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DTC codes / P0Exx · Hybrid propulsion / P0E79

P0E79Coolant Bypass Valve B Control Circuit High

P0E79 indicates that the powertrain control module (PCM) has detected an abnormally high voltage on the control circuit for the coolant bypass valve B, which is used to manage coolant flow for faster engine warm-up and improved efficiency. The PCM commands the valve to a certain position and monitors the circuit; a high voltage reading typically means the circuit is open, shorted to power, or the valve's internal actuator has failed.

SystemPowertrain
Code typeGeneric (all makes)
SeverityMedium
Fix difficultyIntermediate
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the check engine light (malfunction indicator lamp) with no other noticeable symptoms
  • Prolonged engine warm-up time or the temperature gauge rising more slowly than usual
  • Reduced fuel economy or slightly higher emissions during cold starts
  • In some cases, the engine may enter a limp mode or set related codes for coolant temperature
  • A faint clicking or buzzing noise from the valve area if the actuator is stuck
  • In rare instances, overheating if the bypass valve fails in a position that restricts coolant flow

Common causes, most likely first

  1. Open circuit in the control wire between the PCM and the coolant bypass valve B (broken wire, corroded connector, or loose terminal)
  2. Short to voltage on the control circuit, often due to chafed wiring or moisture intrusion
  3. Failed coolant bypass valve B actuator (solenoid or motor) causing an open internal circuit
  4. Poor electrical connection at the valve or PCM connector, including backed-out pins or corrosion
  5. Faulty PCM driver for the control circuit (less common but possible)
  6. Aftermarket or incorrect valve installed that does not match the PCM's expected resistance

How to diagnose and fix it

  1. Verify the code with a scan tool and check for any related codes (e.g., P0E78, P0E7A) to confirm the circuit fault.
  2. Inspect the wiring harness and connectors at the coolant bypass valve B for damage, corrosion, or loose pins; repair as needed.
  3. Measure resistance across the valve actuator terminals and compare to the manufacturer's specification; replace the valve if out of range.
  4. Check for short to voltage on the control circuit by disconnecting the PCM and valve, then measuring voltage with the key on; repair any short.
  5. Perform a continuity test on the control wire between the PCM and valve to rule out an open circuit; repair the wire if broken.
  6. If all wiring and the valve test good, suspect the PCM; consult service information for further diagnostics before replacement.
  7. Clear the code and perform a road test to verify the repair, ensuring the valve operates through its full range.

Don't overlook the wiring harness

  • A high voltage code often results from an open circuit rather than a shorted valve, so a simple continuity test can save you from replacing a good part.
  • The coolant bypass valve B is sometimes part of a larger coolant control module; check if the entire assembly needs replacement rather than just the valve.
  • Intermittent faults can be caused by vibration or heat cycling, so wiggle-test the harness while monitoring live data to catch a failing connection.
  • This code can be set by a low coolant level or air pocket causing the valve to overheat and fail electrically, so always check the cooling system first.
  • Some vehicles use the bypass valve for cabin heater control, so a failure may also affect heater performance without setting a separate code.

Can I keep driving with P0E79?

You can generally continue driving with this code, but avoid extended idling and monitor the engine temperature gauge; if you notice overheating or a significant loss of power, stop and have the vehicle towed to prevent engine damage.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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