John Deere Engines SPN637FMI6 Fault Code: Crankshaft Position Circuit Has Low Resistance
Also called Crankshaft Position Circuit has Low Resistance
Crankshaft Position Circuit Has Low Resistance · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 637 FMI 6 means the John Deere ECU has detected low resistance (or a short/open circuit) in the crankshaft position sensor wiring during a periodic diagnostic check or Harness Diagnostic Mode Test in Service ADVISOR. The alarm level is Warning, and the ECU falls back to the camshaft position sensor to keep tracking engine speed while the fault is active.
Medium severity. This is logged as a Warning level code, and the ECU has a backup strategy using the camshaft position sensor to keep the engine running and controlled. It is not an immediate shutdown condition, but crankshaft position signal problems can affect starting, timing, and smooth running if left unaddressed, so it should be diagnosed without much delay.
What does John Deere Engines error code SPN637FMI6 mean?
SPN 637 FMI 6 points to the crankshaft position sensor circuit on John Deere engines. This sensor tells the ECU exactly where the crankshaft is in its rotation, which the ECU uses for fuel injection timing and to calculate engine speed. FMI 6 specifically flags low resistance, which can show up as a short circuit or an open circuit in that wiring loop.
The ECU picks this up during its own periodic diagnostic check while the ignition is on and the engine is stopped, and it can also be triggered during a Harness Diagnostic Mode Test run in Service ADVISOR. When the fault is active, the ECU does not simply ignore engine speed data. Instead it switches over to using the camshaft position sensor as a backup source for engine speed, so the engine can keep running while the underlying wiring or sensor problem gets sorted out.
Because this is logged as a Warning rather than a shutdown-level alarm, many operators will not notice any dramatic performance change right away. That does not mean it should be ignored, since a degraded crankshaft position circuit can eventually affect starting reliability or cause the code to reappear as an open or intermittent fault.
Common causes of SPN637FMI6
- Bad terminals or connector at the crankshaft position sensor
- Bad crankshaft position sensor
- Bad harness between the ECU and the crankshaft position sensor
- Bad terminals or connector at the ECU
How to troubleshoot John Deere Engines SPN637FMI6: first checks
- With the ignition on and engine stopped, inspect the connector and terminals at the crankshaft position sensor for corrosion, looseness, or damaged pins.
- Inspect the connector and terminals at the ECU end of the circuit for the same kind of corrosion or terminal damage.
- Trace and inspect the wiring harness between the ECU and the crankshaft position sensor for chafing, pinched sections, or shorted conductors.
- Test the crankshaft position sensor itself if the wiring and connectors check out, since a bad sensor is one of the four listed causes.
- Run the Harness Diagnostic Mode Test in Service ADVISOR to help pinpoint whether the fault is being triggered by a short or an open condition in the circuit.
How the code clears
No separate clearing step is listed for this code. Once the bad terminals, connector, harness section, or sensor causing the low resistance condition is repaired, the ECU's periodic diagnostic check should stop detecting the fault on subsequent cycles.
Frequently asked questions
What does SPN 637 FMI 6 mean on a John Deere engine?
It means the ECU has detected low resistance, which can include a short or open circuit, in the crankshaft position sensor wiring circuit. The ECU finds this during its periodic diagnostic check or during a Harness Diagnostic Mode Test in Service ADVISOR.
Is it safe to keep running the engine with this code active?
The alarm level is Warning, and the ECU switches to using the camshaft position sensor for engine speed so the engine can keep operating. It is still best to diagnose and repair the circuit soon rather than run indefinitely with a degraded crankshaft position signal.
What usually causes SPN 637 FMI 6?
John Deere lists four possible causes: bad terminals or connector at the crankshaft position sensor, a bad crankshaft position sensor itself, a bad harness between the ECU and the sensor, or bad terminals or connector at the ECU.
Will this code affect engine starting?
The crankshaft position sensor circuit feeds information used for engine speed and timing. While the ECU falls back on the camshaft position sensor when this fault is active, a genuinely bad connection or sensor can still affect starting or running smoothness, so it is worth checking promptly.
How do I test for SPN 637 FMI 6 on a John Deere engine?
Start by inspecting the connectors and terminals at both the crankshaft position sensor and the ECU for corrosion or damage, then check the harness between them for shorts or opens. Running the Harness Diagnostic Mode Test in Service ADVISOR is also part of the recommended diagnostic sequence.
Does this code trigger only while driving?
No. It is set when the ignition is on and the engine is stopped, as the ECU performs its periodic diagnostic check, or during a Harness Diagnostic Mode Test in Service ADVISOR.