Diy Starter Motor Relay Test When Check Engine Light Blinks Once Then Stays on Obd2

Your check engine light blinks once, then stays solid. You reach for your OBD2 scanner, and now you're wondering if the starter motor relay could be the culprit. This specific symptom a single flash followed by a steady light points to an electrical communication issue between your starter circuit and the engine control module. Testing the starter motor relay yourself saves time and money, and it gives you real answers before you start replacing parts that might not be broken.

What Does It Mean When the Check Engine Light Blinks Once Then Stays On?

A check engine light that flashes once and then remains solid usually signals an intermittent fault that has been detected by the OBD2 system. Unlike a continuously flashing check engine light (which often indicates active misfires), a single blink followed by a steady glow can mean the ECM detected a momentary voltage drop or communication error in a circuit and the starter motor relay is one of the most common sources of this behavior.

When you crank the engine, the starter relay draws a large amount of current. If the relay is failing or has corroded contacts, it can cause a brief power interruption that the ECM reads as a fault. The OBD2 system logs a diagnostic trouble code (DTC), and the light follows the pattern you're seeing. Understanding the difference between a flashing and solid check engine light in relation to the starter motor helps you narrow down whether you're dealing with a relay problem or something deeper in the starting system.

Why Should You Test the Starter Motor Relay Yourself?

Testing the starter motor relay is one of the easiest diagnostic tasks you can do at home with basic tools. A shop might charge you a diagnostic fee just to tell you the relay is bad a part that typically costs between $10 and $30. If you can locate the relay, remove it, and test it with a multimeter, you skip the guesswork entirely.

You also avoid the common trap of replacing the entire starter motor when only the relay is faulty. The relay is a small electromagnetic switch that sends power from the battery to the starter solenoid. When it fails, it mimics many symptoms of a bad starter no crank, slow crank, or intermittent starting but the fix is far simpler and cheaper.

What Tools Do You Need for This Test?

A basic digital multimeter (set to resistance/ohms and continuity)
Your vehicle's owner manual or a diagram of the under-hood fuse box
A 12V test light (optional but helpful)
Needle-nose pliers for removing the relay
OBD2 scanner to read and clear codes after testing

You do not need a professional-grade scan tool. A simple code reader that reads and clears DTCs is enough for this job.

How Do You Locate the Starter Motor Relay?

The starter relay is usually inside the under-hood fuse and relay box. Look for a black rectangular box near the battery or along the fender wall. The lid of the fuse box typically has a diagram printed on it showing the position and function of each relay. If the diagram is missing or faded, check your owner's manual or look up the fuse box layout for your specific year, make, and model.

The starter relay is often labeled "STARTER," "ST," or "STR." In some vehicles, it shares the fuse box with the fuel pump relay and horn relay these are usually the same form factor, so you can sometimes swap them temporarily to test if the relay is the problem.

How Do You Test the Starter Relay with a Multimeter?

Step 1: Remove the relay

Pull the relay straight out of its socket using needle-nose pliers. Don't yank it at an angle, or you could bend the pins.

Step 2: Check the relay coil resistance

Set your multimeter to ohms (Ω). Identify the coil pins these are typically pins 85 and 86 on a standard four-pin automotive relay. Place one probe on each coil pin. A good relay should read between 50 and 120 ohms. An open reading (OL) means the coil is burned out. A reading near zero means the coil is shorted.

Step 3: Test for continuity across the switch pins

Place your probes on pins 30 and 87 (the switch contacts). With no power applied, you should read no continuity (OL). If you get continuity with the relay unpowered, the contacts are stuck closed the relay is bad.

Step 4: Apply 12V to the coil pins to energize the relay

Use a 12V power source (a spare battery or jumper wires from your car battery). Apply positive to pin 85 and negative to pin 86. You should hear a faint click. While energized, check continuity between pins 30 and 87. You should now have continuity (near 0 ohms). If the relay clicks but there's no continuity across the switch pins, the internal contacts are worn or corroded.

What OBD2 Codes Should You Expect With a Bad Starter Relay?

