CAT Engine 5456 Fault Code: Ether Start Relay short to ground
Also called Ether Start Relay Open short to ground, Ether Start Relay Short to Ground
Ether Start Relay short to ground · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 5456 (SPN 545, FMI 6) means the ECM has detected an open or short to ground at the ether start relay output while the driver output was on, battery voltage was above 9 VDC for at least two seconds, and the ECM had been powered for at least three seconds. The system logs the code and disables ether injection, which can cause hard starting in cold weather. It affects the C10, C12, C15, C16, and C27 engines.
Medium severity. Ether injection is disabled once this code sets, which by itself will not stop the engine from running, but it can cause hard starting or no-start conditions in cold weather when the engine needs the ether assist most. Diagnose before the next cold start rather than letting it sit.
What does CAT Engine error code 5456 mean?
Fault code 5456 (SPN 545, FMI 6) is set by the Electronic Control Module (ECM) when it senses an open circuit or a short to ground on the wiring that drives the ether start relay. The ether injection system exists to help the engine start in cold weather by injecting a metered dose of ether into the intake during startup or cranking.
When the ECM detects this wiring fault while the relay driver output is commanded on, it logs the code and disables ether injection entirely as a protective response. The practical result operators notice is hard starting in cold conditions, since the ether assist that would normally help fire the engine is no longer available.
One version of the diagnostic text also describes the physical relay setup: the ECM sends +Battery voltage to the ether start relay coil, with the other side of the coil tied to -Battery. A second output controls an ether hold relay that keeps the solenoid energized at reduced voltage through a dropping resistor and diode assembly, since continuous full battery voltage would damage the solenoid.
What triggers a CAT Engine 5456 code?
The ECM sets this code when all of the following are true: an open or short to ground is detected at the ether relay output for at least 2 seconds, the driver output is on, battery voltage is greater than 9 VDC for the last 2 seconds, and the ECM has been powered for at least 3 seconds. One version of the diagnostic text simplifies this to: the ECM has been powered for 2 seconds and detects a short circuit to ground at the ether start relay output.
Common causes of 5456
- Connector, pins, or socket not fully inserted or coupled at the relay, solenoid, or switch connections
- Corrosion, abrasion, or pinch points in the harness or wiring between the ECM, relay, solenoid, and starting aid switch
- Faulty or failing ether start relay that needs to be replaced
- Faulty ether solenoid needing replacement
- Problem with the ether canister
- Resistor and diode assembly (used in the ether hold relay circuit) needing replacement
- ECM not properly reading the signal from the starting aid switch, or a faulty starting aid switch itself
- Open circuit or short circuit anywhere in the harness between the ECM, relay, and +Battery
- Short between the ECM and the ether relay
- Faulty ECM that may need to be replaced (a less common but listed cause)
How to troubleshoot CAT Engine 5456: first checks
- Inspect all connectors at the ether relay, ether solenoid, starting aid switch, and ECM for full seating, corrosion, and damage. Reconnect and clean any suspect terminals.
- Visually trace the harness between the ECM, ether relay, ether solenoid, and +Battery for chafe points, pinches, or abrasion damage, especially anywhere the harness contacts the frame or moving parts.
- Check resistance across the relay's terminal 86 (solenoid return) and terminal 87 (solenoid power) to look for an open or shorted coil.
- Test the starting aid switch for correct operation in both AUTO and MANUAL modes and confirm the ECM is receiving a clean signal from it.
- Check the ether hold relay's dropping resistor and diode assembly if the machine uses that configuration, since a failure there can affect the circuit the ECM is monitoring.
- With power removed, check for a short to ground on the ether relay driver circuit using a multimeter before replacing any parts.
How the code clears
No separate clearing step is listed beyond fixing the underlying wiring, connector, relay, solenoid, or switch fault. Once the open or short to ground is repaired, the ECM should stop detecting the fault condition and the code should not re-log on the next key cycle. No reset procedure is listed for this code.
Affected models and serial ranges
5456 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 5456 mean?
It means the ECM detected an open circuit or a short to ground at the output that drives the ether start relay, while the driver output was commanded on and battery voltage was above 9 VDC. The ECM logs the fault and disables ether injection.
Will code 5456 stop my engine from running?
No. The code disables ether injection, not the engine itself. The main symptom is hard starting, particularly in cold weather when the ether assist would normally help the engine fire.
Which CAT engines can set this code?
The C10, C12, C15, C16, and C27 engines are listed as affected by this fault code.
What's the difference between AUTO and MANUAL ether injection modes?
In AUTO mode the ECM decides when to inject ether based on coolant temperature, intake air temperature, atmospheric pressure, and engine rpm. In MANUAL mode the operator presses the starting aid switch to request ether, subject to similar rpm and temperature conditions.
Can a bad starting aid switch cause this code?
Yes. A faulty starting aid switch, or the ECM misreading its signal, is listed as one of the possible causes, along with wiring problems between the switch and the ECM.
Is this an electrical problem or a relay problem?
It can be either. The most common causes are wiring and connector issues (corrosion, poor connections, chafed harness) or a failed ether relay or solenoid. Checking resistance across the relay terminals is a good early step to narrow it down.
Do I need to replace the ECM to fix code 5456?
Usually not. A faulty ECM is listed as a possible cause, but it's one of the less common ones. Wiring, connectors, the relay, and the ether solenoid should be ruled out first.