What are the most likely causes of SPN 168?

Alternator output; Battery cables/ground; Failing batteries

How do you diagnose code SPN 168?

1) Charging voltage at idle and 1200 rpm

What part is usually replaced for SPN 168 and how much does it cost?

Alternator: $300–$1,200 USD (aftermarket to dealer), plus 1–2.5 h of labor.

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SPN 168

CAUTION

Battery / charging voltage

The truck's control module (ECU) reports a fault on: Battery / charging voltage.

SPN 168 means a truck control module (ECU) flagged a fault on battery / charging voltage, part of the Electrical / J1939 system on heavy-duty diesel trucks. DERATE rates it CAUTION: you can keep driving carefully, but get it checked soon. Part usually replaced: Alternator, $300–$1,200 (aftermarket to dealer) plus 1–2.5 h labor.

Electrical / J1939

Can I keep driving?

Yes, carefully. It does not usually cut power, but get it checked soon.

Most likely causes

Alternator output
Battery cables/ground
Failing batteries

Recommended tests

1. Charging voltage at idle and 1200 rpm

Part usually replaced

Alternator

$300–$1,200 (aftermarket to dealer)

Labor: 1–2.5 h

USD, part only. Labor is billed at the shop's hourly rate.

Data: DERATE verified base, updated October 2026

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