John Deere Engines SPN177FMI2 Fault Code: Transmission Oil Temperature Erratic
Transmission Oil Temperature Erratic · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
SPN 177 FMI 2 means the engine ECU is receiving an erratic transmission oil temperature signal over the CAN network from the RCU (Reverser Control Unit). When this happens, the ECU defaults the transmission oil temperature reading to a high value and runs the cooling fan to try to protect the transmission from a possible overheat condition.
Medium severity. The code itself does not indicate an actual overheat, it means the ECU cannot trust the temperature signal it is getting and is defaulting to a protective, worst-case assumption. Treat it as a diagnose-soon issue rather than an immediate shutdown, but do not ignore it since the fan running constantly and an unreliable temperature reading can mask a real overheat problem building elsewhere.
What does John Deere Engines error code SPN177FMI2 mean?
SPN 177 FMI 2 points to the transmission oil temperature signal, not a direct engine sensor fault. On these machines, the RCU has its own temperature sensor that measures transmission oil temperature and sends that reading to the engine ECU over the CAN network. The engine ECU does not measure this temperature itself, it just monitors the message coming in from the RCU.
When the ECU decides that message is erratic, meaning it does not make sense or is jumping around in a way that fails the ECU's internal checks, it stops trusting the value. Instead of guessing at a real number, the ECU defaults the transmission oil temperature to a high value as a safety margin. This causes the cooling fan to run in an attempt to prevent an overheat condition, even if the transmission may not actually be running hot.
In short, this code is a communication and data quality problem between the RCU and the ECU over CAN, not necessarily proof of an overheating transmission. The real cause could be on the RCU side, the wiring or connectors between the RCU and ECU, or a related coolant temperature problem that is worth ruling out first.
Common causes of SPN177FMI2
- An erratic transmission oil temperature message being sent to the ECU over CAN from the RCU. No specific list of underlying causes is given for this code.
How to troubleshoot John Deere Engines SPN177FMI2: first checks
- Check for other related trouble codes before doing anything else. If SPN 110 FMI 0, SPN 110 FMI 16, or SPN 111 FMI 1 are also present, diagnose those first since they point to high coolant temperature and may explain or relate to the erratic transmission oil temperature reading.
- On 8.1 L engines, follow the coolant temperature above normal diagnostic procedure in the 8.1L Diesel Engines Base Engine Manual (CTM 86), Section 04, Group 150.
- On 6.8 L engines, follow the coolant temperature above normal diagnostic procedure in the 4.5 L and 6.8 L Diesel Engines Base Engine Manual (CTM 104), Section 04, Group 150.
- Check for any trouble codes logged on the RCU itself. If the RCU has its own fault codes, diagnose those, since a problem at the RCU or its sensor is a likely source of the erratic signal being passed to the ECU.
- Inspect the CAN wiring and connectors between the RCU and the ECU for corrosion, loose pins, or damage, since a communication fault along that circuit can cause an erratic or dropped-out temperature signal.
How the code clears
No separate clearing step is listed for this code. Diagnose and resolve the underlying cause, whether that is a related coolant temperature code, an RCU fault code, or a wiring issue between the RCU and ECU, and confirm the transmission oil temperature signal reads normally again.
Frequently asked questions
What does SPN 177 FMI 2 mean on a John Deere engine?
It means the engine's ECU is getting an erratic transmission oil temperature signal sent over the CAN network from the RCU (Reverser Control Unit). The ECU cannot trust the value it is receiving.
Does SPN 177 FMI 2 mean my transmission is overheating?
Not necessarily. The code means the temperature signal itself is erratic, not confirmed high. However, when this code sets, the ECU defaults the transmission oil temperature to a high value and runs the cooling fan as a precaution, so it behaves as if an overheat might be happening even if it is not.
Why does the cooling fan run constantly when this code is active?
Because the ECU no longer has a reliable transmission oil temperature reading, it defaults to a high value to protect the transmission. Running the cooling fan is the ECU's built-in safeguard against a possible overheat condition while the real temperature is unknown.
What should I check first if I get SPN 177 FMI 2?
First check for related coolant temperature codes such as SPN 110 FMI 0, SPN 110 FMI 16, or SPN 111 FMI 1. If any of those are present, diagnose them first since they can point to the real cause of a high coolant or transmission temperature situation. Also check for any trouble codes stored on the RCU.
Is SPN 177 FMI 2 an engine problem or a transmission problem?
It is really a communication problem. The transmission oil temperature is measured by the RCU, not the engine ECU, and sent over CAN. The fault means that message is erratic, so the root cause could be at the RCU, its sensor, the CAN wiring, or in some cases a related coolant temperature issue.
Which manual should I use to diagnose this further?
For 8.1 L engines, use the coolant temperature above normal procedure in the 8.1L Diesel Engines Base Engine Manual (CTM 86), Section 04, Group 150. For 6.8 L engines, use the same style procedure in the 4.5 L and 6.8 L Diesel Engines Base Engine Manual (CTM 104), Section 04, Group 150.
Is it safe to keep operating the machine with this code active?
Since the actual transmission temperature is uncertain and the system is running in a protective default mode, it is best to diagnose the code within the same shift rather than continuing to run indefinitely. Have the related codes, RCU, and wiring checked before assuming everything is fine.