CAT Engine 18342 Fault Code: Ignition Key Switch loss of signal
Also called Ignition Key Switch Loss of Signal
Ignition Key Switch loss of signal · ai-assisted, editor-reviewed · Last updated 2026-07-13
TL;DR
CAT fault code 18342 (SPN 157 / FMI 2) means the ECM saw an erratic keyswitch signal, either because the keyswitch was cycled at least three times within the last second, or because three voltage readings for the keyswitch in the last one second were below an acceptable voltage while the engine was not cranking. The ECM logs the code, may light a warning lamp, and stops energizing the injector solenoids, shutting the engine down.
High severity. This code shuts the engine down by stopping injector solenoid energizing, so it stops the machine immediately. It is not a slow-developing wear issue, but it can also be triggered by harmless rapid key cycling used to prompt flash codes, so severity depends on whether the shutdown is a real electrical fault or a diagnostic side effect.
What does CAT Engine error code 18342 mean?
This code covers the keyswitch circuit that tells the ECM the operator wants the engine running. When the ECM gets a clean, stable signal from that circuit, it energizes the injector solenoids and lets the engine run. When the signal looks erratic or drops below an acceptable voltage, the ECM assumes something is wrong with the key circuit and shuts the engine down rather than keep running on bad input.
There are two distinct trigger conditions described for this code. One is a rapid-cycling condition: the keyswitch was cycled at least three times within the last second. The other is a low-voltage condition: three voltage readings for the keyswitch in the last one second were below an acceptable voltage while the engine was not cranking.
In every version of this fault, the practical result is the same: the ECM logs the code, a warning lamp or display message may appear, and the ECM stops energizing the injector solenoids, which shuts the engine down.
What triggers a CAT Engine 18342 code?
The ECM sets this code under either of two conditions: (1) the keyswitch signal was erratic because the keyswitch was cycled at least three times within the last second, or (2) three voltage readings for the keyswitch during the last one second were below an acceptable voltage while the engine was not cranking.
Common causes of 18342
- Damaged connectors or wiring in the keyswitch circuit
- A problem with the wiring harness itself, including excessive resistance in wiring or connections between the batteries and ECM
- An intermittent problem in the circuit that only shows up under certain conditions
- Faulty batteries or a problem with the charging system
- A faulty ECM
- A faulty relay in the keyswitch circuit
- Incorrect or out-of-range voltage at the ECM, including an open circuit between the batteries and ECM
- Battery voltage not present on terminal 4 at the keyswitch connector
How to troubleshoot CAT Engine 18342: first checks
- Ask the operator whether the key was cycled rapidly right before the fault, since rapid cycling to prompt flash codes can set this code on its own with no wiring fault present
- Inspect the keyswitch connector and related wiring for damage, corrosion, or loose pins
- Check battery condition and charging system output, since low or unstable battery voltage can cause the keyswitch signal to read below an acceptable voltage
- Verify battery voltage is actually present on terminal 4 at the keyswitch connector
- Check the harness between the batteries and ECM for excessive resistance, loose connections, or an open circuit
- Inspect any relay in the keyswitch circuit for proper operation
- If everything in the circuit checks out and the code recurs without rapid key cycling, consider the ECM itself as a cause
How the code clears
No separate clearing step beyond normal diagnostic code clearing is listed. If the code was set only because the keyswitch was rapidly cycled to prompt flash codes, clear the logged diagnostic code afterward to prevent confusion or an incorrect diagnosis later. If the code was set from low voltage readings while not cranking, resolve the wiring, battery, or charging issue first before clearing the code, since it will likely return if the underlying condition is not fixed.
Affected models and serial ranges
18342 appears in our records across 6 CAT Engine models. Match your machine by model and serial number.
| Model | Serial ranges |
|---|---|
| C13 | Serial range not listed in source records |
| C15 | Serial range not listed in source records |
| C175 | Serial range not listed in source records |
| C18 | 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 18342 mean?
It means the ECM detected an erratic signal from the keyswitch, either from rapid cycling of the key or from low voltage readings on the keyswitch circuit while the engine was not cranking, and responded by shutting the engine down.
Will rapidly turning the key cause this code by itself?
Yes. Cycling the keyswitch at least three times within one second is enough to set this code, and some control modules require this rapid cycling to prompt flash codes. If that is what happened, clear the logged code afterward so it does not cause confusion later.
Why did my engine shut down with no warning?
The ECM stops energizing the injector solenoids as soon as it decides the keyswitch signal is unreliable, so the shutdown happens as part of the fault response itself, not as a separate failure.
Where should I check first if this keeps happening without key cycling?
Start with the keyswitch connector and wiring for damage or corrosion, then check battery voltage and the charging system, and verify battery voltage is present on terminal 4 at the keyswitch connector.
Could a bad battery or charging system cause this code?
Yes. Low or unstable voltage from the batteries or charging system can produce keyswitch voltage readings below an acceptable level, which triggers the low-voltage version of this code.
Is a faulty ECM a likely cause?
It is listed as a possible cause, but it should be considered only after wiring, connectors, batteries, charging system, and any relay in the circuit have been checked and ruled out.
Which CAT engines can set code 18342?
It applies to the C13, C15, C175, C18, C27, and C32 engine models.