No Spark? No Problem. Diagnosing a Customer’s Wiring Issue That Wasn’t

Listen up, keyboard warriors and weekend mechanics—before you start claiming our harness “killed your spark,” maybe, just maybe, take five minutes to figure out that diagnosing electrical issues, patience and proper tools go a long way.

All jokes aside Bo was a good dude, and we we're happy we resolved his issue! With that being said... Lets move forward.

A recent case we helped troubleshoot is a perfect example of why assumptions can lead to unnecessary headaches. A customer contacted us, convinced that the engine harness he purchased from us was causing a “no spark” issue. After hours of back-and-forth troubleshooting, we discovered the real culprit—incorrect fuel injectors running on a stock ECU and a faulty test light giving false readings.

The Initial Claim: "Your Harness Killed My Spark"

The customer reached out with concerns that after installing our harness, his car wouldn’t spark. He claimed the engine block had 12V with the ignition off, something that shouldn’t happen. Naturally, we took this seriously and jumped into troubleshooting mode.

Step one? Verify the claim.

We had the customer check ground continuity at the engine block. With the ECU disconnected, the engine block still showed power—definitely strange. But something wasn’t adding up. We asked him to double-check with a volt meter instead of a test light. The moment of truth came when he measured 0V at the block after disconnecting the battery, confirming what we suspected…

His test light was faulty. It was giving a false positive, misleading him into thinking power was present when it wasn’t.

Okay, But Was There Really No Spark?

With the bogus test light debunked, we moved on to the original complaint—no spark.

To verify, we had him pull a spark plug, ground it, and crank the engine while recording a video. Lo and behold, there was spark! The issue was never a lack of spark—it was something else.

At this point, we noticed something critical: his spark plugs were fouled. The next logical step was to check the injectors. And this is when the real issue was uncovered.

Wrong Injectors + Stock ECU = Recipe for Failure

After asking more questions, the customer casually mentioned that he was running different injectors. The stock ECU wasn’t tuned for these larger injectors, meaning the engine was running excessively rich, flooding the cylinders, and fouling the plugs.

His response? “But it ran fine with the old hacked up stock harness.”

Nope. Just because it "ran" doesn’t mean it ran correctly. The stock harness may have had slightly different resistance characteristics, leading to slightly less fuel delivery, but the issue was always there—our harness simply exposed it further.

The Fix and Final Thoughts

We walked the customer through cleaning the plugs and testing the injectors outside the manifold. Every single injector was spraying fuel—confirming excess fuel delivery was the root cause.

His options?
✔ Swap in stock injectors to match the ECU.
✔ Get a proper tune to accommodate the larger injectors.

Before jumping to conclusions and blaming a product, take a step back and verify the actual problem. In this case, a faulty test light, fouled plugs, and mismatched injectors turned a simple issue into hours of unnecessary troubleshooting.

At the end of the day, we’re always here to help, but diagnostics take time, and we didn't get paid for it. Diagnosing an issue that wasn't even related to our product. If you're installing custom parts, make sure you understand how they interact with your setup. Otherwise, you'll be chasing ghosts instead of solving real problems.

Lesson learned? Use the right tools. Check your components. And never assume—verify.

PS: And hey, if you guys need a B or D series harness that actually works—not some cheap eBay knockoff with incorrect pinouts—you can purchase them here 😃.

UPDATE: Customer installed stock injectors, and the car fired up. The balance to the world had been restored!