Komatsu Wheel Loader CA559 Fault Code: Supply Pump Pressure Very Low Error
Also called Rail Press Low Error, Rail Pressure Low Error, Supply Pump Pressure Very Low Error (Engine Controller System), Supply Pump Pressure Very Low Error 1, Supply Pump Pressure Very Low Error 1 (Engine Controller System), Supply pump pressure very low error, Supply pump pressure very low error 1 (Engine controller system)
Supply Pump Pressure Very Low Error · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CA559 means the engine controller has detected a no-pressure feed or low pressure condition in the common rail fuel circuit. It sets the centralized warning lamp and alarm buzzer ON, and can cause hard starting, black exhaust smoke, and reduced engine output. It affects the WA150 through WA500 Komatsu wheel loader lineup.
High severity. CA559 points to a fuel delivery or common rail pressure problem that can prevent the engine from starting or running properly. It is not an immediate shutdown-level emergency in every case, but ignoring it risks injector damage, supply pump damage, or being stranded with a no-start machine. Diagnose before returning the loader to normal duty.
What does Komatsu Wheel Loader error code CA559 mean?
CA559 is triggered when the engine controller detects that the common rail fuel circuit is not building or holding the pressure it expects, described as a rail pressure low error or a no-pressure feed condition. The common rail system stores high-pressure fuel and meters it to each injector, so if pressure is low the engine can't fuel properly.
When this code sets, the centralized warning lamp and alarm buzzer turn on to alert the operator. On the machine you may notice the engine won't start or is hard to start, exhaust smoke turns black, and engine output drops noticeably under load.
This is a fuel system integrity issue, not just a sensor glitch. It can stem from anything between the fuel tank and the injectors, including the tank itself, the filters, the low-pressure lines, the supply pump, the pressure limiter, the injectors, or the rail pressure sensor circuit.
Common causes of CA559
- Improper or contaminated fuel
- Fuel leakage to outside the system
- Defective low-pressure circuit devices (includes low fuel volume, clogged tank breather, leaking or clogged low-pressure piping, clogged fuel pre-filter, clogged fuel main filter)
- Defective pressure limiter
- Defective injector, including the high-pressure piping in the cylinder head
- Defective supply pump, including defects in the supply pump plunger
- Defective common rail pressure sensor system, including its electrical wiring
- Defective electrical system of the supply pump EMV (electromagnetic valve)
- Air trapped in the low-pressure circuit
- Defective related system components generally
How to troubleshoot Komatsu Wheel Loader CA559: first checks
- Check the remaining fuel volume in the tank; running low or near empty can trigger a low-pressure condition
- Inspect the fuel tank breather for clogging, which can restrict fuel flow to the low-pressure circuit
- Check low-pressure fuel piping for leaks or clogging between the tank and the supply pump
- Inspect and, if needed, replace the fuel pre-filter and fuel main filter; if clogging is found and cleared but the code persists, replace the fuel filter
- Look for visible fuel leakage to the outside of the system, at fittings, lines, and the pressure limiter
- Check the common rail pressure and the common rail pressure sensor voltage using the monitoring function (Code 36400 RAIL PRESS for pressure, Code 36402 RAIL PRESS for voltage)
- Bleed the low-pressure circuit if air is suspected, since trapped air can mimic a low pressure feed
- Verify the electrical connections and wiring to the supply pump EMV are intact and free of corrosion
How the code clears
No separate clearing step is listed beyond correcting the underlying cause. The listed method of reproducing the failure code is to start the engine, so after repairs, start the engine and confirm the common rail pressure reads normally on the monitoring function (Code 36400 RAIL PRESS) before returning the loader to service. If the low-pressure circuit was clogged and cleaning does not clear the code, replace the fuel filter as directed.
Affected models and serial ranges
CA559 appears in our records across 11 Komatsu Wheel Loader models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| WA150 | Serial range not listed in source records |
| WA200 | Serial range not listed in source records |
| WA250 | Serial range not listed in source records |
| WA270 | Serial range not listed in source records |
| WA320 | Serial range not listed in source records |
| WA380 | Serial range not listed in source records |
| WA430 | Serial range not listed in source records |
| WA450 | Serial range not listed in source records |
| WA470 | Serial range not listed in source records |
| WA480 | Serial range not listed in source records |
| WA500 | Serial range not listed in source records |
Frequently asked questions
What does Komatsu fault code CA559 mean?
It means the engine controller detected a rail pressure low error or no-pressure feed condition in the common rail fuel circuit. This turns on the centralized warning lamp and alarm buzzer and can cause hard starting, black exhaust smoke, and lost engine power.
Can I keep operating the loader with CA559 active?
It's not recommended. With low common rail pressure the engine may not start reliably, exhaust smoke turns black, and output drops. Continued operation risks further damage to the injectors or supply pump.
What is the most common cause of CA559 on Komatsu wheel loaders?
Low-pressure circuit problems are commonly checked first: low fuel volume, a clogged fuel tank breather, clogged or leaking low-pressure piping, and clogged fuel pre-filter or main filter. Clearing these clogs often resolves the code.
How do I check common rail pressure on a Komatsu wheel loader?
Use the machine's monitoring function. Rail pressure itself is viewed under Code 36400 RAIL PRESS, and the sensor's voltage signal is viewed under Code 36402 RAIL PRESS.
Will replacing the fuel filter fix CA559?
It can, if the cause is a clogged fuel pre-filter or main filter. If clogging in the filters or related components is removed and the failure code still doesn't clear, the fuel filter should be replaced.
Does air in the fuel system cause CA559?
Yes, air trapped in the low-pressure circuit is a listed possible cause and can prevent the supply pump from building proper rail pressure.
How is CA559 diagnosed after repairs are made?
The listed method of reproducing the failure code is simply starting the engine. After addressing the suspected cause, start the engine and check that rail pressure reads correctly on the monitoring function before putting the loader back to work.