Caterpillar · Camshaft/Timing Reference SensorHigh

Caterpillar SPN 637 FMI 2: Camshaft Timing Sensor Erratic

SPN 637 FMI 2 (FMI 2) — Engine Timing Sensor - Erratic, Intermittent, or Incorrect

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Quick Answer

SPN 637 FMI 2 means the Cat camshaft/timing reference sensor is sending an erratic or intermittent signal instead of a clean pulse. It's a High severity fault that typically causes intermittent hard starting or misfire rather than an immediate shutdown.

Overview: Caterpillar SPN 637 FMI 2

Caterpillar engines use the camshaft timing sensor alongside the crankshaft sensor to confirm exactly which cylinder is on its compression stroke, which the ECM needs to fire injectors correctly. SPN 637 FMI 2 flags a signal quality problem rather than a total loss of signal, which is why symptoms tend to be intermittent and often worse when the engine is fully warmed up. This guide from ProfessionalDieselRepair.com covers the harness and connector checks that resolve the majority of these faults before a sensor replacement is ever needed.

Repair Snapshot

Time

1.5 - 3 Hours

Difficulty

Intermediate

Parts Cost

$90 - $240

Dealer Labor

$400+

Tools Required

  • Cat ET (Electronic Technician)
  • Multimeter
  • Jaltest Diagnostic Tool
  • Diesel Laptops Diagnostic Software

What SPN 637 FMI 2 Means

SPN 637 FMI 2 means the Cat ECM is seeing an erratic or noisy signal from the camshaft timing reference sensor rather than a stable pulse train. Since the ECM cross-references this signal with the crankshaft sensor to confirm cylinder identification for injection timing, an erratic signal can cause hard starting, misfire, or in severe cases a no-start. It illuminates the amber lamp and can occasionally cause a stall if the signal drops out completely.

Symptoms Drivers Notice

  • Intermittent hard starting, especially when the engine is warm
  • Random misfire or rough running that comes and goes
  • Amber diagnostic lamp illuminated
  • Possible correlation fault alongside a crank position code

Most Likely Root Causes

Chafed sensor harness near the front gear housing

Common

Engine vibration wears through wire insulation where the harness runs close to the timing gear cover, introducing signal noise.

Corroded or loose sensor connector

Common

Moisture and heat cycling degrade the connector seal over time, creating intermittent high-resistance contact points.

Failing camshaft sensor element

Occasional

Internal sensor degradation, often heat-related, causes an intermittent signal that worsens as engine temperature rises.

Timing gear wear affecting reluctor ring accuracy

Rare

Excessive gear train wear can physically alter the timing reference the sensor reads, producing a genuinely erratic mechanical signal.

Common Misdiagnoses (Don't Waste Parts)

Replacing the sensor without checking the harness first

Why it happens: It's the fastest parts-cannon fix and the sensor is cheap enough that techs often just swap it and hope.

Do this instead: Ohm out the sensor and inspect the full harness run before replacing the part, since a chafed wire will produce the identical fault on a brand new sensor.

Chasing a fuel delivery problem for intermittent hard starting

Why it happens: Hard starting looks like a fuel or air-in-system issue at first glance.

Do this instead: Always pull codes before assuming a fuel problem - a timing sensor fault produces identical cranking symptoms without any fuel system involvement.

Overlooking a crank sensor correlation fault stored alongside this code

Why it happens: Techs focus only on the named cam sensor code and miss the bigger picture.

Do this instead: Check for any correlation or crank position codes logged at the same time, since these can indicate a shared harness routing or connector problem.

Master Tech Commentary

Mechanic's Notes

Shop tip: ProfessionalDieselRepair.com sees this misdiagnosed as a fuel delivery problem more often than any other timing sensor fault, because the symptom - intermittent hard starting - looks exactly like air in the fuel system. Always scan for codes before chasing a fuel primer or filter change; a noisy cam signal will produce identical cranking symptoms without a single drop of air anywhere in the fuel circuit.

Step-by-Step Diagnostic Procedure

  1. Step 1: Connect Cat ET and pull the active and logged fault codes, checking for an accompanying crank position correlation fault.
  2. Step 2: Inspect the camshaft sensor connector for corrosion or a loose pin, and verify the sensor is fully seated against its mounting boss.
  3. Step 3: Unplug the sensor and measure resistance across its terminals with a multimeter, comparing to the Cat spec for your engine family.
  4. Step 4: Trace the sensor harness back toward the ECM, checking for chafing near the front gear housing or accessory drive where vibration is highest.
  5. Step 5: Clear codes with Cat ET, road test while monitoring live timing signal quality, and confirm the fault does not return through a full RPM range.

Live Data & Freeze-Frame Tips

On Cat ET, monitor the engine timing signal status parameter and watch for dropouts correlated with engine vibration or temperature. If your diagnostic setup includes an oscilloscope function through Jaltest, a clean signal trace will show consistent pulse spacing, while a chafed harness typically produces sporadic noise spikes rather than a steady dropout.

Safety Warnings

  • Never work near the front gear train or accessory drive with the engine running.
  • Allow the engine to cool before accessing sensors near the timing cover.
  • Disconnect batteries before disturbing timing sensor wiring.
  • Keep loose clothing and tools clear of rotating components during inspection.

When to Limp In vs Tow

Towing is rarely necessary since the symptoms are usually intermittent rather than a complete stall. Consider towing only if the engine begins stalling repeatedly and will not reliably restart.

Prevention Tips for Fleets & Owner-Operators

  • Inspect camshaft sensor harnesses for chafing at every major PM interval.
  • Apply dielectric grease to the connector when reassembling after any front engine service.
  • Route the harness away from sharp edges any time the gear housing or accessory drive is serviced.
  • Address intermittent hard-starting complaints early before they progress to a hard fault.

Related Fault Codes

SPN 636 FMI 2

Crankshaft position sensor erratic signal, the companion timing sensor the ECM cross-checks against this one.

SPN 723 FMI 9

Engine speed sensor #2 abnormal update rate, related to the same timing reference system on Cat platforms.

SPN 190 FMI 2

Engine speed erratic, which can result from a compromised timing signal feeding the ECM's RPM calculation.

Frequently Asked Questions

Can I drive with SPN 637 FMI 2 active?

Generally yes, since the fault tends to cause intermittent hard starting rather than a stall while driving, but it should be diagnosed promptly to avoid being stranded with a no-start.

How do I reset this code without a scanner?

A battery disconnect may temporarily clear the stored fault, but it will return as soon as the erratic signal condition reoccurs if the underlying wiring issue isn't repaired.

Does this trigger a power derate?

This particular fault doesn't typically trigger an emissions-related derate, but a severely degraded timing signal can cause the engine to run poorly or fail to start reliably.

Why does my truck only hard-start when it's warm?

Heat causes connector and wiring materials to expand, which can turn a marginal connection into an intermittent open or noisy signal specifically at operating temperature.

Is the camshaft sensor expensive to replace?

No, the sensor itself is typically inexpensive, usually under $150, though labor cost depends on how accessible it is on your specific Cat engine platform.

Can this code cause a complete no-start?

In severe cases where the signal drops out entirely rather than just being erratic, yes, the ECM may not be able to confirm cylinder position well enough to start the engine reliably.