or loose connection can mimic a bad coil.
- After identifying a faulty coil, replace it (and related parts if recommended by the manufacturer: boots, gaskets, etc.). Clear codes and run a test drive to confirm the issue is resolved and that no new codes appear.
Note: When performing electrical tests, follow the vehicle manufacturer’s procedures and safety guidelines. If you’re not comfortable with high-voltage ignition systems, seek a qualified technician.
Summary
Yes—an OBD-II code reader can detect a bad coil pack if the ECU has logged a related fault, such as P0300/P030x misfire codes or P035x coil primary circuit faults. The reliability of detection depends on the reader’s capabilities and whether the misfire or coil fault has been recorded. For a conclusive diagnosis, use a capable scan tool to view live data, perform targeted tests (swap tests, resistance checks, spark tests), and inspect the wiring and connectors. In many cases, a combination of codes and direct electrical testing is needed to confirm a bad coil pack and avoid unnecessary part replacement.
Can OBD2 detect a bad ignition coil?
OBD2 Scanners Can Recognize Ignition Problems
They can also identify problems with ignition coils, spark plugs, and other components via codes such as P035X (ignition coil issues) or P030X (specific cylinder misfires).
Will bad coil packs throw a code?
Yes, a bad coil pack will almost certainly throw a check engine light and corresponding diagnostic trouble code (DTC). The most common codes are P0300-P0312 for a misfire in a specific cylinder (e.g., P0301 for cylinder 1) or P0350-P0362 for a coil circuit malfunction.
Common codes from a bad coil pack
- P0300-P0312: These codes indicate a misfire. The number at the end of the code corresponds to the cylinder number (e.g., P0302 for cylinder 2).
- P0350-P0362: These codes signal a more general ignition coil circuit problem, which could be due to a faulty coil, wiring, or a connector issue.
Other signs of a bad coil pack
- Rough engine idle: A misfire can cause the engine to run unevenly.
- Stalling or poor performance: The engine will feel sluggish and may even stall, as one or more cylinders are not firing properly.
- Backfiring: Unburnt fuel can be expelled from the exhaust, leading to loud pops.
- Decreased fuel efficiency: The engine's efficiency is reduced when a cylinder isn't firing correctly.
- Smell of gasoline: Unburnt fuel can be smelled coming from the exhaust.
This video explains how to diagnose misfiring issues related to spark plugs and ignition coils: 56sFixITYouTube · Oct 2, 2024
What to do
- Use an OBD-II scanner to read the specific code. Most auto parts stores will read these codes for free.
- The code will often point to a specific cylinder, but it won't tell you whether the problem is the coil or the spark plug.
- It's often best to replace the spark plug in that cylinder along with the coil, as the two are often related.
Is there a way to test a coil pack to see if it's bad?
To test the coil pack, first locate it near the distributor or firewall. Use a multimeter to check primary and secondary resistance against specifications. Inspect for visible cracks or corrosion. If the coil shows abnormal readings or physical damage, replace it.
What is the code for ignition coil failure?
Common ignition coil error codes include P0350-P0362, which signal a malfunction in the ignition coil's primary/secondary circuit, and P0300-P0308, which indicate misfires that can be caused by a failing coil. The specific code will often include a cylinder number (e.g., P0351 for cylinder 1), but a misfire code (e.g., P0301 for cylinder 1) can also point to a bad coil. A faulty ignition coil can lead to poor engine performance, rough idling, and a lit check engine light.
You can watch this video to learn about the P035X codes and their relation to ignition coil problems: 51s8020 AutomotiveYouTube · Jul 20, 2025
Types of codes
- P035X (Circuit Malfunction): This is a specific code that points to a problem with the ignition coil's electrical circuit for a particular cylinder. The "X" is replaced by the cylinder number (e.g., P0351 for cylinder 1, P0352 for cylinder 2).
- This indicates an electrical issue, such as a break in the wiring, a bad connector, or a short circuit, in addition to a potentially failing coil.
- P030X (Misfire): This code indicates the engine is not firing correctly in a specific cylinder. The "X" is the cylinder number (e.g., P0301 for cylinder 1).
- A bad ignition coil is a very common cause of these misfire codes.
- A P0300 code is a "random" or "multiple cylinder" misfire, which can occur if multiple coils are failing or if there is another engine issue.
Common causes of ignition coil error codes
- Faulty ignition coil: The coil itself may be defective and no longer able to produce the necessary spark.
- Wiring or connector issues: A broken wire, a short circuit, or a loose or corroded connector can disrupt the electrical signal to the coil.
- Bad spark plug or wire: A problem with the spark plug or its wire can prevent a proper spark, sometimes being mistaken for an ignition coil problem.
- Other electrical problems: Issues with fuses, relays, or the powertrain control module (PCM) can also trigger these codes.
This video demonstrates how to locate the ignition coil for cylinder number 3: 53sHVAC Mechanic YouTube · Feb 20, 2024
What to do
- Note the specific code: An error code like P0351 is more specific than P0300.
- Check your cylinder's firing order: To identify which cylinder the code refers to (e.g., coil "A" or "B"), search online for your vehicle's engine and firing order.
- Perform a visual inspection: Look for any obvious damage to the ignition coil, wires, or connectors.
- Consider swapping the coil: If the ignition coil is easy to access, you can swap it with one from a different cylinder. If the misfire code moves with the coil, the coil is faulty.
- Get a professional diagnosis: Because error codes can have multiple causes, it's best to have a qualified technician diagnose the problem to ensure correct repair.
