Diagnosing Multiple Misfire Codes P0302, P0305, and P0308 with a Black & Decker Smart Scan Tool

Encountering engine trouble can be frustrating, especially when a single problem seems to multiply into a series of issues. One of our website visitors recently reached out with a perplexing situation involving their Jeep Patriot and a cascade of misfire codes. Initially facing a P0302 code, indicating a misfire in cylinder 2, their troubleshooting journey led to the emergence of additional codes – P0305 and P0308 – after spark plug and coil replacements. They were using a , model AD925, to read these codes. Let’s delve into their problem, analyze the diagnostic steps taken, and offer expert advice on how to resolve these multiple misfire codes.

The Initial Problem: P0302 Misfire

The user described experiencing a sudden onset of engine shaking, rough idling, poor acceleration, and limp mode after encountering a bump on the road. Using their Black & Decker Smart Scan car diagnostic tool, they pulled a P0302 code. This code specifically points to a misfire detected in cylinder 2.

To isolate the issue, they wisely followed standard diagnostic procedures by:

• Swapping Ignition Coils: They exchanged the ignition coil from cylinder 2 with cylinder 3 to see if the misfire followed the coil. The subsequent codes P0305 and P0308 appeared, suggesting initially that the coil was the culprit.
• Replacing Spark Plugs: Based on the shifted codes, they deduced the spark plugs might be the issue and replaced all four. However, the rough idling and limp mode persisted, and the codes remained, along with the original P0302, P0305 and P0308.
• Replacing Ignition Coil on Cylinder 2: Despite the spark plug replacement, the user then installed a new ignition coil on cylinder 2, the cylinder initially flagged by the P0302 code.

After these steps, the engine reportedly ran “great” and “completely normal.” However, the Black & Decker Smart Scan tool continued to display the P0302, P0305, and P0308 codes, and they were unable to clear them using the tool.

Understanding Misfire Codes and Diagnostic Steps

Misfire codes, such as P0302, P0305, and P0308, indicate that one or more cylinders are not firing correctly. This can be due to a variety of reasons, ranging from ignition system problems to fuel delivery issues, vacuum leaks, or even internal engine problems.

The user referenced a service manual for their Jeep Patriot, which is a crucial step in proper diagnostics. The manual outlines a diagnostic inspection procedure for a P0302 code, which is relevant to their situation even with the additional codes. Let’s break down the diagnostic inspection points from the service manual and relate them to the user’s experience:

