A 2025 Song Plus DM-i just landed in your bay with a flashing CEL and a misfire on cylinder 2. The owner says the shop across town installed a new coil pack three weeks ago. The code now reads P0300 and P0302. Sound familiar? That’s the price of replacing ignition parts without testing the circuit first.
BYD’s 1.5L DM-i powertrain uses a compact pencil coil per cylinder. Toss one in, clear the code, and you might get lucky. But luck doesn’t pay for comebacks. Here’s the order of operations that actually holds up.
Check the resistance spec before you even pull the coil. On BYD’s 1.5L and 1.5T applications, primary resistance typically lands between 0.5 and 1.5 ohms across the two connector pins. Secondary resistance—measured from the high-tension spring to the coil’s positive pin—should read in the 8.0 to 15.0 k-ohm range. Every BYD coil doesn’t share identical spec, so pull the factory number or your aftermarket catalog first.
But here’s where a lot of techs stop. An ohmmeter will not catch a coil that breaks down under load or under heat. It only proves the winding isn’t shorted or open at room temperature. I’ve tested coils that read perfectly on the bench and then died fifteen minutes later once real heat settled in.
The scope tells the truth. Hook a secondary pickup on the coil and crank it. The firing line should be tall and sharp—roughly 8 to 12 kV depending on load. The burn line should slope gently and hold for about 1.0 to 1.5 milliseconds. When that burn time drops below 0.6 ms, the coil is running out of energy. You’ll also see extra oscillations after the spark event when the coil’s insulation starts to break down. That’s ringing, and it means the coil is on its way out. If the firing line measures low and the fuel mixture is correct, suspect a weak coil or a failing spark plug before you blame the injector.
Test it hot. Heat is the enemy of resin and copper windings. Unless you’re paying attention, a thermal failure hides behind a clean cold reading. Use a heat gun or let the engine idle to operating temp, then scope the coil again. The waveform will flatten as the coil fades.
Pro tip: A cold resistance test is a starting point, not a verdict. Scope the coil under load, or swap it to a known-good cylinder and monitor misfire counts. If the miss moves, you’ve found your part. If it stays, stop selling coils and start chasing connectors, grounds, and compression.
Don’t skip the small stuff. BYD’s coil connector pins corrode, particularly after a few pressure washes or when the cowl drains back up. Pull the connector, look for green pins, sealed terminal, and a solid ground strap before you condemn anything. Carbon tracks on the coil boot are another silent failure—no code tells you that directly, but the scope shows it as a short, fat burn line with inconsistency across cycles.
If you do find a bad coil, replace the boot and spring along with it. Some shops sell just the coil top and wonder why moisture creeps back under the boot. And check the spark plug too. A worn plug can kill a healthy coil in a few thousand miles—the wider gap spikes the secondary voltage demand and cooks the coil from the inside.
While the car is on the lift and the CEL is the only thing screaming, keep an eye on the tire pressure light. BYD’s 2025 lineup moved to direct TPMS sensors with a non-serviceable battery. When the sensor dies, you get a flashing ‘TPMS’ warning and no spare part on the shelf. The BL-ST-096 | BH-Sens 433MHz TPMS Tire Pressure Sensor | Fits BYD (2025+) | OE# EUEA3609200B, 110964, BLST096 BL-ST-096 covers that scenario—it’s the OE-style 433MHz replacement for BYD 2025+, matching part numbers EUEA3609200B and 110964.
If you’re doing a regular rotation or seasonal tire change on a newer BYD, throw a quick TPMS check in with the coil test. The sensor is already off the wheel, the battery isn’t replaceable, and the customer hears about it now instead of three weeks later when the light starts flashing on the highway. Keep the BL-ST-096 in stock—it programs quickly and won’t leave you hunting factory tooling for a basic air-pressure sensor.
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