CAT Engine 1681 Fault Code: System Voltage Low
Also called Battery Voltage Below Normal, Electrical System Voltage : Low, Electrical System Voltage Low, Electrical System Voltage Low - Most Severe, Electrical System Voltage Low - Most Severe (3), Electrical System Voltage low, Electrical System Voltage low - most severe (3)
System Voltage Low · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT code 1681 (SPN 168 / FMI 1) means the ECM has detected battery voltage below normal while the engine is running or right after power-up. It points to a weak battery, bad connection, or wiring problem feeding the ECM, and can cause misfires, derates, or full engine shutdown if voltage drops too far.
High severity. This code can trigger engine misfire, a 100 percent power derate, or a complete engine shutdown while low voltage conditions are present. Treat it as an urgent electrical problem, not a nuisance code.
What does CAT Engine error code 1681 mean?
Code 1681 (SPN 168, FMI 1) is CAT's way of saying the voltage the ECM sees on its battery supply circuit has dropped below the level it needs to run reliably. The ECM runs on battery power fed through a circuit breaker and the keyswitch circuit, and if that supply sags, the ECM can misbehave, log the fault, and in some cases shut the engine down.
Different CAT ECM platforms set slightly different thresholds and timers for this code, but they all describe the same basic failure: a low or unstable voltage on the ECM's battery input while the key is on and, on most versions, while the engine is running or just after startup.
This is treated as an electrical system fault rather than a charging system fault code by itself, but a weak battery or bad alternator output is one of the most common root causes, so the charging and starting systems should be checked as part of any diagnosis.
What triggers a CAT Engine 1681 code?
CAT documents several versions of this trigger depending on ECM and machine platform. Common set-conditions include: battery voltage below 6 VDC for 2 seconds on 12-volt systems, or below 9 VDC for 2 seconds on 24-volt systems, with the engine not cranking. Another version sets the code when the ECM has been powered for 3 seconds, the engine is not cranking, and battery voltage is below 9 VDC for 2 seconds. A separate condition set uses battery supply voltage below 18 VDC for 0.5 seconds on 24-volt systems (or below 9 Volts for more than 0.5 seconds on 12-volt systems) after the ECM has been powered for 3 seconds, engine not cranking. Yet another version requires the engine to be running for more than 3 seconds with voltage below 9 Volts (12V systems) or below 18 Volts (24V systems) for more than 0.5 seconds. If battery voltage disappears completely without returning, the ECM will not log the code at all, and the engine will simply shut down.
Common causes of 1681
- Connector, pin, or socket not fully coupled or inserted at the ECM or in the battery/keyswitch circuit
- Harness or wiring damage: corrosion, abrasion, or pinch points anywhere between the batteries, circuit breaker, keyswitch, and ECM
- Open circuit or excessive resistance in wiring or connections between the batteries and the ECM
- Negative battery wires needing repair, or bad block/chassis ground connections with corrosion or damage
- Faulty, weak, or discharged batteries that need to be recharged or replaced
- Wiring between the 15 amp circuit breaker and the ECM connector (P1) needing repair
- Wiring between the battery and the 15 amp circuit breaker needing repair
- Wire between the keyswitch and the ECM needing repair, including wire 308-YL on some platforms
- Wire between +Battery and the keyswitch needing repair, or the circuit protection for that circuit
- Faulty keyswitch, verified by checking for voltage present on terminal B but missing on terminal R
- Main power relay damaged or needing replacement
- Damaged alternator (charging system problem)
- Faulty ECM (checked last, after wiring, batteries, and relays are confirmed good)
- Intermittent problems, sometimes caused by rapidly cycling the keyswitch, which can log the code without a real fault present
How to troubleshoot CAT Engine 1681: first checks
- Before testing this code, rule out separate starting system or charging system problems since a bad alternator or weak battery bank is a very common root cause
- Check whether the engine shutdown was accompanied by loss of other electrical accessories. If accessories also dropped out, suspect the keyswitch or load reduction relay circuit rather than ECM wiring
- Inspect all connectors in the battery-to-ECM circuit for corrosion, damage, or partial insertion, including at the ECM connector (J1/P1 on some platforms)
- Check the harness and wiring for abrasion, pinch points, or corrosion between the batteries, the 15 amp circuit breaker, the keyswitch, and the ECM
- Test battery condition and state of charge, and recharge or replace batteries as needed
- Check block and chassis ground connections for corrosion or damage
- Verify the main power relay is functioning and not damaged
- Check continuity of the keyswitch wiring: confirm voltage is present on terminal B, then confirm it reaches terminal R when the key is ON. If terminal B has no voltage, repair the circuit between +Battery and the keyswitch. If terminal B is good but terminal R is not, replace the keyswitch
- If the problem is intermittent, try temporarily bypassing the machine wiring harness to see if the shutdown symptoms disappear, which would point to the machine wiring rather than the ECM
How the code clears
No separate clearing step is listed beyond fixing the root cause and rechecking. Because rapidly cycling the keyswitch can generate this code without an underlying fault, clear any logged diagnostic codes after repairs so old codes do not cause confusion during future diagnosis.
Affected models and serial ranges
1681 appears in our records across 8 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C11 | Serial range not listed in source records |
| C12 | Serial range not listed in source records |
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C16 | Serial range not listed in source records |
| C175 | Serial range not listed in source records |
| C27 | Serial range not listed in source records |
| C32 | Serial range not listed in source records |
Frequently asked questions
What does CAT code 1681 mean?
It means the engine's ECM has detected that battery voltage supplying it has dropped below the level needed for reliable operation, either while the engine is running or shortly after the ECM powers up. It is logged as SPN 168, FMI 1.
Can low voltage code 1681 cause the engine to shut down?
Yes. CAT's documentation lists engine misfire, a full 100 percent power derate, rpm changes, and both intermittent and complete engine shutdowns as possible results while the low voltage condition is present. If voltage disappears entirely without returning, the engine shuts down and the code may not even get logged.
Is this a battery problem or a wiring problem?
It can be either. CAT lists weak or faulty batteries, corroded or damaged connectors, chafed or pinched wiring, bad grounds, a failing alternator, a bad main power relay, and a faulty keyswitch as possible causes. The ECM itself is only suspected after all of these are ruled out.
Why does cycling the keyswitch matter for this code?
CAT notes that rapidly cycling the keyswitch can generate some of these codes on its own, without a real electrical fault. If you suspect that happened, clear the logged codes so they do not cause a misdiagnosis later.
What voltage triggers code 1681?
It depends on the ECM platform. Documented thresholds include below 6 VDC for 2 seconds (12V systems) or below 9 VDC for 2 seconds (24V systems), below 9 Volts for more than 0.5 seconds, and below 18 Volts for more than 0.5 seconds on 24-volt systems, generally combined with the ECM being powered for 3 seconds and the engine not cranking.
Should I check the charging system if I see this code?
Yes. CAT specifically instructs verifying there are no problems with the starting system or charging system before working through the wiring test for this code, since a weak alternator or battery is a common underlying cause.
What wiring should a mechanic check first for 1681?
Start with the battery-to-ECM circuit: the connectors at the ECM (J1/P1 on some platforms), the 15 amp circuit breaker wiring, the keyswitch circuit including wire 308-YL where applicable, and ground connections at the block and chassis.