OBD-II Diagnostic Trouble Code
P1638 — 5 Volt Reference 2 Circuit
P1638 is a manufacturer-specific code that commonly means the 5-volt reference 2 circuit is out of range. It supplies a separate group of sensors from reference 1, so a short on any of them can trigger it. Meaning can vary by OEM.
Severity 3/5
Truck-relevant
Service this week
Severity
Service this week
5 Volt Reference 2 Circuit
Code
P1638
Category
Powertrain
Subsystem
Network
Truck-relevant
Yes
What this means for truckers
Service within a few days on a Cummins X15, Detroit DD15, PACCAR MX-13. Performance is likely degraded. Continuing to drive without service risks additional component damage.
Before you replace parts — try this first
P1638 on the Network system is most often shorted sensor on the 5v reference 2 circuit — not a dead part. On commercial diesels especially, work these in order before spending money on parts:
- Confirm the symptoms. Match what your truck is doing to the Symptoms section below. If the code is set but the symptoms are not there, clear the code and re-test after a drive cycle — it may not return.
- Clean and dry the sensor connector. Water, road salt, or oil seeping into a connector is the #1 fake sensor failure. Disconnect, look for green corrosion or moisture, spray with electrical contact cleaner, dry, reseat. Many faults clear here for $0.
- Inspect the wiring harness. Trace the wire from the sensor toward the ECM — look for chafing against the engine block, melted insulation near the turbo or exhaust, or pinch points where a clamp has rubbed through.
- Test the sensor before swapping it. Most Common Causes (below) list the expected voltage or resistance range — a $20 multimeter saves a $300 sensor swap that does not fix the problem.
- Only then, replace the part. Use the OEM or a tier-1 aftermarket. Cheap sensors throw the same code in two weeks.
Symptoms
Several sensor codes setting together, reduced power, possible throttle or pedal faults, and a check-engine light.
Common causes
- CommonShorted sensor on the 5V reference 2 circuit ($100–$500)
- CommonChafed wiring shorting to ground or voltage ($100–$500)
- OccasionalCorroded or water-intruded connector ($80–$300)
- RareECM internal fault ($800–$2500)
How to fix
Measure 5V reference 2 voltage at the ECM and sensors
When to use: First step
Disconnect sensors one at a time to isolate the short
When to use: When voltage is low
Repair wiring or replace the faulty sensor
When to use: When the cause is found
Replace the ECM
When to use: When the fault remains with all sensors unplugged
Affects most
No specific vehicle pattern found in our complaint database. This code can appear on any vehicle that supports OBD-II diagnostics.
Related
- → Browse all DTC trouble codes
- → Decode a VIN to check codes for your truck
- → Owner-operator maintenance guides
- → P1607 — Internal Control Module Performance
- → P1635 — 5 Volt Reference 1 Circuit
- → P1654 — AC Refrigerant Pressure Switch Performance
- → P1690 — Wastegate Control Boost Pressure Out of Range
- → P1693 — Glow Plug Control Module Communication Lost
Frequently Asked Questions
Can I drive with code P1638?
P1638 is severity 3 — plan service within a few days. Performance is likely degraded already (you may feel reduced power, harsher shifts, or slightly worse fuel economy). Continuing to drive risks additional damage to adjacent components.
How much does it cost to fix P1638 on a commercial truck?
Total repair cost ranges $80–$2500 depending on the root cause. The cheapest fix is usually the first one listed below — Shorted sensor on the 5V reference 2 circuit — which is also the most common. On commercial trucks the cost is usually higher than passenger vehicles due to part availability and shop labor rates ($120–185/hour at most truck shops, vs $90–130 at general auto shops). DIY-capable fixes (sensors, hoses, filters) are at the low end; major component replacements (turbo actuator, high-pressure fuel pump, transmission solenoid pack) are at the high end. Always diagnose with a scan tool BEFORE replacing parts — guessing wrong on a $1,500 turbo actuator when the actual issue was a $30 boost hose is a common expensive mistake.
Will P1638 fail a DOT inspection?
P1638 is generally not a direct DOT inspection fail item by itself. Inspectors focus on safety-critical systems (brakes, steering, tires, lights, hours-of-service). However, the check engine light alone can trigger additional inspection scrutiny. Best practice: fix the code and clear it before a planned inspection.
What's the difference between P1638 and related codes?
P1638 is closely related to: P1607 (Internal Control Module Performance), P1635 (5 Volt Reference 1 Circuit), P1654 (AC Refrigerant Pressure Switch Performance), P1690 (Wastegate Control Boost Pressure Out of Range). These codes share the same subsystem (network) and often appear together. If you see this code, scan for the full code list — fixing the root cause usually clears multiple companion codes at once. If only P1638 is set without the companions, the issue is more isolated.
How do I prevent P1638 from coming back after a repair?
For P1638 (5 Volt Reference 2 Circuit), prevention centers on the network system. Pay attention to ground connections at every PM — bad grounds are 70% of communication codes. Inspect harness routing for chafe points. Don't disconnect modules with the key on.