2010 Lexus RX350 Ignition Coil Problems: Symptoms and How to Diagnose Them

If your 2010 RX350 suddenly starts running rough and throws both a check engine light and a traction control light at the same time, don’t assume the worst — this is exactly the pattern of a single bad ignition coil, and it’s a quick, cheap diagnosis before you touch anything.

Symptoms

• Noticeably rough, uneven idle
• Check engine light on
• Traction control light on at the same time as the check engine light

Diagnose it with a code reader

1. Plug in an OBD2 reader (the creator uses a Blue Driver) and read codes. Reading just the check-engine-related codes is much faster than a full code sweep.
2. Note the cylinder letter the misfire code points to. The reader reports it as a letter (A, B, C…), not a number — use the reader’s own cylinder diagram to translate that letter into a physical cylinder position on your engine.
3. Don’t assume a recently-replaced coil is fine. In this case the coil at fault had only ~15,000 miles on it since replacement.

Rule out a bad connector first

Before buying a new coil, check the connector on the suspect coil — it can mimic a bad coil:

1. Unplug and reseat the coil’s connector. It should click and seat against a rubber seal that keeps it from backing out on its own.
2. Clear the codes and see if the symptom returns. If it comes right back, the coil itself is the problem, not the connector.

Getting to a rear-bank coil

The rear coils on this engine sit behind the windshield wiper assembly and intake, the same area covered in the spark plug and ignition coil replacement guide. The good news here: unlike a full plug-and-coil job on all six cylinders, reaching a single rear coil didn’t require pulling the intake in this case.

1. Remove the windshield wiper tray/cowl assembly to open up access to the back of the engine.
2. Clear the wiring harness bracket blocking the coil — there’s a single screw holding it out of the way.
3. Unbolt the coil (the creator describes it as one bolt once you can reach it) and disconnect its connector.

Swap the coil and confirm the fix

1. Install the new coil and reconnect everything, including the wiring harness bracket you moved out of the way.
2. Clear the codes with your code reader.
3. Start the car and check for smooth idle with no check engine or traction control light. If the light stays off after a drive, the coil was the fix.

A dealer or shop would likely charge somewhere in the $300–$400 range just for this diagnosis and swap; done yourself, it’s the cost of one coil and an OBD2 reader you’ll reuse for the next code that pops up.