P26AC

Code

Generic P — Powertrain

Engine Coolant Bypass Valve B Control Circuit/Open

Brand: Generic

Type: P — Powertrain

Views: UK: 23 EN: 74 RU: 30

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Page language: EN

Causes

Open or damaged wiring in the bypass valve B control circuit
Corroded, loose, or disconnected connector at the valve or harness
Failed Engine Coolant Bypass Valve B (stuck open/coil open)
Blown fuse, damaged relay or loss of power/ground to the circuit
High resistance in circuit from damaged insulation or terminal corrosion
Faulty ECM or PCM driver output (rare)

Symptoms

Malfunction Indicator Lamp (MIL) illuminated
Engine warm-up or temperature control issues (overheating or slow warm-up)
Poor HVAC heater performance or erratic cabin temperature
Engine running in a protective mode on some systems (reduced performance)
Noisy or abnormal coolant routing behavior in some models

What to check

Retrieve freeze frame and related codes; confirm P26AC is current and reproducible
Visually inspect valve B connector and wiring for damage, corrosion, or water intrusion
Check fuses and relays associated with coolant control and ECM power/ground
Backprobe the valve connector with key ON to check for power and control signal
Measure continuity between the ECM control pin and the valve control pin
Measure coil resistance of the bypass valve (compare to service manual spec)

Signal parameters

Expected control: ECM supplies a switched ground or PWM control to the valve solenoid (verify with service manual)
Typical command voltage range: 0–12 V on power supply; control line may be low-side switching to ground
Coil resistance (typical solenoid range): roughly 5–40 ohms (vehicle-specific — confirm spec)
Open-circuit condition: infinite resistance or no continuity between valve coil pins
When commanded ON, expect voltage change or PWM pattern at control pin (use scope)

Diagnostic algorithm

Verify code: Connect a scan tool, confirm P26AC and note freeze frame data and related coolant/temperature codes.
Visual inspection: With ignition OFF, inspect harness and connector at Bypass Valve B for damage, corrosion, or loose pins. Repair obvious issues.
Check power/ground: With key ON (engine OFF), backprobe connector to verify battery + supply and ground are present where applicable. Repair any missing supply or ground.
Coil resistance: Disconnect connector and measure resistance across the valve coil. An open (OL/infinite) reading indicates a failed valve. Compare to manufacturer spec.
Continuity to ECU: Check continuity between the valve control pin and the ECU/PCM pin. An open indicates wiring fault; repair broken wire or connector.
Signal verification: Reconnect and use a scan tool or oscilloscope. Command the valve ON/OFF while monitoring the control wire for switching or PWM. If ECM does not command, investigate ECM outputs and related inputs.
Wiggle test: With connector connected and system powered, gently wiggle harness and connectors while monitoring voltage/signal and scan data to find intermittent opens.
Repair and retest: Repair any wiring, connector, fuse, or valve faults found. Clear codes, perform functional tests and a road/idle test to confirm code does not return.
ECM check: If wiring and valve verify good and the ECM output never changes when commanded, consider ECM/PCM driver fault and follow manufacturer procedures for module testing/repair.

Likely causes

Broken or pinched wire between the ECM and bypass valve connector
Corroded connector terminals at the valve or a water intrusion point
Open/failed solenoid coil inside the bypass valve
Missing power or ground due to blown fuse or disconnected harness
Intermittent open caused by broken strand at a connector or splice

Fault status

⚠️ Status

Control circuit open for Engine Coolant Bypass Valve B. ECM reports no continuity or expected control signal to valve B — circuit open or high resistance.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 0.5-2.0 hours