CAT 42 Fault Code: Primary to Secondary Engine Speed Signal Calibration
Primary to Secondary Engine Speed Signal Calibration · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 42, SPN 637 FMI 11, means the ECM found the primary and secondary engine speed/timing sensors reading more than 3 degrees apart after a timing calibration completed with battery voltage above 9 volts. It affects C11, C13, and C15 engines and points to a sensor, wiring, or mechanical timing alignment mismatch, not necessarily an immediate shutdown.
Medium severity. The engine can run on a single working speed/timing sensor without a noticeable performance change, so code 42 alone usually does not stop the engine. But if both sensor signals are lost during operation, the ECM will terminate injection and shut the engine down, and if both are lost at start-up the engine will not start. Diagnose this code before it progresses to a dual-signal loss.
What does CAT error code 42 mean?
This code is about how the ECM keeps track of crankshaft position and engine speed. The engine uses two speed/timing sensors that read a unique pattern on the camshaft gear. The ECM counts the time between pulses to figure rpm and uses the pulse pattern to determine crankshaft position, which tells it exactly when to fire each injector.
Code 42 sets when the ECM completes a timing calibration but finds the primary and secondary sensor signals disagreeing by more than 3 degrees, while battery voltage is above 9 volts. That disagreement means the two sensors are not agreeing on where the camshaft actually is, even though the calibration process finished.
The two sensors are both magnetic sensors but they are not interchangeable and must not have their positions swapped. If either sensor needs replacement, both must be replaced as a pair. A timing calibration is only required after replacing an ECM that will not communicate, not after simply replacing the sensors.
What triggers a CAT 42 code?
The ECM sets this code when a timing calibration has completed successfully, the primary and secondary engine speed/timing sensors are reading more than 3 degrees apart, and battery voltage is above 9 volts at the time of the check.
Common causes of 42
- ECM connection issue (loose, corroded, or damaged connector)
- Engine speed sensor failure or a sensor installed improperly
- Engine speed sensor harness damage or wiring fault
- ECM failure
- Improper front timing alignment
- Damaged cluster gear
- Backlash issue at the camshaft
- Engine speed/timing sensors or their wiring out of spec
How to troubleshoot CAT 42: first checks
- Inspect the ECM connector for corrosion, looseness, or damaged pins before touching the sensors.
- Check both engine speed/timing sensor connectors and the wiring harness between the sensors and the ECM for chafing, corrosion, or loose terminals.
- Confirm neither sensor was swapped in position during a prior service. The two sensors are not interchangeable.
- Verify front timing alignment and inspect the cluster gear and camshaft backlash for mechanical damage if wiring and connections check out.
- If sensors were replaced, confirm no unnecessary timing calibration was performed since replacing sensors does not require one, only ECM replacement (when the ECM will not communicate) does.
How the code clears
No separate clearing step is listed for this code. Correcting the underlying cause, whether that is a connector, sensor, harness, ECM, or mechanical timing issue, should allow the ECM to complete a clean calibration without the greater than 3 degree offset. No reset procedure beyond fixing the fault is described.
Affected models and serial ranges
42 appears in our records across 3 CAT models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C11 | Serial range not listed in source records |
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
Frequently asked questions
What does CAT fault code 42 mean?
It means SPN 637 FMI 11, a mismatch of more than 3 degrees between the primary and secondary engine speed/timing sensor signals after a timing calibration completed with battery voltage above 9 volts. It affects C11, C13, and C15 engines.
Will code 42 shut down my engine?
Not by itself. The engine can start and run on just one working speed/timing sensor with no noticeable performance change. It only shuts down if both sensor signals are lost while running, and it will fail to start if both are lost at start-up.
Can I swap the positions of the two speed/timing sensors?
No. The two sensors are not interchangeable and their positions must not be switched. If replacement is needed, replace both sensors as a pair.
Do I need to recalibrate timing after replacing the sensors?
No. Timing calibration is not necessary after replacing the sensors. It is only required after replacing an ECM that will not communicate.
What mechanical issues can cause code 42 besides sensors?
Improper front timing alignment, a damaged cluster gear, or camshaft backlash problems can all cause the primary and secondary signals to disagree, in addition to sensor or wiring faults and ECM issues.
Which CAT engines can throw fault code 42?
C11, C13, and C15 engines are listed as affected by this code.