P1937 — Additional Heater Control Circuit Open

P1937 — Additional Heater Control Circuit Open. P1937 can leave the cabin without adequate heat or defrosting.

For Ford vehicles

open circuit in additional heater control circuit

vehicle

Is it safe to drive?

Prompt repair recommended P1937 can leave the cabin without adequate heat or defrosting. Driving may be possible in mild weather, but cold-weather use should be avoided until the additional heater control circuit is repaired.

Typical symptoms

The additional heater does not activate when heat is requested
The cabin remains cold even when the HVAC blower is running
Heater output may be noticeably reduced or intermittent
P1937 remains stored or returns as a pending code
Windshield defrosting may be weak during cold or damp conditions

Common causes

Broken or disconnected wiring in the additional heater control circuit
Corroded, loose, or heat-damaged electrical connectors
A failed additional heater control relay
A blown fuse or inadequate ground in the heater control circuit
An internal open failure within the additional heater control module

How to diagnose with an OBD scanner

Confirm P1937 with an OBD2 scanner Connect an OBD2 scanner such as an ELM327 adapter and open AIScanAuto or the scanner interface. Read stored, pending, and freeze-frame data to confirm P1937 and determine whether the additional heater fault is active.
Inspect fuses, grounds, and connectors With the ignition off and the heater wiring cool, check the circuit fuse, ground points, relay sockets, and connector terminals. Look for corrosion, separated pins, crushed wiring, or moisture intrusion.
Test the control circuit Use the vehicle wiring diagram to test voltage, ground, and continuity at the additional heater control relay or module. An open reading on the supply or control line confirms where the circuit is interrupted.
Check the relay and module If the wiring and connectors are intact, test the heater control relay for continuity and proper operation. Verify that the control module receives its command and can complete the ground path before condemning either part.
Repair and verify operation Repair the open wire, connection, fuse, ground, relay, or module as the test results require. Clear P1937, request heater operation again, and confirm that the code does not return.

Approximate repair cost

A rough independent-shop estimate is $150-$900 for diagnosis and repair, depending on whether the fault involves a fuse, connector, wiring, relay, or control module. These figures are estimates and vary by region.

Common questions

Does P1937 mean the HVAC blower is defective?

No. P1937 identifies an open in the additional heater control circuit, so the blower may operate normally while the auxiliary heater fails to receive or complete its control circuit.

Is it safe to drive with P1937 stored?

It may be safe in mild conditions, but reduced heat and defrosting can create visibility and comfort problems. Repair P1937 before extended cold-weather driving.

What should be checked first when P1937 is present?

Start with the heater-circuit fuse, relay connections, grounds, and wiring connectors. These inexpensive items commonly create an open circuit before a control module failure is suspected.

Why does P1937 return after the heater is repaired?

P1937 may return if a connector remains loose, a damaged wire is only temporarily bypassed, or the relay or module still has an internal open. Complete circuit testing before clearing the code again.

P1937 — ---

When your vehicle's computer detects something wrong with the readings coming from a sensor or the behavior of an actuator, it logs a diagnostic trouble code to flag the problem for a technician. Code P1937 is one of these flags — it indicates that a particular circuit is not behaving within the parameters the engine control unit expects. The exact component affected depends on which system your vehicle uses, but the underlying principle is the same: the ECU sees a discrepancy between what it commands and what the circuit reports back.

Modern engines rely on a constant conversation between sensors, actuators, and the ECU. Every fraction of a second, the computer sends a command and expects a specific response. P1937 triggers when this conversation breaks down — either the signal never arrives, arrives corrupted, or the component fails to perform its function. The ECU monitors these circuits continuously while you drive. If the abnormality persists across enough consecutive drive cycles to rule out a fleeting glitch, P1937 gets stored and the Check Engine light activates. The freeze frame data captured alongside the code is particularly valuable — it captures the exact engine speed, load, temperature, and vehicle speed at the moment the fault first appeared, giving you a precise snapshot of the conditions that caused the failure.

Common Symptoms of P1937

The Check Engine light remains illuminated after the fault is detected
Noticeable increase in fuel consumption compared to normal driving patterns
The affected system operates inconsistently or stops functioning entirely
Vehicle may enter reduced-power limp mode depending on which component failed
In some cases, no obvious drivability symptoms occur apart from the warning light

What Triggers P1937

The root causes typically fall into one of four categories, listed here from most to least frequent:

Defective sensor or actuator within the monitored circuit
Damaged, corroded, or loose electrical connector disrupting the signal path
Broken, chafed, or internally shorted wiring in the affected harness
Faulty output driver inside the control module itself

Diagnosing P1937

Follow these steps to pinpoint the exact failure point before replacing any parts:

Plug in an OBD2 scanner, retrieve P1937, and record the associated freeze frame data. This information tells you exactly what the engine was doing when the fault occurred — rpm, temperature, load, and speed all matter.
Scan for any additional trouble codes. Related codes often point to the upstream or downstream component that actually failed, and P1937 may be a consequence rather than the primary fault.
Visually inspect the wiring harness and connectors tied to the affected circuit. Look for worn insulation, bent pins, corrosion buildup, or connectors that are not fully seated.
Test electrical values at the component connector — verify proper reference voltage from the ECU, confirm a solid ground path, and check signal continuity through the circuit.
Perform a wiggle test while monitoring live data. Gently move the wiring harness and connectors while watching the signal reading; any dropouts or spikes point to intermittent wire breaks.
Clear P1937 after your inspection, then drive the vehicle through a complete drive cycle that reproduces the conditions recorded in the freeze frame. If the code returns, the problem is still active.

Repair Options and Costs

The fix depends entirely on which part failed. Always investigate the inexpensive possibilities before committing to major repairs:

Replace the faulty sensor or actuator: $150–$500
Repair damaged wiring or connector terminals: $80–$250
Replace or reprogram the control module: $300–$1,000

These figures reflect typical parts and labor rates at independent shops in the United States. Your actual cost will vary based on vehicle make, model, and local labor rates. An accurate diagnosis before purchasing replacement components is the single best way to keep total costs manageable — the control module replacement scenario represents the worst-case expense and should only be considered after simpler causes have been ruled out.

Can You Drive with P1937?

Most vehicles remain operational when P1937 is stored, though you may notice diminished performance, reduced fuel efficiency, or a disabled auxiliary system. The car will not typically leave you stranded. However, if the underlying fault causes stalling, failure to start, or affects brake-assist or electronic steering functions, treat the situation as urgent and schedule service immediately.