SPN 158 — Keyswitch Battery Potential
System: Electrical
The voltage the engine computer sees on its key-switch (ignition) input — how it knows the key is on and how healthy the switched power feed is.
Can I keep running it?
MEDIUM — fix soon
Low or dropping key-switch voltage can cause stalling, no-starts and a pile of unrelated codes. Fix power and grounds before chasing other faults.
Likely causes
- Weak batteries or poor charging
- Corroded or loose ignition feed, fuse or relay
- Worn ignition switch
- Aftermarket accessories spliced into the ignition circuit
- Poor ECM ground
Checks & fixes (easiest first)
- Load-test the batteries and check charging voltage
- Measure key-on voltage at the ECM ignition pin and compare with battery voltage
- Voltage-drop test the ignition feed and ECM ground under load
- Inspect fuses, relays and any aftermarket splices
Common FMIs with SPN 158
- SPN 158 FMI 1Value too low (most severe)
- SPN 158 FMI 2Data erratic, intermittent, or incorrect
- SPN 158 FMI 17Value too low (least severe)
- SPN 158 FMI 18Value too low (moderately severe)
Related codes
FAQ
What does SPN 158 mean?
SPN 158 is Keyswitch Battery Potential. The voltage the engine computer sees on its key-switch (ignition) input — how it knows the key is on and how healthy the switched power feed is.
Can I keep running with SPN 158?
Low or dropping key-switch voltage can cause stalling, no-starts and a pile of unrelated codes. Fix power and grounds before chasing other faults.
What causes SPN 158?
Weak batteries or poor charging; Corroded or loose ignition feed, fuse or relay; Worn ignition switch; Aftermarket accessories spliced into the ignition circuit; Poor ECM ground.