CAT Engine 42 Fault Code: Cylinder #4 Injector : Erratic, Intermittent, or Incorrect
Also called Check Sensor Calibrations, Cylinder #4 Injector - Erratic, Intermittent, or Incorrect, Cylinder #4 Injector Erratic, Intermittent, or Incorrect, Cylinder #4 Injector erratic, intermittent, or incorrect, Engine Injector Cylinder #04 : Erratic, Intermittent or Incorrect, Engine Injector Cylinder #04 : Erratic, Intermittent, or Incorrect
Cylinder #4 Injector : Erratic, Intermittent, or Incorrect · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT Code 42 flags erratic, intermittent, or incorrect data from the No. 4 cylinder electronic unit injector, or in some older descriptions, unrecalibrated Speed/Timing or Boost Pressure sensors. The ECM will log the code, warn the operator, and derate the engine while it stays active.
High severity. The engine is derated while this code is active, and injector faults on the No. 4 cylinder can affect combustion and engine protection. It should be diagnosed before continued heavy operation, though it is not an immediate shutdown situation on its own.
What does CAT Engine error code 42 mean?
CAT fault code 42 (SPN 654 / FMI 2) most commonly means the ECM has found that data from the electronic unit injector on cylinder No. 4 is out of limits, indicating erratic, intermittent, or incorrect signal behavior from that injector's circuit.
On some Caterpillar models and ECM software versions, code 42 instead relates to sensor calibration status. If the Speed/Timing Sensor or Boost Pressure Sensor has not been correctly calibrated (common right after an ECM replacement), the ECM sets code 42 as a current-only code that is not logged in memory.
Because both the injector version and the sensor-calibration version of this code affect fuel delivery and timing, Caterpillar has the engine respond by derating power (injector version) or defaulting to base timing and zero boost (sensor version) to protect the engine until the issue is resolved.
What triggers a CAT Engine 42 code?
For the injector version: data from the No. 4 cylinder electronic unit injector is out of limits, and diagnostic code 0168-01 is not active, codes 0004-05 and 0004-06 are not active, no 0041 codes are active, no 0262 codes are active, code 0190-08 is not active, and no 0110 codes are active. For the sensor-calibration version: the Speed/Timing Sensor or Boost Pressure Sensor is not correctly calibrated, most often seen after installing a new or replacement ECM.
Common causes of 42
- Faulty ECM
- Faulty injectors (No. 4 cylinder electronic unit injector)
- Disconnected ECM
- Damaged connectors and/or wiring on the injector solenoid circuit
- Short circuit detected in the injector supply or return line
- Open circuit in the injector supply or return line
- Resistance in return wires
- Problem with the wiring harness
- Leakage at the injector
- Damaged hose (boost pressure sensor circuit)
- Water entering the boost pressure sensor module
- Moisture in the boost pressure hose
- Faulty sensors (Speed/Timing or Boost Pressure)
- Leaks in the boost pressure hose
- Faulty transducer module
How to troubleshoot CAT Engine 42: first checks
- If this is a new or replacement ECM, check whether the Speed/Timing Sensor and Boost Pressure Sensor have been calibrated yet; code 42 will show until calibration is completed and is normal in that situation.
- With the engine not running, run the Injector Solenoid Test through the diagnostic tool. This briefly activates each solenoid, listens for an audible click, and reports each circuit as OK, Open, or Short.
- Inspect injector wiring and connectors for damage, corrosion, or looseness, paying attention to the shared power supply and individual return circuit for each injector.
- Check the boost pressure sensor hose and module for moisture, water intrusion, leaks, or physical damage.
- Check for other active diagnostic codes listed in the set conditions (0168-01, 0004-05, 0004-06, 0041, 0262, 0190-08, 0110). If any of these are active, resolve them first since they can mask or relate to the No. 4 injector fault.
- Verify ECM connections are secure, since a disconnected or faulty ECM is listed as a possible cause.
How the code clears
No separate clearing step is listed beyond correcting the underlying cause. For the sensor-calibration version of this code, it should clear on its own once the Speed/Timing Sensor and Boost Pressure Sensor are correctly calibrated. For the injector version, the code is active while the fault condition is present and the ECM logs it; repairing or replacing the faulty injector, connector, or wiring, or the ECM if faulty, should allow the code to clear.
Affected models and serial ranges
42 appears in our records across 1 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C175 | Serial range not listed in source records |
Frequently asked questions
What does CAT code 42 mean?
It usually means the ECM detected erratic, intermittent, or incorrect data from the No. 4 cylinder electronic unit injector. On some models, it instead means the Speed/Timing Sensor or Boost Pressure Sensor has not been calibrated yet, which is common after installing a replacement ECM.
Will code 42 cause a loss of power?
Yes, when it relates to the No. 4 injector, the engine will be derated while the code is active. If it relates to an uncalibrated Boost Pressure Sensor, boost pressure is set to 0 kPa (0 psi) by the ECM, which limits fuel delivery rate and horsepower.
Is code 42 always logged in memory?
No. When code 42 is caused by sensor calibration issues, it is current but not logged in memory. When it is caused by the No. 4 injector data being out of limits, the ECM does log the code and displays a warning.
I just replaced my ECM and code 42 appeared. Is that normal?
Yes. On a new or replacement ECM, code 42 will appear until the Speed/Timing Sensor and Boost Pressure Sensor have been properly calibrated. Complete the calibration procedure and the code should clear.
How do I test the injector solenoid for code 42?
With the engine off, run the Injector Solenoid Test on your diagnostic tool. It briefly activates each solenoid, and you should hear an audible click. The tool reports each circuit's status as OK, Open, or Short, helping you identify which injector or circuit is faulty.
Can wiring cause code 42 instead of the injector itself?
Yes. Damaged connectors, wiring harness problems, open or short circuits, and resistance in return wires can all trigger this code even if the injector itself is fine. Since injectors share power supply circuits in pairs, a short on a neighboring injector's return line can sometimes affect the reading on the injector logging the fault.
What other codes should I check alongside code 42?
Check whether codes 0168-01, 0004-05, 0004-06, 0041, 0262, 0190-08, or 0110 are active. Code 42's set conditions require that none of these are active, so resolving them first may be necessary before properly diagnosing the No. 4 injector issue.