Bobcat M2603 (26-03) Fault Code: Glow Plug Relay Error Off
Also shown on the panel as 26-03
Glow Plug Relay Error Off · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
M2603, shown on the panel as 26-03, sets when the gateway controller commands the glow plug relay on but does not detect voltage back on the feedback wire for 5 seconds. It points to a cold-start glow plug circuit problem, either a wiring fault, a bad relay, or a controller issue.
Medium severity. M2603 will not shut the machine down and it is self resetting, but if the glow plug circuit is not working the engine may be hard to start or run poorly in cold weather. Diagnose it before you rely on the machine for cold-weather starts.
What does Bobcat error code M2603 mean?
M2603 is a Bobcat gateway controller code for the glow plug relay circuit. The description is straightforward: the controller sends a command to energize the glow plug relay, then watches a feedback wire to confirm the relay actually switched on. On some machines that feedback wire is numbered 3420, on others it is numbered 8550, depending on the model and harness variant. If the controller does not see voltage on that feedback wire for 5 seconds after commanding the relay, it sets M2603.
In plain terms, the controller told the glow plug relay to turn on, but got no confirmation that it did. That could mean the relay itself failed, the wiring to or from the relay has a break or short, or the controller's own command or feedback circuit is bad.
Glow plugs help the engine start in cold conditions by preheating the combustion chambers. If this circuit is not confirmed working, cold starts can be harder, slower, or rougher, even though the code by itself does not stop the machine from running.
What triggers a Bobcat M2603 code?
The code sets when the key switch (run/enter) is on, ambient temperature is below 70°F, and the gateway controller commands the glow plug relay but does not detect voltage on the feedback wire (3420 or 8550 depending on model) for 5 seconds.
Common causes of M2603
- Supply wire 1060 open (loss of power feed to the glow plug relay circuit).
- Feedback wire (3420 or 8550, depending on model) shorted to ground or open.
- Glow plug wire 8500 shorted to ground.
- A failed glow plug relay.
- A gateway controller fault.
- Moisture, corrosion, or pushed-back pins in the gateway controller connectors, glow plug relay connectors, or the mainframe harness.
- Intermittent causes: loose connections, corrosion, pushed-back pins, the controller, the wire harness, or a combination of these, which may not be found if the code is not active at the time of testing.
How to troubleshoot Bobcat M2603: first checks
- Verify the key switch is on and check whether ambient temperature is below 70°F, since this is a condition for the code to set.
- Inspect the gateway controller connectors, glow plug relay connectors, and mainframe harness connectors for moisture, corrosion, or pushed-back pins.
- Check supply wire 1060 for continuity and confirm it is not open.
- Check the feedback wire (3420 or 8550 depending on your model) for an open circuit or a short to ground.
- Check glow plug wire 8500 for a short to ground.
- Test the glow plug relay itself for proper operation, and swap or bench test if possible.
- If the code is not currently active, treat it as intermittent: recheck all connectors and harness routing for loose connections or chafing, since the root cause may not show up unless the fault is present during testing.
How the code clears
M2603 is self resetting. No separate clearing step is listed beyond correcting the underlying wiring, relay, or controller fault; once the glow plug relay feedback is detected correctly, the code clears on its own.
Affected models and serial ranges
M2603 appears in our records across 16 Bobcat models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| S510 | SN A3NJ11001-99999, SN A3NK11001-99999, SN ATZC11001-ATZC99999 |
| S530 | SN A7TV11001-99999, SN ATZD11001-99999 |
| S550 | SN A3NK11001-A3NL99999, SN A3NM11001-99999 |
| S570 | SN A7U711001-799999, SN A7U811001-899999 |
| S590 | SN ANMN11001-99999, SN ANMP11001-99999 |
| S630 | SN A3NT10001-12369, SN A3NT12370-99999, SN A3NU11001-11111, SN A3NU11112-99999 |
| S650 | SN A3NV11001-13098, SN A3NV13099-99999, SN A3NW11001-11248, SN A3NW11249-99999 |
| S750 | SN A3P211001-299999 |
| S770 | SN A3P411001-99999 |
| T550 | SN A7UJ11001-AJZV12276 |
| T590 | SN ALJU11001-999999, SN B37811001-999999, SN B3Z711001-999999 |
| T630 | SN A7PU11001-11663, SN A7PU11664-99999 |
| T650 | SN A3P012214-099999, SN A3P111242-199999 |
| T750 | SN ANKA11001-A99999 |
| T770 | SN A3P811001-899999, SN A3P911001-999999 |
| T870 | SN A3PG11001-99999, SN A3PH11001-99999 |
Frequently asked questions
What does Bobcat code M2603 mean?
It means the gateway controller commanded the glow plug relay on but did not detect voltage on the relay's feedback wire for 5 seconds, so it flagged a glow plug relay error off.
Is M2603 the same as panel code 26-03?
Yes. 26-03 is the numeric panel display form of M2603 on affected Bobcat skid steer and compact track loader models.
Will M2603 stop my Bobcat from running?
No. It is self resetting and does not force a shutdown, but it does indicate the glow plug preheat circuit may not be working, which can affect cold starting.
Why does ambient temperature matter for this code?
The code's set conditions include ambient temperature below 70°F, since glow plug operation is tied to cold-start assistance. The controller checks the relay feedback specifically under these conditions.
Which wire numbers should I check for M2603?
Check supply wire 1060, glow plug wire 8500, and the feedback wire, which is either 3420 or 8550 depending on your specific model and harness.
Why does the code come back even after I checked everything?
If the fault is intermittent, it may not be present when you test, so loose connections, corrosion, pushed-back pins, the controller, or the harness can all be intermittent contributors that are hard to catch without the fault active.
Do I need a diagnostic tool to clear M2603?
No separate clearing procedure is listed. The code is self resetting once the underlying wiring, relay, or controller issue is corrected.