A failing starter relay may not always trigger a starter-specific code. The ECM might log codes related to low voltage during cranking, communication bus errors, or power supply issues. Common DTCs you might see include:

P0615 Starter Relay Circuit Malfunction
P0616 Starter Relay Circuit Low
P0617 Starter Relay Circuit High
U0100 Lost Communication with ECM/PCM (caused by voltage drop during cranking)

If your OBD2 scanner shows codes that seem unrelated to the starter, that's still normal. A voltage dip from a bad relay can confuse multiple modules. After replacing or confirming the relay, clear the codes and see which ones return. If you want a deeper look at this, our diagnostic guide on the starter motor causing the check engine light to flash then remain solid covers how these codes interact.

What Are the Most Common Mistakes People Make?

Replacing the starter motor before testing the relay. This is the number one waste of money. A starter motor can cost $150–$400 with labor. The relay costs a fraction of that and takes five minutes to swap.

Not checking the relay socket contacts. Sometimes the relay is fine, but the socket it plugs into has corroded or spread pins. Always inspect the socket when you pull the relay. Look for green corrosion, melted plastic, or pins that feel loose.

Ignoring the ground side of the relay circuit. The relay needs a good ground through pin 85 or 86 (depending on your vehicle's wiring). A corroded ground wire can cause the same symptoms as a bad relay, and testing the relay alone won't reveal this.

Clearing codes without fixing the problem. Clearing DTCs with an OBD2 scanner turns off the light temporarily. If the relay is failing intermittently, the light will come back. Read the codes first, fix the issue, then clear. For more on working through these kinds of intermittent failures, see our OBD2 code scanner troubleshooting guide for intermittent starter motor failure.

How Do You Know If the Relay Is the Real Problem and Not the Starter Solenoid?

This is a fair question, and the symptoms overlap a lot. Here's the key difference:

Bad relay: You turn the key and get nothing no click, no crank. Or you get a single click and nothing more. The relay either sends full power to the solenoid or sends nothing.
Bad starter solenoid: You hear rapid clicking (chatter) when you turn the key. The solenoid is trying to engage but can't hold. Or you hear one loud click the solenoid engaging but the motor doesn't spin.

A simple test: have someone turn the key while you listen near the relay box. If the relay clicks but the starter doesn't engage, the problem is likely downstream the solenoid or the starter motor itself. If the relay does not click, the relay or its control circuit is the issue.

Can You Temporarily Bypass the Starter Relay to Confirm?

Yes, but be careful. You can use a jumper wire to connect the two high-current pins (30 and 87) in the relay socket. This sends battery power directly to the starter solenoid, bypassing the relay. If the engine cranks normally when you do this, the relay is confirmed bad.

Important safety note: Make sure the vehicle is in Park or Neutral with the parking brake set. Bypassing the relay means the starter will engage the moment you connect the jumper the key does not need to be on. Keep hands and tools clear of moving parts.

What Should You Do After Replacing the Relay?

Plug in the new relay and make sure it seats firmly in the socket.
Start the vehicle and confirm normal cranking.
Connect your OBD2 scanner and check for pending or stored codes.
Clear all codes.
Drive the vehicle normally for a few days.
Rescan to see if any codes return. If the check engine light stays off, the relay was the problem.

If the light comes back on with the same codes, the issue may be in the wiring between the relay and the solenoid, the ignition switch, or the neutral safety switch. At that point, testing with a wiring diagram and a multimeter on the circuit itself is the logical next step.

Quick DIY Checklist Before You Start

✅ Read and record your OBD2 codes before doing anything else
✅ Locate the starter relay using the fuse box diagram
✅ Test the relay coil resistance (pins 85 and 86: 50–120 Ω)
✅ Test switch pin continuity with and without power applied
✅ Inspect the relay socket for corrosion or damage
✅ Try a relay swap with a matching relay in the fuse box (like the horn relay)
✅ Only replace the starter motor if the relay tests good and the bypass test fails
✅ Clear codes after the fix and monitor for recurrence

Tip: Keep a spare relay in your glove box. They're small, cheap, and one of the most common no-start failures on the road. If you're looking for a clean way to label your diagnostic notes or create printed checklists for your garage, tools like Montserrat make your printouts easy to read at a glance.