The no start that only happened when it was hot

On hot days the car just wouldn't start. It would crank, but there was no spark, and when I turned the key I couldn't hear the fuel pump prime. On cooler days it started fine.

So I went back to basics. An engine needs spark, fuel, and air. I worked through each one and followed every step with a multimeter, replacing components as I went and ruling things out one at a time.

None of it fixed the problem. But losing spark and fuel at the same time pointed at something upstream of both. So I traced the signals back to the ECU, and then from the ECU back to the immobilizer. The immobilizer is what tells the ECU the key is legit. No okay from the immobilizer, no spark and no fuel.

The thing that cracked it was the pattern. It only failed when it was hot. Heat makes things expand, and a cracked solder joint can separate just enough to lose contact when it warms up, then close back up once it cools down. That fit everything I was seeing.

So I resoldered the entire immobilizer. Sure enough, it fixed it.

Work one system at a time, measure everything, and pay attention to when something fails, not just what fails. The temperature was the clue the whole time.

• Diagnostics
• Multimeter
• Signal Tracing
• ECU
• Soldering