CAT Engine 3422 Fault Code: Loss of Secondary Engine Speed signal
Also called Loss of Secondary Engine Speed Signal, Loss of secondary Engine Speed Signal, Secondary Engine Speed Sensor Erratic, Intermittent, or Incorrect, Secondary Engine Speed Sensor erratic, intermittent, or incorrect
Loss of Secondary Engine Speed signal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 3422 (SPN 723 / FMI 2, also shown as flash code 34 or CID 342-FMI 02) means the ECM lost or detected an intermittent signal from the secondary (bottom) engine speed/timing sensor. The engine keeps running normally on the primary sensor, but if both sensors fail, the ECM cannot fire the injectors and the engine shuts down.
Medium severity. This code alone usually does not change how the engine runs since the primary sensor is still covering timing duty. But it removes your backup, and if the primary sensor also fails you get engine misfire or a full shutdown. Treat it as a repair-soon issue, not an emergency, unless engine misfire or rough starting is already happening.
What does CAT Engine error code 3422 mean?
CAT fault 3422 points to the secondary engine speed/timing sensor, the bottom sensor that reads a timing reference ring on the crankshaft. Its whole job is backup: if the primary (top) sensor ever fails, the secondary sensor lets the ECM keep firing injectors and keep the engine running without interruption.
When the ECM sees the secondary sensor's signal go missing or become erratic, even briefly, it logs 3422. On its own this rarely changes how the engine feels because the primary sensor is still supplying valid timing data. The real risk is that you are now running without a backup: if the primary sensor also drops out, the ECM loses all crankshaft position data, cannot time injector firing, and the engine stalls.
Because of that risk, Caterpillar's guidance is direct: repair the engine speed/timing circuit at the earliest opportunity to avoid unplanned downtime. Don't let this one sit on the fault log.
What triggers a CAT Engine 3422 code?
The ECM logs 3422 only when all of these are true: the engine has been running for 3 seconds and is not cranking, the timing ring pattern is lost twice within 3 minutes without any ramp-down in engine speed, the timing ring pattern returns within 1 second of being lost, and battery voltage is above 9 VDC for the last 2 seconds. A related active code sets when there is an invalid timing ring pattern for 1 second while the primary sensor still shows a valid pattern and a signal above 100 rpm, with battery voltage again above 9 VDC for the last 2 seconds. In other listings, the trigger is simply the secondary sensor signal being intermittent or lost for less than 1 second.
Common causes of 3422
- A connector, pin, or socket at the sensor or ECM that is not fully inserted or coupled
- Harness or wiring damage: corrosion, abrasion, or pinch points along the routing
- Sensor flange not sitting flush against the engine block or housing
- Sensor O-ring missing or damaged, allowing debris or moisture intrusion
- Short circuit in the harness or connector
- Open circuit or excessive resistance in the harness or connector
- Faulty secondary speed/timing sensor assembly itself
- Intermittent connection issue that comes and goes with vibration or heat
- Faulty ECM (less common, but listed as a possible cause)
How to troubleshoot CAT Engine 3422: first checks
- Inspect the secondary (bottom) engine speed/timing sensor connector for full seating, corrosion, or bent pins.
- Trace the harness from the sensor to the ECM looking for chafe points, pinches, or corrosion, especially anywhere the harness crosses moving components or hot surfaces.
- Pull the sensor and check that its flange sits flush against the engine and that the O-ring is present and undamaged.
- Check for short or open circuit conditions in the wiring using standard continuity and resistance checks on the harness and connector.
- Check battery voltage and charging system condition, since the ECM requires battery voltage above 9 VDC to log or set this code and low voltage can complicate diagnosis.
- If wiring and connectors check out, test or swap the sensor assembly itself before condemning the ECM.
How the code clears
No separate clearing step is listed. Once the underlying wiring, connector, or sensor issue is repaired and the ECM sees a stable, valid signal from the secondary sensor, the active code should clear on its own. Confirm with a diagnostic scan tool that the code is no longer active and check stored/logged codes separately since those may need to be cleared manually depending on your scan tool procedure.
Affected models and serial ranges
3422 appears in our records across 5 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C10 | Serial range not listed in source records |
| C12 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C16 | Serial range not listed in source records |
| C27 | Serial range not listed in source records |
Frequently asked questions
What does CAT code 3422 mean?
It means the ECM detected that the secondary (backup) engine speed/timing sensor signal was lost or intermittent. The primary sensor is likely still working, so the engine keeps running, but the backup coverage is gone.
Will code 3422 shut down my engine?
Not by itself. The engine only shuts down if both the primary and secondary engine speed/timing sensors fail. If only the secondary sensor is affected, you may see engine misfire or rough running at startup, but the engine keeps operating on the primary sensor.
Is CAT flash code 34 the same as 3422?
Yes. On the diagnostic lamp this fault displays as flash code 34, on the Cat Data Link it appears as CID 342-FMI 02, and on the J1939 data link it appears as SPN 723-FMI 02. These are all the same underlying fault.
What models does this code affect?
This code applies to Caterpillar C10, C12, C15, C16, and C27 engine platforms.
What should I check first for code 3422?
Start with the secondary sensor's connector and wiring: look for loose connections, corrosion, pinched wiring, and confirm the sensor flange is flush against the engine with an intact O-ring. Most causes trace back to connectors, harness, or the sensor itself rather than the ECM.
Can I keep operating the machine with this code active?
Generally yes in the short term, since the primary sensor keeps the engine running normally. But Caterpillar recommends repairing the engine speed/timing circuit at the earliest opportunity, because losing the backup sensor increases the risk of a full shutdown if the primary sensor later fails.
Does low battery voltage affect this code?
Yes. The ECM only logs or sets this code when battery voltage is above 9 VDC for the preceding 2 seconds. Weak batteries or charging issues can influence when and how this code appears.