CAT Engine 37 Fault Code: Cylinder #3 Injector : Not Responding Properly
Also called Cylinder #3 Injector - Not Responding Properly, Cylinder #3 Injector Not Responding, Cylinder #3 Injector not Responding, Cylinder #4 Injector : Erratic, Intermittent, or Incorrect, Fuel Pressure Sensor Fault
Cylinder #3 Injector : Not Responding Properly · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 37 (SPN 653 / FMI 7) means the ECM has determined that an electronic unit injector, most commonly cylinder #3, is no longer capable of delivering the correct amount of fuel. The engine will derate and the warning light will come on. This is found on C175 engines and requires running the Fuel System Verification Test and often replacing the suspect injector.
High severity. The engine derates as soon as this code is active, and the ECM logs it along with a warning light. It is not an immediate shutdown condition, but continued operation on a derated engine with a fuel delivery problem risks further injector or cylinder damage, so it should be diagnosed promptly rather than run for an extended period.
What does CAT Engine error code 37 mean?
Fault code 37 relates to an electronic unit injector, on the C175 typically identified with cylinder #3, that the ECM has judged incapable of delivering the correct amount of fuel. The common rail fuel system uses a solenoid inside each injector, and the ECM sends 105 V pulses to that solenoid at a timing and duration matched to engine load and speed. When those pulses do not produce the expected injector response, the ECM sets this code.
Every injector on the engine shares its power supply circuit with another injector, but each injector has its own separate return (ground) circuit back to the ECM. Because of this shared wiring arrangement, a short circuit to ground on the return line for one injector can sometimes affect how the ECM reads the paired injector, meaning one injector may work fine while the ECM reports a fault on the other. This is why a specific Injector Solenoid Test is used to isolate which circuit is actually at fault.
On C175-20 engines specifically, cylinders 9, 10, 19, and 20 do not share a power supply circuit with another injector. Those four cylinders have their own individual power and return circuits, so the shared-circuit troubleshooting logic does not apply to them the same way it does to paired cylinders.
Common causes of 37
- Leakage at the injector or associated fuel system components
- Damaged connectors and/or wiring in the injector circuit
- A short circuit detected on the injector solenoid supply or return line
- An open circuit detected on the injector solenoid supply or return line
- A problem with the ECM itself
- A problem with the wiring harness, including the external engine harness
- Resistance building up in the return wires
- A bad or failed injector
- Low current detected in the injector solenoid circuit, which can point to connector, wiring, ECM wiring, or injector harness problems under the valve cover
How to troubleshoot CAT Engine 37: first checks
- Connect the electronic service tool and confirm the active/logged diagnostic code and which cylinder is affected.
- Run the Injector Solenoid Test with the engine off. This briefly activates each solenoid; a good solenoid clicks audibly, and the tool reports each circuit as OK, Open, or Short.
- Inspect connectors and wiring at the suspect injector and along the engine harness for corrosion, chafing, or damage, since wiring problems that only show up during vibration are a known cause.
- Check the shared power supply circuit and the individual return circuit for the affected injector and its paired injector to determine which side is actually causing the fault.
- Run the Fuel System Verification Test using the electronic service tool after any repairs to confirm whether the diagnostic code is still active.
- If the code remains active after wiring checks, plan to replace the suspect electronic unit injector following the Disassembly and Assembly procedures for injector removal and installation.
How the code clears
No separate clearing step is listed beyond running the Fuel System Verification Test. If the diagnostic code is still active after that test, the suspect electronic unit injector should be replaced, then the Fuel System Verification Test should be run again to confirm the fault is resolved. When an injector is replaced, the new injector's trim code must be programmed into the ECM; if the ECM itself is replaced, all injector trim codes must be reprogrammed into the new ECM.
Affected models and serial ranges
37 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 fault code 37 mean?
It means the ECM has detected that an electronic unit injector, typically cylinder #3 on the C175, can no longer deliver the correct amount of fuel, or that the injector solenoid circuit is not responding properly. The engine will derate and the warning light will come on.
Will my engine keep running with code 37 active?
Yes, but it will run in a derated state. The ECM logs the code, turns on the warning light if equipped, and reduces engine power output while the fault is present.
How do I find out if the problem is wiring or the injector itself?
Run the Injector Solenoid Test through the electronic service tool with the engine off. It activates each solenoid individually and reports the circuit status as OK, Open, or Short, which helps isolate whether the fault is in the wiring or the injector.
Why would one injector show a fault when its paired injector seems fine?
Injectors share a common power supply circuit in pairs, but each has its own separate return circuit. A short circuit to ground on one injector's return line can sometimes cause the ECM to log a fault on the paired injector even though that injector is working normally.
Do I need to reprogram anything after replacing the injector?
Yes. Each replacement injector has a trim code that must be programmed into the ECM. If the ECM itself is replaced, every injector's trim code needs to be reprogrammed into the new ECM.
What test should I run after making a repair?
Use the electronic service tool to run the Fuel System Verification Test. If the code is still active after that test, the suspect injector likely needs to be replaced.
Is code 37 specific to one engine model?
The documented occurrences of this code are tied to the C175 engine, including the C175-20 variant, which has a slightly different wiring arrangement for cylinders 9, 10, 19, and 20.