2007 Chevy Classic Knock Sensor Diagnosis: Aftermarket vs. OEM Parts

Initial Diagnosis and Suspicions

• 💡 The vehicle presented with a knock sensor code (P0327), despite previous replacement of knock sensors and harness.
• ⚠️ A key diagnostic finding was that the knock sensor signal dropout occurred only when the PCM was plugged in, leading to suspicion of a faulty PCM.
• 💬 Discussions with Keith Perkins from L1 Auto Diagnostics and research on Identifix revealed no common cases of PCM failure due to knock sensor codes, raising further questions.

Diagnostic Testing and Circuit Switching

• 🔬 The diagnostic strategy involved switching knock sensor circuits to determine if the issue was with the PCM or the sensor itself.
• ⚡ Testing with a scope showed that the known good circuit (light blue wire) produced a good signal when isolated, but the signal disappeared when the suspect sensor (dark blue wire) was connected to it.
• 🧐 Conversely, when the suspect sensor was connected to the known good circuit, the signal was present but with lower amplitude, and when the known good sensor was connected to the suspect circuit, the signal disappeared.

Identifying the Faulty Part

• 🛠️ After extensive testing and ruling out the PCM, the focus shifted back to the knock sensors, specifically the one on the dark blue wire (front sensor).
• 🔍 Visual inspection revealed what appeared to be cracks in the porcelain of the suspect sensor, though this was later determined to be a manufacturing mark.
• 🚀 The definitive test involved swapping the sensors and circuits: the problem followed the suspect sensor to the known good circuit, confirming it was the faulty component.

Aftermarket vs. OEM Parts

• ⚠️ The suspect sensors were identified as aftermarket (likely Dorman), indicated by "Made in Taiwan" and a part number consistent with Dorman kits.
• ✅ Genuine GM OEM sensors were ordered and installed, resolving the issue.
• 📈 The video highlights the significant difference in signal amplitude and reliability between the aftermarket and OEM knock sensors, emphasizing the importance of using OEM parts for this component.

Key Takeaways and Lessons Learned

• 🔑 Always question your diagnosis and ask, "If this doesn't work, what's my next step?" This proactive approach prevents unnecessary part replacements.
• 💡 Resistance checks and open circuit tests can be misleading; testing under load (with the PCM plugged in) is crucial for accurate diagnosis.
• 🗣️ Switching circuits is a valuable diagnostic technique to isolate component failures from wiring or module issues.
• 💰 The shop that installed the aftermarket parts was contacted, and Dorman agreed to cover the cost of the parts and some labor, as the customer was charged for OEM but received aftermarket.