Detroit Diesel SPN 723 FMI 2: Camshaft Position Sensor Erratic
SPN 723 FMI 2 (FMI 2) — Engine Camshaft Speed/Position Sensor - Data Erratic, Intermittent, or Incorrect
Quick Answer
SPN 723 FMI 2 means the backup camshaft speed/position sensor signal is erratic or intermittent. It's Medium severity - most drivers notice no drivability change since this is a secondary signal to the primary crankshaft sensor.
Overview: Detroit Diesel SPN 723 FMI 2
ProfessionalDieselRepair.com treats this as an important-but-not-urgent repair - the redundancy this sensor provides matters most if the primary crank sensor ever fails.
Repair Snapshot
Time
1 - 2 Hours
Difficulty
Intermediate
Parts Cost
$120 - $320
Dealer Labor
$400+
Tools Required
- Detroit Diesel Diagnostic Link (DDDL)
- Multimeter
- Jaltest
What SPN 723 FMI 2 Means
SPN 723 FMI 2 means the secondary camshaft speed/position signal is dropping out or reading erratically. Since this sensor backs up the primary crankshaft signal, most drivers won't notice any drivability change - it's Medium severity mainly because ignoring it removes a layer of redundancy the ECM relies on if the crank sensor ever fails.
Symptoms Drivers Notice
- Amber check engine light with no noticeable drivability change in most cases
- Possible hard start if the primary crank signal is also marginal
- No power loss under normal operation
- Fault may appear and disappear across drive cycles
Most Likely Root Causes
Corroded or loose camshaft sensor connector
CommonElectrical fault - a poor connection produces the intermittent signal described by this code.
Harness chafing near the exhaust manifold or gear housing
CommonElectrical fault - heat and vibration in this location are known to degrade insulation over time.
Failing camshaft position sensor
OccasionalA genuine sensor hardware issue confirmed after ruling out the connector and harness.
Damaged camshaft tone wheel teeth
RareMechanical fault - physical damage to the timing gear reluctor pattern produces erratic signal readings even with a good sensor.
Common Misdiagnoses (Don't Waste Parts)
Replacing the sensor without checking the connector or harness first
Why it happens: It feels like the fastest way to resolve a sensor-named code.
Do this instead: This is a parts-cannon shortcut - an intermittent signal is a classic connector or chafed-wire symptom. Inspect those first before ordering a new sensor.
Treating this as an urgent roadside repair
Why it happens: Any camshaft-related code sounds serious to drivers unfamiliar with the redundancy built into the system.
Do this instead: Explain that this is a backup signal with no drivability impact in most cases, and schedule the repair at the next convenient service interval rather than an emergency stop.
Master Tech Commentary
Mechanic's Notes
ProfessionalDieselRepair.com note: this sensor is a backup to the primary crank signal, so don't let a driver talk you into an emergency repair for a code with zero drivability symptoms - schedule it as routine maintenance, but don't skip it entirely since redundancy matters if the primary sensor ever fails.
Step-by-Step Diagnostic Procedure
- Step 1: Connect DDDL and confirm whether the fault is active or intermittent, and check the fault occurrence counter to gauge how often it's happening.
- Step 2: Locate the camshaft position sensor at the rear of the cylinder head near the gear housing and inspect the connector for corrosion or a loose lock tab.
- Step 3: Back-probe the sensor's signal circuit with a multimeter and check for a clean, consistent waveform while cranking, or measure sensor resistance if it's a passive reluctor-style unit.
- Step 4: Inspect the wiring harness between the sensor and ECM for chafing against nearby brackets or heat damage from proximity to the exhaust manifold.
- Step 5: If the sensor and harness test clean but the fault persists, check the camshaft tone wheel for damaged or missing teeth through the sensor bore.
Live Data & Freeze-Frame Tips
In DDDL, log the camshaft position sensor signal continuously during a normal drive cycle and note conditions (temperature, RPM range) when dropouts occur - this often points directly to a heat-related harness issue near the exhaust.
Safety Warnings
- Allow the engine to cool before working near the camshaft sensor at the rear of the cylinder head.
- Disconnect the battery before disconnecting sensor connectors to avoid an unintended crank event.
When to Limp In vs Tow
Not necessary - this fault doesn't affect drivability in the vast majority of cases since it's a backup signal only.
Prevention Tips for Fleets & Owner-Operators
- Inspect camshaft sensor connectors for corrosion during PM service.
- Route new harness repairs away from exhaust manifold heat.
- Address intermittent sensor faults promptly rather than letting them persist for years.
Related Fault Codes
SPN 190 FMI 2
Erratic primary crankshaft speed signal - a more serious version of a similar signal-quality problem.
SPN 636 FMI 2
Crankshaft position sensor circuit fault - the primary signal this camshaft sensor exists to back up.
Frequently Asked Questions
Can I keep driving with this code active?
Yes - this fault doesn't typically affect drivability since it's a backup signal, though it's worth fixing to maintain full redundancy.
How do I reset it without a scanner?
It may clear temporarily, but it will likely return if the connector or wiring issue isn't fixed. Confirm the repair with DDDL.
Does this code trigger a derate?
No - this is a signal quality issue on a backup sensor, not a fault severe enough to warrant a power reduction.
Is this always a connector problem?
Most of the time, yes, but always rule out a genuinely failing sensor or damaged tone wheel if the connector and harness check out clean.
How much does this repair cost?
A connector repair is often under $100 in parts. A full sensor replacement typically runs $250-$320 including labor.
Why does this fault come and go?
Intermittent connector or wiring issues often only show up under specific conditions like heat, vibration, or engine movement, which is why the symptom appears and disappears unpredictably.