Bobcat M0421 (04-21) Fault Code: Engine Oil Pressure Out of Range High
Also shown on the panel as 04-21
Engine Oil Pressure Out of Range High · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
M0421 (panel display 04-21) sets when Bobcat's gateway controller sees a signal voltage above 4.8 volts for 30 seconds from the engine oil pressure sensor circuit, on a sensor supplied with 5.0 volts. This points to a wiring short to supply voltage, an open ground, or a failed sensor, controller, or poppet relief valve, rather than an actual oil pressure problem.
High severity. This is a sensor circuit fault, not proof of low oil pressure, but you cannot trust the displayed oil pressure reading while this code is active. Treat the gauge as unreliable and verify engine oil pressure and condition by other means before extended operation.
What does Bobcat error code M0421 mean?
M0421 is an electrical fault code from the gateway controller on Bobcat skid-steer and compact track loader models including the S510 through S770 and T550 through T870 series. It fires when the engine oil pressure sensor's signal wire (3140 on some harness variants, 3520 on others) reports a voltage greater than 4.8 volts for a full 30 seconds. Since the sensor runs on a 5.0 volt supply, a reading that high and that steady almost always means the signal is being pulled up toward supply voltage rather than reflecting real oil pressure.
In plain terms, the controller is not being told 'oil pressure is too high.' It is seeing a voltage signature that looks electrically wrong, most often caused by a short in the wiring or a failed sensor, and it flags that as an out-of-range-high condition rather than trusting the number.
Because engine oil pressure monitoring exists to protect the engine from running dry or starved of oil, any fault here means you lose confidence in that protection. The machine may still show a pressure reading on the dash, but it should not be trusted until the fault is diagnosed.
What triggers a Bobcat M0421 code?
The code sets when the gateway controller detects a voltage greater than 4.8 volts for 30 seconds on the engine oil pressure sensor signal wire, with the key switch or run/enter switch on. The sensor's supply voltage is 5.0 volts.
Common causes of M0421
- 5 volt supply wire (5140 or 3500 depending on harness variant) shorted to supply voltage
- Signal wire (3140 or 3520) shorted to a supply voltage
- Ground wire (2140 or 3510) open
- Faulty engine oil pressure sensor
- Faulty poppet relief valve
- Faulty gateway controller
- Moisture, corrosion, or pushed-back pins in the gateway controller connectors, engine oil pressure sensor connector, or mainframe harness
- Intermittent faults: if the code is not active at the time of testing, the cause may not be found right away. Likely culprits are loose connections, corrosion, pushed-back pins, the controller itself, the wire harness, or a combination of these
How to troubleshoot Bobcat M0421: first checks
- Turn the key switch to run/enter and check whether the code is currently active or was only stored, since some causes are intermittent and may not show up during a static test
- Inspect the engine oil pressure sensor connector and the gateway controller connectors for moisture, corrosion, or pins that have been pushed back out of their sockets
- With a meter, check the 5 volt supply wire for a short to supply voltage and check the signal wire for the same condition
- Check the ground wire for the engine oil pressure sensor circuit for continuity to confirm it has not gone open
- Inspect the mainframe harness along its full run for chafing, pinches, or moisture intrusion that could cause a short
- If wiring and connectors check out, test the engine oil pressure sensor itself and inspect the poppet relief valve for proper operation
- Verify actual engine oil level and condition by dipstick as a safety check while the sensor circuit is unverified
How the code clears
This code is self resetting. No separate clearing step is listed. Once the underlying wiring, connector, sensor, poppet relief valve, or controller issue is repaired and the voltage on the signal circuit returns to a normal range, the code should clear on its own during normal operation.
Affected models and serial ranges
M0421 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 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 M0421 mean?
It means the gateway controller detected an engine oil pressure sensor signal voltage above 4.8 volts for 30 seconds, which is out of range high for a sensor running on a 5.0 volt supply. It usually points to a wiring or sensor problem, not necessarily an actual oil pressure issue.
Is it safe to keep running my Bobcat with M0421 active?
You should not rely on the displayed oil pressure while this code is set, since the sensor circuit is reporting an electrical fault. Check the oil level and condition manually and get the circuit diagnosed before continuing extended operation.
What is the panel display number for M0421?
On the machine's panel it displays as 04-21.
Which Bobcat models can show M0421?
It applies to a range of skid-steer and compact track loader models, including the S510, S530, S550, S570, S590, S630, S650, S750, S770 and the T550, T590, T630, T650, T750, T770, and T870.
Do I need to manually clear M0421 after repair?
No separate clearing step is listed. The code is self resetting once the fault condition on the sensor circuit is resolved.
Could a bad ground cause M0421?
Yes, an open ground wire on the engine oil pressure sensor circuit is one of the listed causes, along with shorts on the signal or 5 volt supply wires.
Why does M0421 come and go intermittently?
Intermittent M0421 codes are often caused by loose connections, corrosion, or pins pushed back in a connector. If the code is not active when you test, the exact cause may not be identifiable until it recurs.