C1117
RPM Input Circuit Failure
Chassis Speed/Idle Control RPM Signal Loss 🔴 Serious — Stop or limit driving
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What This Actually Means
In plain language — no jargon

The engine control computer can't read the engine speed (RPM) signal properly, like a tachometer with a broken wire. Without this critical input, the engine struggles to run smoothly or may not start at all.

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Symptoms You May Notice
3 known symptoms for this code
Engine stalls or won't start
Rough idle or unstable RPM
Check engine light illuminated
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Embedded Systems Insight
What the ECU/ECM is actually computing

The ECU monitors the crankshaft position sensor or ignition signal to calculate engine RPM in real-time. It compares the signal frequency and pattern against expected thresholds to detect open circuits, shorts, or signal dropout. When RPM data is missing or erratic, the ECU cannot control fuel injection timing and ignition properly.

Voltage & Parameter Thresholds

Parameter Normal Range Fault Condition
RPM Signal Frequency Continuous signal proportional to engine speed No signal, intermittent signal, or constant dropout
Signal Voltage 0.5-5.0V alternating pattern Stuck voltage, open circuit, or short to ground
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Diagnostic & DIY Fix Guide
Check these in order — from cheapest to most complex
1
Crankshaft position sensor connector
Inspect and reseat the CKP sensor connector at the engine block for corrosion or loose pins.
2
Engine wiring harness
Check the RPM signal wiring for damage, pinches, or water intrusion between the sensor and ECU.
3
Crankshaft position sensor
Replace the CKP sensor if connectors are clean but signal remains absent or erratic.