1. Worn Serpentine Belt: The user correctly assessed that the serpentine belt seemed fine and unlikely to be the cause. A worn belt could potentially affect the alternator or other driven components, but is less directly linked to misfires.
2. Misalignment or Binding Water Pump, P/S Pump, or A/C Compressor Pulleys: Similar to the serpentine belt, while pulley issues can cause noise and system malfunctions, they are not primary causes of misfires. The user reasonably ruled this out.
3. Improper CKP, CMP, MAP, or TP Sensor Mounting: These sensors – Crankshaft Position (CKP), Camshaft Position (CMP), Manifold Absolute Pressure (MAP), and Throttle Position (TP) – are critical for engine management. While the user noted these are often associated with P219A codes (related to air-fuel ratio imbalance), sensor issues can indirectly cause misfires if they provide incorrect data to the engine control module (PCM). It’s important to note that sensor problems can lead to misfire codes, even if they are not directly P030X codes. Checking sensor connections and potentially testing sensor outputs is a valid step.
4. Poor Connector/Terminal to Component Connection: This is a highly relevant point. Loose, corroded, or damaged connectors to ignition coils, fuel injectors, and sensors (CKP, CMP, MAP, TP) can disrupt signals and cause misfires. The user visually inspected for corrosion, which is a good start, but a more thorough check might be needed, potentially including contact cleaner and ensuring secure connections.
5. Corroded PCM Power or Ground Circuits: The PCM (Powertrain Control Module) is the brain of the engine management system. Poor power or ground connections to the PCM can cause a wide range of issues, including misfires. The user checked ground cables and found them in good condition, which is positive, but verifying PCM power and ground, as per the service manual, is a more in-depth check.
6. Vacuum Leaks: Vacuum leaks are a common cause of misfires. Unmetered air entering the intake manifold can lean out the air-fuel mixture, leading to combustion problems, especially at idle and low speeds. The user visually inspected for leaks, but vacuum leaks can be subtle. Using a vacuum gauge or smoke tester is often necessary to pinpoint them accurately.
7. Restriction in the Air Induction or Exhaust System: A blocked air filter or a restricted exhaust system (e.g., clogged catalytic converter) can cause engine performance issues, including misfires. Checking the air filter is a simple and worthwhile step. Exhaust restrictions are less common but should be considered if other causes are ruled out.
8. Internal Engine Component Malfunction: This is a more serious category, encompassing issues like valve problems, piston ring issues, or cylinder head gasket leaks. While less likely in a sudden onset scenario, internal engine problems can cause misfires. Compression testing and leak-down testing are diagnostic procedures to assess internal engine health, if simpler causes are eliminated.
9. Moisture on Ignition System Components: Moisture can interfere with ignition, especially in older vehicles or in damp conditions. While unlikely to be a persistent issue if the engine is now running well, it’s worth considering if the problem arose after rain or washing the engine.
10. Insufficient Fuel / 11. Low Quality Fuel: Fuel delivery issues are a prime suspect for misfires. Insufficient fuel pressure, clogged fuel injectors, or even contaminated fuel can cause misfires. While the user has fuel in the tank, fuel pressure testing and injector testing might be necessary. Low-quality fuel is less likely to cause sudden misfires but is worth considering if the vehicle has been fueled recently at an unfamiliar station.
11. Manual Transmission Bog / 13. Towing Overload: These are less relevant as the vehicle is not a manual transmission and has not been towing.

Addressing the User’s Questions

Let’s directly answer the user’s questions based on the information provided and expert knowledge:

1. Do I need to replace more ignition coils? (replaced CYL2, should I do 1,3,4?)

It’s possible, but not the first step now that the engine is running smoothly. While you initially suspected a coil issue, and replacing the coil on cylinder 2 seemed to improve the engine’s running condition, the persistent codes suggest the problem might be more complex or that the codes are not clearing correctly. Before replacing more coils, focus on systematically checking the other diagnostic points, especially vacuum leaks, sensor connections, and fuel delivery. If misfires return or persist after addressing these areas, then further coil testing or replacement might be warranted.

2. What do P0305 and P0308 mean on a four-cylinder? These are NOT outlined in the service manual.

This is a crucial observation and points to a potential issue with the diagnostic tool or a misunderstanding. P0305 and P0308 codes on a four-cylinder engine are not standard misfire codes. Standard misfire codes for a four-cylinder engine would be P0300 (random/multiple cylinder misfire), P0301 (cylinder 1), P0302 (cylinder 2), P0303 (cylinder 3), and P0304 (cylinder 4). Codes P0305 and P0308 are typically associated with engines having more than four cylinders, such as 6-cylinder or 8-cylinder engines, referring to cylinders 5 and 8 respectively.

This discrepancy suggests one of the following:

• Incorrect Code Reading: The Black & Decker Smart Scan tool might be misinterpreting or incorrectly displaying the codes. It’s less likely, but possible.
• PCM Issue: In a more unusual scenario, the PCM itself could be malfunctioning and incorrectly reporting codes.
• Generic OBD-II Interpretation: Generic OBD-II readers sometimes provide a general code range (P0300-P0304 for misfires) and the specific cylinder number is based on the vehicle’s configuration. However, displaying P0305 and P0308 on a 4-cylinder is still anomalous.

Action: Double-check the codes with a different OBD-II scanner to confirm if P0305 and P0308 are indeed being reported. A parts store or repair shop can often perform a free code scan. If the same codes are consistently reported by multiple scanners, it would point towards a more unusual PCM or wiring issue. If a different scanner only shows P0302 or P0300, then the initial reading might be erroneous.

