CAT Engine 5455 Fault Code: Ether Start Relay open/short to +batt
Also called Ether Start Relay Open/Short to +Batt, Ether Start Relay Open/short to +batt, Ether Start Relay open/short to batt
Ether Start Relay open/short to +batt · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 5455 (SPN 545 / FMI 5) sets when the ECM sees an open circuit or a short to +Battery at the ether start relay output for at least 2 seconds, with the driver output off, battery voltage above 9 VDC, and the ECM powered up for at least 3 seconds. The ECM logs the code and disables ether injection, which can cause hard cold starting on C10, C12, C15, C16, and C27 engines.
Medium severity. This code does not stop the engine from running, but it disables the cold-start ether system. In cold climates that means hard starting or failure to start, so it should be diagnosed before the next cold-weather start, not left for a routine service interval.
What does CAT Engine error code 5455 mean?
This code points to a wiring or relay problem in the ether start circuit, the system that injects a shot of ether into the intake to help a cold engine fire up and run smoothly until it warms. The ECM watches the ether relay output circuit and expects it to behave like an open driver when it is not commanded on. If it instead reads as an open circuit or shorted to +Battery for a sustained period while the driver is off and battery voltage is normal, it flags this code.
The ether system on these engines runs in either Automatic Mode, controlled by the ECM based on coolant or intake air temperature and atmospheric pressure, or Manual Mode, triggered by the operator pressing the starting aid switch. In both modes the ECM cycles the relay on for 3 seconds and off for 3 seconds within a 6 second cycle. Some versions of this control logic describe AUTO mode activating ether injection for as little as 10 seconds at 10 °C (50 °F) up to 130 seconds at -40 °C (-40 °F), and disabling automatically above 1900 RPM or once coolant temperature exceeds 10 °C (50 °F). Manual mode on that same logic allows injection up to 130 seconds per switch toggle under similar rpm and temperature limits.
Two relays are typically involved: an ether start relay that applies 24 VDC to energize the solenoid initially, and an ether hold relay that maintains a reduced voltage through a dropping resistor and diode assembly. Continuous full battery voltage will damage the solenoid, which is why the relay logic and this fault check exist.
What triggers a CAT Engine 5455 code?
The ECM sets this code when all of the following are true at once: an open circuit or a short to +Battery is detected at the ether relay output for at least 2 seconds, the driver output is off, battery voltage is greater than 9 VDC for the last 2 seconds, and the ECM has been powered for at least 3 seconds.
Common causes of 5455
- Ether relay connector or wiring pins not fully inserted or seated, or otherwise damaged
- Corrosion, abrasion, or pinch points in the harness between the ECM, relay, and solenoid
- Faulty or failing ether start relay that needs replacement
- Faulty ether solenoid, or a resistor and diode dropping assembly (on hold-relay equipped systems) that needs replacement
- Problem with the starting aid switch, or the ECM not properly reading the switch signal
- Open circuit or short circuit in the wiring between the ether relay and +Battery
- Short between the ECM and the ether relay, or damaged wiring between the switch and the ECM
- Faulty resistance across the relay coil terminals (terminal 86 solenoid return and terminal 87 solenoid power)
- Problem with the ether canister itself on systems that reference canister-related faults
- ECM internal fault requiring replacement, once wiring, relay, switch, and solenoid are ruled out
How to troubleshoot CAT Engine 5455: first checks
- Inspect the ether relay connector and the ECM connector for full seating, corrosion, or bent pins, and reseat or clean as needed
- Visually trace the harness from the ECM to the ether relay and from the relay to the ether solenoid, looking for chafe points, pinches, or abrasion damage
- Check battery voltage is stable and above 9 VDC while cranking or with the key on, since low or unstable voltage can affect readings
- Test the starting aid switch operation and confirm the ECM is receiving a clean signal when the switch is pressed
- Measure resistance across the ether relay coil terminals (86 and 87) to confirm the relay coil is within spec and not open or shorted
- Check the wiring and connection between the ether relay and +Battery for an open or short condition
- Inspect the ether solenoid and, on hold-relay equipped systems, the dropping resistor and diode assembly for damage
How the code clears
No separate clearing step is listed beyond repairing the open or short condition. The ECM logs the code and disables ether injection while the fault is active; once the wiring, relay, switch, or solenoid issue is corrected and the circuit reads normally, the code should stop being active. Clear the stored code with a diagnostic tool and confirm it does not return during a cold start attempt or bench test of the ether system.
Affected models and serial ranges
5455 appears in our records across 5 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C10 | Serial range not listed in source records |
| C12 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C16 | Serial range not listed in source records |
| C27 | Serial range not listed in source records |
Frequently asked questions
What does CAT code 5455 mean?
It means the ECM detected an open circuit or a short to +Battery at the ether start relay output for at least 2 seconds while the relay driver was off and battery voltage was above 9 VDC. The ECM logs the fault and disables ether injection.
Will code 5455 stop my engine from running?
No. This code only disables the ether cold-start assist system. The engine will still run and can still be started, but cold starting may become harder without ether injection available.
Why is my engine hard to start after this code appears?
Because the ECM disables ether injection once it detects the fault. Ether injection exists specifically to help cold engines start, so losing it in cold weather often shows up as hard starting or extended cranking.
What should I check first for code 5455?
Start with the ether relay and ECM connectors for corrosion or loose pins, then visually inspect the harness between the ECM, relay, and solenoid for chafe or pinch damage. Also check the starting aid switch and relay coil resistance.
Can a bad ether relay alone cause this code?
Yes. A faulty or failing ether relay is listed as a direct possible cause, along with a faulty ether solenoid, damaged wiring, or a bad starting aid switch.
Is this code specific to certain CAT engines?
It applies to C10, C12, C15, C16, and C27 engines using this ECM-controlled ether injection system.
Do I need to replace the ECM to fix this code?
Not usually. ECM replacement is listed as a possible cause, but it is typically the last thing to check after ruling out wiring, connectors, the relay, the solenoid, and the starting aid switch.