P0B38 — Motor Electronics Coolant Pump "B" Control Circuit/Open

How do I diagnose P0B38 myself?

Start with a scan: confirm P0B38, save the freeze frame and note any companion codes. Then inspect the wiring and connectors related to the engine cooling and temperature control, and verify the suspect component against live data before buying parts.

Is it safe to drive with P0B38?

Drive with caution and watch the temperature gauge. A thermostat stuck open mainly costs fuel; any sign of overheating means stop immediately — driving an overheating engine destroys head gaskets and worse.

Will P0B38 clear itself?

Sometimes P0B38 clears after a few drive cycles if the fault was intermittent — for example a loose connector or a tank of bad fuel. The underlying cause usually remains, so the code tends to return. Clear it once; if it comes back, diagnose rather than erase it repeatedly.

How much does it cost to fix P0B38?

Replace the thermostat — $150–$400; Cooling system repair (pump, fan, radiator) — $300–$1,200; Replace the coolant temperature sensor — $100–$300. Exact cost depends on the root cause, your vehicle and local labor rates — a proper diagnosis is the cheapest first step.

What are the symptoms of P0B38?

Drivers most often notice: Check Engine Light on, increased fuel consumption, temperature gauge reads low or never reaches normal, poor cabin heat.

When your scanner pulls code P0B38, it's telling you the powertrain control module has detected a malfunction in the motor electronics coolant pump "B" control circuit — specifically an open circuit condition. This falls within the engine cooling and temperature regulation group that OBD2 monitors across all makes and models. The fault essentially means the control module sent a command to the secondary coolant pump but received no valid response back, either because the circuit is broken, the pump has failed, or the signal cannot reach its destination.

The secondary electric coolant pump serves a specific purpose in hybrid and modern engine designs: it keeps coolant circulating through the motor electronics housing when the main engine is off but the system still needs thermal management. Think of it as the pump that prevents heat buildup in the inverter, power electronics, or motor assembly during idle-stop phases or when the internal combustion engine isn't running. The ECU tracks this pump's operation continuously. When the measured circuit voltage or pump feedback doesn't match expected values for a set number of drive cycles, the module logs P0B38, records the freeze frame snapshot, and illuminates the check engine light to alert you.

That freeze frame data is your diagnostic starting point. It captures engine speed, load, coolant temperature, and vehicle speed at the exact moment the fault occurred — information that tells you whether the problem happened during cold startup, highway cruising, or a specific operating condition. Use it to replicate the scenario during your verification drive.

Common symptoms of P0B38

• Check Engine Light remains illuminated after the fault occurs
• Fuel economy drops noticeably compared to your normal driving pattern
• Coolant temperature gauge reads lower than usual or fails to reach normal operating range
• Cabin heater produces weak or inadequate warmth
• Engine cooling fans run continuously even at low ambient temperatures

What causes P0B38?

• The coolant temperature sensor has failed or is sending erratic readings
• Coolant level is low or air pockets exist in the cooling system
• The water pump, radiator, or associated components have developed faults
• Thermostat is stuck in the open or closed position
• Cooling fan motor or fan relay has failed
• Wiring harness or connector damage in the sensor or pump control circuit

How to diagnose P0B38

1. Plug in your OBD2 scanner, confirm P0B38 is present in the stored codes, and save the freeze frame data immediately — this snapshot reveals the exact engine conditions when the module first detected the fault.
2. Scan for any related trouble codes before clearing anything. Intermittent or secondary codes often point to the root cause rather than the symptom. Also check manufacturer technical service bulletins for your specific vehicle — some P0xxx codes have known factory fixes or require software calibration updates that resolve the issue without parts replacement.
3. Perform a thorough visual inspection of the wiring harness and electrical connectors tied to the engine cooling and temperature control system. Look for damaged insulation, corrosion buildup, broken pins, loose connections, or chafed wires that could create an open circuit.
4. Monitor coolant temperature through live data on your scanner during a warmup cycle. Normal operation shows temperature climbing smoothly and stabilizing near operating temperature within a predictable timeframe.
5. Physically verify coolant level in the reservoir and squeeze the upper and lower radiator hoses to confirm the thermostat opens and allows circulation at the correct temperature.
6. Clear the code, complete a full drive cycle that reproduces the conditions recorded in the freeze frame, and verify P0B38 does not reappear — a one-time event may indicate a temporary electrical glitch rather than a failed component.

Repair options with costs

Address the most probable causes before committing to expensive component replacement:

• Thermostat replacement — typically $150–$400 depending on vehicle make and labor rates
• Cooling system repair involving the pump, fan, or radiator — generally $300–$1,200 depending on which component fails and whether the radiator requires replacement
• Coolant temperature sensor replacement — usually $100–$300 for parts and installation

How much does it cost to fix P0B38?

Total repair expense varies based on which underlying component triggered the code. Parts and labor estimates at independent shops in USD:

Thermostat replacement runs $150–$400 in most cases. Cooling system repair encompassing pump, fan, or radiator work ranges $300–$1,200 depending on the scope and your vehicle's design. Coolant temperature sensor replacement costs $100–$300 for the complete job.

For most vehicles, the total bill to resolve P0B38 falls between $100 and $1,200. The wide range reflects how differently these components fail. An accurate diagnosis before purchasing parts is essential — the $1,200 scenario is almost never the first thing to check, and skipping diagnosis to replace the most expensive part first wastes money.

Can you drive with P0B38?

Exercise caution while driving and monitor your temperature gauge closely. A stuck-open thermostat primarily affects fuel economy and cabin comfort. However, any indication of overheating — temperature climbing past normal, steam from the hood, temperature warning light — means stop immediately and discontinue driving. Continuing with an overheating engine risks catastrophic head gasket failure, warped cylinder heads, and engine destruction that far exceeds the cost of any cooling system repair.

Related OBD2 codes

• P0B00: Auxiliary Transmission Fluid Pump Motor Phase U Current
• P0B01: Auxiliary Transmission Fluid Pump Motor Phase U Current Low
• P0B02: Auxiliary Transmission Fluid Pump Motor Phase U Current High
• P0B03: Auxiliary Transmission Fluid Pump Motor Phase V Current
• P0B04: Auxiliary Transmission Fluid Pump Motor Phase V Current Low
• P0B05: Auxiliary Transmission Fluid Pump Motor Phase V Current High