The Correct Meaning of Bobcat M6604 on M-Series Loaders
Bobcat M6604 shows up when the loader’s controllers lose contact with the engine ECU. Get the OEM label, the sibling codes, and what owners checked first.
Table of Contents
Bobcat M6604 means the ECU stopped talking to the machine
Bobcat M6604 is “ECU No Communication.” Bobcat’s Diagnostic Service Codes list (Rev. 10-20) assigns it to M-Series loaders. The loader’s controllers have lost contact with the engine control unit (ECU), the computer that runs the engine.
It is not a DPF code, and it is not a hydraulic sensor code. You can check battery, fuse, and connector condition as your Operation & Maintenance (O&M) manual describes. Confirming whether the ECU responds takes dealer diagnostic software.
What the code says
| Code | Bobcat label | Applies To | Who confirms it |
|---|---|---|---|
| M6604 | ECU No Communication | M-Series loaders (S450–S850, T450–T870) | Dealer diagnostic software |
Source: Bobcat Diagnostic Service Codes, revised 10-20. Verified September 24, 2026.
Where to go next
- If you’ve seen it called a DPF code, see why M6604 gets mislabeled.
- If your display shows M6505, M6506 or M6507 instead, see the ECU code family.
- If the machine won’t run and has to be moved, see running or moving it.
ℹ️ Safety Note: Checking this fault means working at the battery and in the engine compartment. A tool bridging a battery terminal and the frame can arc, cause burns, and ignite the explosive gas batteries give off. Engine-area parts also stay hot after shutdown. Never reach under raised lift arms or a tilted cab without Bobcat’s approved support in place. Treat this article as general understanding: follow your O&M manual, and have a certified heavy equipment technician do anything beyond a visual check.
Why M6604 gets mislabeled as a DPF or sensor code
Several pages ranking for this code give it the wrong meaning, and a wrong meaning sends you to the wrong part of the machine.
The DPF confusion
A diesel particulate filter (DPF) cleaning business lists M6604 in the title of a fault-code page whose content covers passenger-car regeneration warnings. Bobcat’s list gives M6604 no DPF meaning.
The hydraulic-sensor confusion
One AI-generated Q&A summary calls M6604 a hydraulic pressure sensor fault. The human technician in the same thread says it means no communication with the engine controller. Another summary ties it to coolant and fuel-pressure sensors, and neither matches Bobcat’s label.
Where DPF faults actually show up
Bobcat’s DPF-related switch codes are nearby in the numbering:
- M5902: DPF Regeneration Switch Error On
- M6002: DPF Inhibit Regen Switch Error On
- M6102: Remote Parked Regeneration Switch Error On
Because they sit a few numbers from M6604, check every digit against your display. Our guide to how Bobcat M-codes are structured shows how to read the number.
M6604 vs M6505, M6506, M6507 and M6904
Bobcat separates “the ECU has a power problem” from “the ECU isn’t communicating,” and the code number tells you which one you have.
| Code | Bobcat label | Applies To | Points to |
|---|---|---|---|
| M6505 | ECU Power Short to Battery | M-Series loaders and excavators | ECU power circuit |
| M6506 | ECU Power Short to Ground | M-Series loaders and excavators | ECU power circuit |
| M6507 | ECU Power Open Circuit | M-Series loaders | ECU power circuit |
| M6604 | ECU No Communication | M-Series loaders | ECU communication |
| M6904 | DCU No Communication | M-Series loaders | DCU communication |
Source: Bobcat Diagnostic Service Codes, revised 10-20. Verified September 24, 2026.
ECU power codes
M6505 through M6507 describe the circuit feeding the ECU. The circuit is either shorted, or open, meaning broken so current can’t flow. If voltage codes appear alongside them, our breakdown of Bobcat M0309 and M4109 voltage codes covers the charging side.
Communication codes
M6604 means the ECU isn’t communicating. M6904 carries the same label for the DCU, a separate engine controller that Bobcat lists for its D34 engine. Bobcat’s list covers three engine controllers: V1 (Delphi ECU), V2 (Bosch ECU) and D34 (DCU). Check which code list covers your serial number before assuming which controller your loader has.
What owners found behind M6604
Documented owner reports point to the power supply and connections first. None of the public threads we reviewed recorded a confirmed fix.
Reports by model and year
| Model | Year (owner-stated) | What the owner reported | Thread date |
|---|---|---|---|
| T590 | 2014 | Fuses good, but no power reaching the ECM fuse | Dec 2024 |
| T770 | 2014 | Replaced a burnt ECU fuse and cleaned connections; code stayed | Oct 2021 |
| T590 | 2016 | Battery connections checked and looked good | Feb 2021 |
| T590 | Not stated | Machine shuts down with the code | Oct 2023 |
| T770 / T590 | Not stated | Technician booked; owner hoped for an ECU software update | Jul 2024 |
Source: five public threads on JustAnswer and SkidSteerForum with a model and date attached, reviewed September 24, 2026.
What the reports have in common
- Fuses and battery connections were the first things owners checked.
- In the 2014 T770 report, replacing the fuse alone didn’t clear the code.
- In the 2014 T590 report, the fuse itself was good, but power wasn’t reaching it.
