The Problem
2022 Audi e-tron GT quattro (41,100 miles) brought in with intermittent 'Powertrain Fault — Performance Reduced' and P0C73 after a rear left wheel bearing replacement at a general repair shop one week prior. Fault appeared only above 60 mph.
Diagnostic Procedure
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1. Confirm P0C73. Intermittent, speed-dependent (>60 mph). Speed-dependent faults strongly suggest a mechanical or sensor signal issue rather than a hard electrical failure.
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2. Check rear motor resolver signal quality via oscilloscope during road test. At low speed: resolver sine/cosine output clean. Above 60 mph: noise spikes visible on resolver signal, correlating with wheel rotation frequency.
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3. The noise frequency matches the rear left wheel speed sensor tooth frequency at corresponding speed — electromagnetic interference from the adjacent ABS wheel speed sensor.
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4. Inspect rear left wheel speed sensor installation (site of recent wheel bearing work). Sensor bracket bolt torqued to approximately 4 Nm — spec is 9 Nm. Under-torqued bracket allows sensor to oscillate at high speed.
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5. Oscillating wheel speed sensor generates a variable magnetic field that induces noise into the resolver winding — the two sensors share a common proximity due to packaging.
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6. Torque wheel speed sensor bracket to spec (9 Nm). Apply thread locker per Audi specification.
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7. Road test: 70 mph sustained. Resolver signal clean. P0C73 does not return.
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8. No return after 500-mile customer follow-up.
Resolution & Root Cause
Rear left wheel speed sensor bracket torqued to specification. Resolver signal noise eliminated. P0C73 resolved.
💡 Key Lesson
P0C73 resolver fault appearing after adjacent wheel bearing or suspension work — always check the wheel speed sensor torque spec before any motor or resolver diagnosis. An under-torqued wheel speed sensor bracket vibrates at road speed and can induce noise into the nearby resolver signal, mimicking a resolver hardware failure. The fix is a torque wrench, not a drive unit replacement.
About This Case
This case was solved remotely by an HVDesk specialist with 15+ years of hands-on experience across major EV platforms including Tesla, Hyundai/Kia, Volkswagen ID series, BMW i-series, and Ford EVs. The procedure was provided as structured remote support to an independent auto repair shop.
