SSB Inductive Signature Malfunction
Your engine's secondary shaft position sensor isn't sending a clean signal to the computer, like a radio station losing its broadcast strength. The ECU can't reliably read the shaft's rotational signature to synchronize ignition or fuel timing.
The ECM monitors the inductive signal voltage and frequency from the secondary shaft sensor, typically used on camshaft or variable valve timing systems. It compares signal amplitude and edge transitions against stored thresholds to detect opens, shorts, or degraded sensor output.
| Parameter | Normal Range | Fault Condition |
|---|---|---|
| Signal Amplitude | 0.3–5.0 volts AC minimum | Below 0.3V or no signal detected |
| Edge Rise Time | Within specification for shaft speed | Excessive noise or missing transitions |
Code P1721 is a moderate fault. You can generally drive to a workshop, but avoid long trips or high-load driving (motorway, uphill towing) until it is diagnosed. If the code keeps returning after clearing, or if you notice the symptoms listed above worsening, do not delay professional diagnosis. Many moderate codes have multiple possible root causes — a mechanic with live OBD data can identify the exact fault more efficiently than part-by-part trial and error.
Once the fault is repaired, P1721 can be cleared using any OBD-II scanner. Connect the scanner, navigate to "Clear Codes" or "Erase DTCs," and confirm. The check engine light turns off immediately.
The code will return if the root cause was not actually fixed. The ECM re-detects the fault within 1–3 drive cycles and sets the code again.