P0B38 — Motor Electronics Coolant Pump “B” Control Circuit/Open #

Note: The label for DTC P0B38 may vary depending on the vehicle manufacturer and diagnostic tool. Always refer to your specific OEM (Original Equipment Manufacturer) documentation for confirmation.

• P0B38 – Motor Electronics Coolant Pump “B” Control Circuit/Open (Generic/SAE)
• P0B38 – Hybrid/EV Electronic Coolant Pump B Circuit Open (Toyota, Lexus)
• P0B38 – Engine Electronics Coolant Pump “B” Control Circuit/Open (BMW)
• P0B38 – Auxiliary Coolant Pump “B” Control Circuit/Open (General Motors/Chevrolet)

Summary of the P0B38 Code #

P0B38 is a generic OBD-II diagnostic trouble code signaling an open circuit or fault in the control circuit for the Motor Electronics Coolant Pump “B”. This component, also known as an auxiliary or electronic coolant pump, is responsible for circulating coolant through the power electronics, battery, or motor circuits—especially on hybrid or electric vehicles. When the vehicle’s Engine Control Module (ECM) or Hybrid/EV Control Module detects an abnormal voltage or no continuity in this circuit, it triggers the P0B38 fault. Addressing this code promptly is essential to prevent potential overheating of critical components and to maintain proper vehicle performance.

Frequent Symptoms for Code P0B38 #

When P0B38 is active, drivers and technicians may notice a range of warning signs. While some vehicles may show very few symptoms initially, ignoring them can risk severe damage to the electronics or powertrain. Look out for the following:

• Illuminated Check Engine Light (CEL) or Hybrid System Warning
• Reduced hybrid/electric system performance
• Intermittent or persistent overheating warnings, especially under load or high ambient temperatures
• Loss of electric drive function (hybrids/EVs may revert to “limp home” mode)
• Unusual fan or coolant pump noises, or silence where pump operation is expected
• HV battery temperature warning (on some hybrids/EVs)

Probable Causes for Code P0B38 #

The P0B38 code generally points to a break or malfunction in the electrical path controlling the auxiliary coolant pump “B.” The most common causes, in order of likelihood, are:

1. Faulty motor electronics coolant pump “B” (internal failure or open windings)
2. Damaged or disconnected wiring (corroded, broken, or loose connectors in the control circuit)
3. Blown fuse or relay associated with the pump circuit
4. Failed control module (ECM/Powertrain Control Module or dedicated Hybrid/Electric Control Module)
5. Sensor or signal line issues (less common, but possible if shared with other circuits)

Step-by-Step Diagnosis for Code P0B38 #

Diagnosing P0B38 requires a methodical approach to avoid unnecessary part replacements. Always prioritize safety, especially when working with high-voltage hybrid/EV systems.

• 1. Confirm the code using a professional-grade scan tool. Record any other related codes.
• 2. Inspect the coolant pump “B” and its connectors for obvious damage, corrosion, or water intrusion.
• 3. Check the pump circuit fuse and relay, replacing as needed. Verify correct amperage ratings.
• 4. Perform a continuity test on the wiring harness between the control module and the pump using a multimeter. Look for open circuits or high resistance.
• 5. Command the pump “B” on/off with a scan tool (if supported) and monitor voltage and current at the connector.
• 6. Test the coolant pump “B” directly by supplying power and ground (per OEM procedure) to check for operation.
• 7. If all else checks out, consider a control module fault and follow OEM diagnostic flowcharts for further testing.

Possible Repairs for Code P0B38 #

Once the issue is pinpointed, repairs should address the root cause to prevent recurrence. The most common solutions include:

• Replacing a faulty coolant pump “B”
• Repairing or replacing damaged wiring/connectors in the pump control circuit
• Replacing a blown fuse or faulty relay specific to the pump circuit
• Reflashing or replacing the ECM or Hybrid Control Module (rare, only after confirming module failure)
• Cleaning or securing ground connections related to the pump circuit

Can You Drive with Code P0B38? #

Driving is generally NOT recommended with an active P0B38 code, especially on hybrid or electric vehicles. The fault indicates the cooling system for critical electronics isn’t functioning, risking overheating and serious component damage (such as power inverters, batteries, or electric motors). In some cases, the vehicle may enter a limited power mode to protect itself, but continued driving could lead to expensive repairs or sudden loss of propulsion. Address this code as soon as possible.

Vehicles Most Commonly Affected by Code P0B38 #

While P0B38 can appear on any modern vehicle using an auxiliary or electronic coolant pump, it is most frequently reported on:

• Toyota Prius (various hybrid models)
• Lexus hybrid vehicles (e.g., RX, ES, CT series)
• BMW hybrid and i-series (i3, i8, plug-in hybrids)
• Chevrolet Volt and other GM plug-in hybrids
• Ford hybrid and electric vehicles (Fusion Hybrid, C-Max, Focus Electric)
• Hyundai and Kia hybrid models

Common Mistakes & Best Practices with Code P0B38 #

Avoiding costly errors when diagnosing and repairing P0B38 requires attention to detail. The most frequent pitfalls and expert tips include:

• Replacing the coolant pump before testing the circuit—Always verify power and ground at the connector first.
• Overlooking corroded or damaged connectors, especially in high-moisture environments.
• Ignoring related fuses and relays—A blown fuse may indicate an underlying short.
• Attempting module replacement without confirming all wiring and pump functions first.
• Best practice: Always clear the code and road test the vehicle after repairs to ensure the issue is fully resolved.

Quick FAQ for P0B38 #

Q: Can a bad battery cause P0B38?
A: Rarely. P0B38 specifically targets the coolant pump control circuit, not the 12V or hybrid battery. However, a weak battery can sometimes cause false codes if voltage drops are significant.

Q: Is P0B38 covered under warranty?
A: In many cases, yes—especially on hybrid/EV powertrain warranties (often longer than standard). Check with your OEM dealer for specifics.

Q: Will P0B38 cause my hybrid or EV to stop running?
A: It can. Many vehicles enter a fail-safe or limp mode to prevent overheating, which may limit or disable electric propulsion until repairs are made.