The P0340 code in your 2012 Subaru Impreza indicates a camshaft position sensor circuit malfunction, which on the 2.0L FB20 boxer engine can be caused by timing belt tensioner failure affecting camshaft timing accuracy. When the tensioner fails to maintain proper belt tension, the camshaft position varies from expected values, triggering P0340.
Understanding the Timing System
The 2012 Impreza's FB20 engine uses a timing chain (not belt), unlike older Subaru engines. However, if your Impreza has the older EJ-series engine in certain markets or configurations, it uses a timing belt with a hydraulic tensioner. The tensioner keeps constant pressure on the timing belt to prevent slack. When it fails, belt tension varies, causing camshaft timing to fluctuate.
Recognizing Tensioner Failure
Symptoms include a rattling or slapping noise from the front of the engine at startup or idle, the engine may run rough as valve timing fluctuates, and P0340 sets as the camshaft position doesn't match expected values. In severe cases, a completely failed tensioner can allow the belt to skip teeth, potentially causing valve-to-piston contact on this interference engine.
Diagnostic Process
First verify which engine your Impreza has—FB20 uses timing chain, EJ-series uses belt. Listen for timing area noise at cold startup. Monitor camshaft position sensor waveforms with an oscilloscope if available. On belt-equipped engines, visual inspection of the tensioner (if accessible) may reveal fluid leakage or abnormal movement. Check timing marks alignment to verify belt hasn't slipped.
Repair Requirements
If the tensioner is failing, timing belt replacement with a complete kit (belt, tensioner, idler pulleys, water pump) is recommended. On boxer engines, this requires significant disassembly. If timing has slipped due to tensioner failure, check for valve damage before reassembly. This is not a DIY job for most owners—the boxer engine layout and timing complexity require experience.