P26E4: Starter Relay B Circuit

What it means

The control circuit for the secondary starter relay (used on twin-starter heavy diesels or stop-start systems with a dedicated stop-start starter) returns no usable signal.

At a glance

• Usually lights the check-engine light

How it works

Starter relay B switches power to a secondary starter circuit, which may serve a second starter or a dedicated stop-start starter. The control module monitors the relay B control circuit and sets this code when it does not receive a usable signal.

Common causes

Most likely first.

1. Open coil in the relay
2. Broken control wire

Symptoms

• Engine may crank slowly or fail to start
• Stop-start function may be unavailable
• A starter or powertrain warning light may appear

How to diagnose it

1. Use a scan tool to record freeze-frame data and check for related starter, charging, or stop-start codes before clearing anything.
2. Check battery condition and cable connections, then note whether the engine cranks slowly, fails to crank, or starts normally.
3. Inspect starter relay B, its socket, and the control wire for loose connections, corrosion, heat damage, or a broken wire.
4. Use the vehicle wiring diagram to test continuity through the relay B control circuit and check the relay coil using the manufacturer's procedure.
5. If the relay and wiring pass, check the control module command and output at the specified test points.

Common fixes

• Repair a broken or loose relay B control wire or connection.
• Clean or repair corroded or heat-damaged relay socket terminals.
• Replace starter relay B if its coil or contacts fail the specified test.
• Repair the control module circuit if its output is absent despite a valid command.

Can I keep driving?

Starting may be unreliable, and a stop-start function that uses relay B may be unavailable. Avoid repeated long cranking attempts, which can overheat the starter and cables.