P1938 — Additional Heater Supply Voltage Too Low

P1938 — Additional Heater Supply Voltage Too Low. P1938 does not usually require immediate stopping, but the additional heater may not operate.

For Ford vehicles

additional heater control circuit – supply voltage too low

vehicle

Is it safe to drive?

Caution: additional heat may be unavailable P1938 does not usually require immediate stopping, but the additional heater may not operate. In severe cold weather, reduced cabin heating and defrost can impair visibility, so the supply and ground circuits should be inspected promptly.

Typical symptoms

P1938 is stored as a current, pending, or historic fault
The climate system may request additional heat but the heater does not activate
On some vehicles, the blower operates while the additional heater remains silent
The cabin warms slowly or cannot maintain the selected temperature in cold weather
The fault may appear intermittently when the heater is commanded under load

Common causes

Low battery or charging-system voltage reaching the additional heater controller
A blown fuse or faulty power relay feeding the additional heater control circuit
Corroded or loose power and ground connections creating excessive voltage drop
Damaged wiring with an open or high-resistance section between the power source and heater controller
An internally degraded additional heater control module or switching component

How to diagnose with an OBD scanner

Confirm P1938 with an OBD scanner Connect an ELM327 adapter with AIScanAuto or another OBD2 scanner, place the ignition on, and read stored, pending, and historic codes. Record any freeze-frame data or supported voltage readings before clearing P1938.
Inspect the heater power circuit With the vehicle safely prepared for service, inspect the additional heater fuse, relay, wiring, connectors, and ground points. Look for corrosion, heat damage, loose terminals, or evidence of a previously replaced fuse.
Perform supply and voltage-drop tests Command the climate system for heat and measure voltage at the heater controller and at the battery or upstream power source. Compare both readings with manufacturer specifications; good battery voltage with low heater-side voltage indicates a circuit voltage drop.
Check the ground and charging system Check for excessive voltage drop between the heater controller ground and battery negative, then verify battery and alternator operation. A weak charging system can cause P1938 when the heater circuit is energized.
Isolate the component and verify repair If the supply, ground, fuse, relay, and wiring pass testing, follow the manufacturer pinpoint procedure for the additional heater controller. After the repair, clear P1938 and confirm that voltage remains stable while heat is requested.

Approximate repair cost

A rough independent-shop estimate in the USA is $125–$300 for diagnosis and $250–$900 for repair, depending on whether the fault is a fuse, relay, wiring repair, or control-module replacement. These figures are estimates only and vary by region, vehicle complexity, and labor rates.

Common questions

What does P1938 specifically identify?

P1938 identifies an additional heater control circuit receiving less supply voltage than specified, so power, ground, and connection testing should occur before condemning the heater.

Does P1938 always mean the heater element is bad?

No. A blown fuse, failed relay, corrosion, wiring resistance, or low charging voltage can all cause P1938 even when the heater element itself is serviceable.

Can I clear P1938 and continue driving?

You may clear the code, but clearing it does not correct the low supply voltage. The additional heater could remain inoperative, particularly when defrost or cabin heating is needed.

Why might P1938 appear only during cold weather?

Cold-weather operation commands the additional heater and places greater electrical demand on its circuit, allowing a marginal fuse, relay, connector, or charging system to trigger P1938.

P1938 — ---

When your vehicle's engine control unit detects that a sensor and actuator circuit is not behaving as expected, it logs OBD2 trouble code P1938. This code indicates the electrical feedback loop between a sensing element and its controlled device has fallen outside factory-calibrated parameters. Rather than pinpointing one specific failed part, P1938 flags a discrepancy in how signals travel through a circuit — the ECU sees readings that do not match what should be happening based on current operating conditions.

Modern vehicles rely on constant communication between sensors and actuators to manage everything from emissions systems to transmission behavior. The ECU continuously monitors voltage, resistance, and signal patterns from these components. When live data contradicts expected values, the module stores P1938, typically across multiple drive cycles to rule out temporary glitches. It also captures a freeze frame — a snapshot of engine speed, load, coolant temperature, and vehicle speed at the exact moment the fault occurred. That snapshot is invaluable during diagnosis because it tells you exactly what conditions triggered the code.

Setting P1938 means the ECU has confirmed a genuine electrical irregularity, not a momentary spike. Depending on which circuit is affected, you may notice drivability issues or the system may simply disable a non-critical function while leaving the car otherwise operational.

Symptoms of P1938

The system tied to the faulted circuit operates intermittently or stops working entirely
Fuel economy increases noticeably in some vehicle applications
Limp-home mode may activate, limiting engine power or specific functions
Check Engine Light remains illuminated on the dashboard

Causes of P1938

A blown fuse feeding the affected circuit interrupts power delivery
Corroded, loose, or damaged connector pins prevent reliable electrical contact
Open or shorted wiring breaks signal continuity or creates unwanted current paths
Poor ground connection causes voltage irregularities at the component
The sensor or actuator itself has failed and sends incorrect data

How to Diagnose P1938

Connect an OBD2 scanner and confirm P1938 is stored. Save the freeze frame data immediately — it shows engine conditions when the fault first appeared.
Look for related trouble codes that may point to the underlying problem. Check manufacturer technical service bulletins for your specific make and model — many P0xxx codes have documented factory fixes or software updates available through the manufacturer website or NHTSA database.
Perform a visual inspection of the wiring harness and connectors associated with the faulted circuit. Look for chafed insulation, corrosion buildup, broken wire strands, or pins that are pushed out of position.
Use a multimeter to check reference voltage, ground integrity, and signal continuity at the component connector against published specifications.
Conduct a wiggle test on the harness while monitoring live data. Intermittent signal drops indicate broken wires that only fail under movement or vibration.
Clear the code, perform a complete drive cycle matching the freeze frame conditions, and verify P1938 does not return.

Repair Options and Costs

Address the most common and least expensive possibilities before replacing major components:

Repair damaged wiring or connectors: $80–$250
Replace the faulty sensor or actuator: $150–$500
Control module diagnosis and repair: $300–$1,000

Total Repair Cost for P1938

The final bill depends entirely on which part actually failed. Parts and labor at independent shop rates:

Wiring or connector repair: $80–$250
Sensor or actuator replacement: $150–$500
Control module work: $300–$1,000

Most repairs for P1938 fall between $80 and $1,000 total. Proper diagnosis before purchasing parts is the single most effective way to control costs — the control module scenario, while possible, is rarely the culprit.

Driving Safety Verdict

Severity level: short. In most vehicles, P1938 does not prevent driving. You may experience reduced performance or a disabled auxiliary system, but the car remains operational. If the vehicle stalls, fails to start, or displays brake and steering warnings alongside this code, treat it as urgent and stop driving immediately.