3. Will addressing P0302 further fix these codes?

Potentially, but it’s not guaranteed, especially given the uncertainty around P0305 and P0308. If the P0305 and P0308 readings are indeed incorrect and the underlying issue is primarily a misfire related to cylinder 2 (P0302), then resolving the root cause of the P0302 misfire might clear all misfire codes. However, it’s crucial to accurately diagnose the cause of the P0302, not just treat the symptom. Systematically work through the diagnostic checklist.

4. If the engine is running fine, but the codes are still showing up, is there another way I can clear them to see if there is actually an issue still?

The Black & Decker Smart Scan AD925 should be able to clear codes. If it’s not clearing them, there could be a few reasons:

• Persistent Fault: The most common reason codes won’t clear is that the underlying fault is still present, even if the engine seems to be running better. The PCM will often re-trigger the code quickly if the problem persists.
• Scanner Limitation: While the AD925 is a functional OBD-II scanner, it might have limitations in clearing certain types of codes or in certain vehicle systems.
• Procedure Error: Double-check the scanner’s manual to ensure you are following the correct procedure for clearing codes. Sometimes, the ignition needs to be in a specific position (e.g., “key on, engine off”) for clearing to work.
• Battery Disconnect (Caution): As a last resort (and with caution), you can try disconnecting the negative battery terminal for a few minutes. This will often reset the PCM and clear codes. However, this is not a recommended diagnostic procedure and should be done with caution. Disconnecting the battery can erase learned settings in the PCM and may cause other issues, such as radio presets being lost or requiring the throttle body to relearn its position. It’s always better to clear codes using a scanner.

Recommended Action:

• Re-attempt Code Clearing: Carefully follow the instructions for your Black & Decker Smart Scan tool to clear codes. Ensure the ignition is in the correct position.
• Verify Codes with Another Scanner: As mentioned, use a different OBD-II scanner to confirm the codes being reported.
• Address Diagnostic Checklist: Systematically work through the diagnostic inspection list from the service manual, focusing on vacuum leaks, sensor connections, and fuel delivery to cylinder 2.
• If Codes Persist After Addressing Potential Issues: If you’ve diligently checked for vacuum leaks, sensor issues, and fuel delivery problems and the codes still persist and cannot be cleared, and especially if the engine runs perfectly fine, there’s a possibility of a PCM issue or a more unusual electrical fault. In this case, seeking professional diagnostic help from a qualified mechanic is recommended.

Conclusion and Next Steps

Troubleshooting engine misfires can be methodical. Using a Black & Decker Smart Scan car diagnostic tool is a great first step in understanding the problem by retrieving codes. However, code reading is just the beginning. A systematic approach to diagnosis, like the one outlined in the service manual, is essential.

In this case, the user has made good initial steps by replacing spark plugs and a coil. The next crucial steps are:

1. Verify the Accuracy of Codes: Confirm if P0305 and P0308 are genuinely being reported or if there is a misinterpretation. Use a second OBD-II scanner.
2. Systematically Check for Vacuum Leaks: Use appropriate tools and methods to thoroughly check for vacuum leaks.
3. Inspect Sensor Connections: Carefully inspect connectors to CKP, CMP, MAP, TP sensors, fuel injectors, and ignition coils. Clean and secure connections.
4. Consider Fuel Delivery: If vacuum leaks and sensor connections are ruled out, investigate fuel delivery to cylinder 2 (and potentially other cylinders), including fuel pressure and injector function.

By following these steps, and potentially re-scanning with their Black & Decker Smart Scan tool after each step, the user should be able to pinpoint the root cause of the misfires and resolve the issue, even if those unusual P0305 and P0308 codes are still a bit of a mystery. Remember, when in doubt, professional diagnostic assistance can save time and prevent unnecessary parts replacements.