What an owner can check before calling the dealer
Keep your own checks visual and follow the O&M manual. Anything involving live circuits or the ECU itself belongs to a technician.
Checks the O&M manual allows
- Battery terminals and cables: clean, tight, and free of corrosion.
- Battery and charging condition: a system voltage code such as M0309 would point here.
- ECU fuse and relays: find them using your model’s O&M fuse chart, not a forum photo.
- Harness near the ECU connectors: look for chafing, rodent damage, and pulled or corroded plugs.
📜 The Rule: OSHA 29 CFR 1926.441(a)(5) requires that face shields, aprons, and rubber gloves be provided for construction workers who handle batteries (OSHA 1926.441, verified September 24, 2026). If your crew handles the battery during these checks, that gear belongs on the truck. Regulations change; recheck the citation before relying on it.
Where the dealer tool takes over
Confirming whether the ECU responds requires dealer diagnostic software. Harness pin testing, ECU replacement, and programming go to a certified heavy equipment technician.
⚠️ Hazard: A wrench that touches a live terminal and the frame at the same time can arc and ignite battery gas. Don’t disconnect, jump, or probe the system except as your O&M manual directs. Leave circuit testing to a certified technician.
✅ Do This: Call your Bobcat dealer with the full serial number, the code, and every check you’ve done. Ask the technician to verify power and ground at the ECU, and whether the ECU responds on their tool, before any controller is replaced.
Can you keep running or move a machine showing M6604?
Treat M6604 as a machine-down code until a technician clears it. Bobcat’s list gives only the label, and owner reports describe the loader shutting down.
If it stopped
Repeated cranking won’t restore a lost data link, and it runs the battery down. For how M6604 compares with other faults, see which Bobcat codes stop work.
Getting a dead machine on a trailer
A loader that won’t run can’t drive onto a trailer. The towing or brake-release steps for your model are in the O&M manual. Have your dealer or a licensed carrier with winch equipment do it.
⚠️ Hazard: A dead machine being winched can shift or roll unexpectedly. Stay clear of the winch line and the load path, and leave the procedure to the carrier.
The dealer repair path and Machine IQ for M6604
Most of the diagnosis happens on dealer software, so the most useful thing you can bring is information.
If the ECU itself is the fault
Bobcat’s list includes M4028, “Wrong ECU Detected,” for M-Series loaders, so a replacement ECU has to be the correct unit. Replacement ECUs also need programming with proper diagnostic tools before installation. Stick to dealer-programmed controllers, and get a written quote that separates harness, power, and ECU work.
Seeing codes remotely
On connected machines, Bobcat Machine IQ reports machine data to the Bobcat Owner Portal. Fault code alerts come with the Health and Security subscription. At its October 2020 US launch it was listed at $200 a year plus a $100 activation fee.
Bobcat M6604 FAQ
1. What does Bobcat M6604 mean?
Bobcat M6604 means “ECU No Communication”: on M-Series loaders, the machine’s controllers have lost contact with the engine control unit.
2. Is Bobcat M6604 a DPF code?
No. Bobcat’s list defines M6604 as ECU No Communication; its DPF switch codes are M5902, M6002, and M6102.
3. Is M6604 a hydraulic pressure sensor fault?
No. That claim comes from an AI-generated summary; Bobcat’s own list labels M6604 ECU No Communication, not a sensor fault.
4. What’s the difference between M6505 and M6604?
M6505 is ECU power shorted to battery; Bobcat M6604 is ECU No Communication — a power-circuit fault versus a communication fault.
5. What does M6904 mean compared with M6604?
M6904 is “DCU No Communication,” the same fault type as M6604 but for the DCU controller Bobcat lists for D34 engines.
6. What causes Bobcat M6604?
Owner reports point to battery terminals, ECU fuses, and connectors, but no public thread we reviewed recorded a confirmed fix for M6604.
7. Can a blown fuse cause M6604?
Owners have found ECU fuse problems with Bobcat M6604, but one 2014 T770 owner replaced the fuse and the code stayed. Have a certified technician trace power upstream.
8. Can I keep running a Bobcat with M6604?
Treat Bobcat M6604 as machine-down: owner reports describe shutdowns, and repeated cranking won’t restore a lost data link. Call your dealer before restarting.
9. Does a replacement ECU for M6604 need programming?
Yes. A replacement ECU must be programmed with proper tools, and Bobcat logs M4028 when it detects the wrong ECU. Use your Bobcat dealer.
10. Can Machine IQ alert me to codes like M6604?
On connected machines, Machine IQ Health and Security sends fault code alerts through the Bobcat Owner Portal, so M6604-type faults can reach you remotely.
Next steps if your Bobcat shows M6604
You’re looking at a communication fault, not a DPF or sensor problem. Your checks stop at battery, fuses, and visible connectors.
Next, confirm which code list covers your serial number, then compare M6604 with other work-stopping faults in our Bobcat codes-by-machine reference. No calculator applies to this code.
Ask a certified heavy equipment technician one question first: does the ECU respond on your tool, and are power and ground in spec? This page is compiled from Bobcat’s code list and dated owner reports, not first-hand operation.
