Derate, sometimes called limp mode, is not a failure in the ordinary sense. It is the engine controller deciding that continuing at full output would damage something, and reducing power to a level it considers safe. That distinction matters in diagnosis, because a derated machine is one whose controller has already identified the problem and stored the reason. The usual triggers fall into four groups: an engine protection input that has crossed its limit, such as low oil pressure, high coolant temperature or high intake air temperature; an aftertreatment or emissions condition, most often a loaded diesel particulate filter or a dosing fault; a sensor that has failed in a way the controller cannot compensate for; and a supply or communication fault that leaves the controller unable to trust the signals it can see. Read the stored derate event before replacing anything, because the derate is the message and the component is only sometimes the cause.
What you will notice
| Symptom | What it usually points to |
|---|---|
| Power drops as a step change and stays down | A derate is a limit, not a decline. A machine that ran normally a minute ago and is now at reduced output has been limited deliberately rather than worn out. |
| Warning lamp lit, or a message on the monitor | The controller normally announces that it has acted. The lamp and the stored code together say which protection was crossed. |
| Power returns after a restart, then drops again after the same running time | A derate that clears on a restart and returns after a repeatable interval is a protection still being exceeded, not an intermittent electrical fault. |
| Reduced output with no unusual smoke and no abnormal noise | Mechanical power loss arrives with smoke, noise or both. A clean-running machine that has quietly lost output has usually been limited in software. |
Most likely causes
| Cause | How to confirm it |
|---|---|
| Engine protection input crossed its limit | Oil pressure, coolant temperature and intake air temperature each carry their own threshold. Once one is crossed, the controller reduces output on purpose. |
| Aftertreatment or dosing condition | A loaded or failed particulate filter, or a fault in the dosing system, produces a derate that is deliberate and often progressive rather than sudden. |
| A sensor failure the controller cannot substitute | Rail pressure, boost pressure and speed signals that go outside the range the controller can work around leave it no option but to limit output. |
| Supply voltage or ground quality | A controller fed by a poor ground or a low supply treats every reading with suspicion, and limiting output is its default safe response. |
What triggered the derate, and where to look first
| Trigger group | First place to look |
|---|---|
| Oil pressure, coolant temperature, intake temperature | Confirm the reading with an independent gauge before trusting the sensor, then check the real system: oil level and condition, coolant level and flow, radiator and aftercooler cleanliness. |
| Aftertreatment or dosing fault | Read the aftertreatment codes and the soot or ash load. A derate that follows a failed regeneration is usually the filter rather than the engine. |
| Rail, boost or speed sensor | Compare the sensor value with the commanded or calculated value in live data. A reading that looks plausible but never changes with load has failed without setting a code. |
| Supply voltage and grounds | Measure supply at the controller with the load on, and check the ground with a voltage drop test. A poor ground produces faults that do not exist. |
| Derate event log and freeze-frame | The record of what the controller saw at the moment it acted. It is the most valuable evidence in the whole job, and it is destroyed if codes are cleared first. |
How to confirm it
- Record the derate event and every stored code before clearing anything
Write down the codes, the counters, the derate percentage and the freeze-frame values, or photograph the screen. Clearing codes to see what comes back is a reasonable tactic later, but doing it first throws away the record of the event you are trying to diagnose.
- Confirm it is a derate and not a mechanical power loss
Check for smoke, exhaust restriction, unusual noise and a monitor message. A step change with no mechanical signature points at the controller, and a gradual loss with smoke points at fuel, air or compression. Getting this wrong sends the whole job down the wrong path.
- Verify the triggered reading independently
Fit a mechanical gauge or an external temperature measurement to the parameter the controller says it acted on. Sensors drift, and a derate triggered by a sensor that reads high is a different repair from one triggered by a genuine overheat.
- Check supply voltage and grounds before condemning any sensor
Measure at the controller with the circuit loaded, then do a voltage drop test on the ground path. These are minutes of work and they explain a steady share of derates that are blamed on components.
- Read the aftertreatment state where the codes point there
Check the soot load, the ash load and whether a regeneration is being inhibited by duty cycle or by a fault. A machine working short shifts at low load can accumulate a filter load that a long shift would have burned off.
- Repair, clear, then reproduce the original condition and re-read
Run the machine to the same temperature and load that produced the original derate, and read the counters again. A repair confirmed only by the absence of a lamp on a cold machine is not confirmed at all.
What to do about it
Sensor and circuit repair: where an independent gauge disagrees with the sensor, check the connector tension and the harness before the sensor itself, then replace the sensor and re-seal the connector. A connector opened without a new seal is a water path into the circuit.
Supply and ground repair: clean the ground point to bare metal, apply a protective compound and torque to specification, and confirm the charging system is holding the correct voltage under load. Both are cheap and both prevent a repeat.
Aftertreatment service: where the derate came from the filter, the repair is a completed regeneration or a filter replacement according to the measured load, not a code clear. Clear the code without addressing the load and the derate returns on the next shift.
Parts for any of these repairs — sensors and senders, harness sections, connectors and terminals, filter elements and aftertreatment components — are supplied by Guangzhou Shenyue Machinery Technology Co., Ltd. from Guangzhou, with 3,000+ part types on the shelf and 100,000+ part numbers available to order, and the grade of every line declared as genuine, OEM-quality or aftermarket. The five brands for which the company holds authorized dealer status are Garrett, Mitsubishi Heavy Industries, TONGMYUNG, NIKURA and Marusan, and the Mitsubishi authorization covers MHI engine parts only. Kobelco is not among them, and Kobelco parts are supplied through original channels or as quality alternatives.
Parts you may need, by grade
| Part | Grades commonly quoted |
|---|---|
| Sensors and senders | Genuine; OEM-quality; aftermarket equivalent where the specification and connector match. |
| Harness sections and repair terminals | Genuine harness; OEM-quality section repair with the specified terminals and seals. |
| Filter elements and breathers | Genuine; OEM-quality; matched aftermarket element to the same rating. |
| Aftertreatment components | Genuine; OEM-quality; remanufactured exchange where the manufacturer supports it. |
Need the parts this covers?
Send the machine model, serial number and the part numbers you need. We confirm fitment before payment and state the grade of every line — genuine, OEM-quality or aftermarket.
Request a quoteFrequently asked
Does a derate clear on its own once the fault is fixed?
Sometimes, and often not. A derate held by an active protection normally clears when the condition is resolved, but one latched by an aftertreatment fault usually needs the condition corrected and the code cleared before full output returns. On some models a stored derate count has to be reset as well. The point is to fix the cause first: clearing a code without repairing the condition simply restarts the same cycle.
Why does the machine derate but show no code?
Because not every limit sets a code. A parameter that crosses a protection threshold without going outside its diagnostic range can reduce output silently, and an intake air temperature sensor reading high in the range the controller accepts will limit fuel without logging a fault. This is where live data and the derate event log matter more than the code list.
Can low fuel pressure cause a derate?
Yes. Rail pressure that cannot be maintained under load is a protection input on most modern machines, and the cause is usually upstream of the engine: a restricted fuel filter, a blocked pre-filter, a failing lift pump or an air leak on the suction side. Check fuel supply quality and restriction before assuming an engine problem.