2018 CX-5. It had the cylinder deactivation 'deactivated' by dealer shortly after purchase with a software update. It is current on all its software. Been to 3 dealers.
181k miles on said car. Religious oil change intervals (5k, Mobile 1 0-20), filters, etc.
In the last 15k miles, the vehicle has had fuel injectors removed and bench cleaned, walnut blasted those intake valves due to build up, new exhaust manifold due to the upstream cat being a 1-piece design, new NGK plugs, new NTK coils, new NTK MAF, new upstream O2 (air fuel mix sensor) and new brake switch. Prior work on this car include resealing the timing case cover (timing chain inspection good) due to oil leaks. Trans service at 75k, 150k - new filter and Mazda fluid. Coolant valve assy was replaced upon failure, then reimbursed by Mazda once it went to recall status. In-tank fuel pump is not (today) part of the current open recall. The failure mode for that is 'it works, or it doesn't and the engine won't start'. So far there, all good.
Car runs well when it does run. Idles fine, revs ok and generally does good, but when it doesn't.
When cold, after starting, usually at the ~40mph speed, the car may stumble/jerk. Some times you can let off the gas and reapply and ease your way past that ~40mph 'wall'. Sometimes, the car freaks out, dies or near-dies, and you coast to the side of the road. Funny, it's not all the time when starting from cold. Or hot either.
Both Mrs and I see/experience a flashing check engine light. Traction control is illuminated (off) and the gauge cluster shows "smart city braking is disabled". Here's the thing: turn off the car and then restart it. Everything goes away like it never happened. No check engine light, traction is on and brake system is fine. Most often, none of this result in any codes being recorded. Not always, but I have pulled the following codes before:
U0401-00: Invalid data received from ECM/PCM 'A' no sub type information
U0316-09: Software incompatibility with vehicle dynamics control module; component failure
U0415-68: Invalid data received from ABS control module; event information
Today, I was also able to see a misfire code for cylinder 1 too - P0301-24. Never seen a '-24' on your standard misfire code.
The last 15k or so miles, the misfires (any random 1 or more of the 4 cylinders) drove a lot of parts cannon repairs. A local Mazda dealer wanted $500 to replace 4 plugs. No kidding. Thieves. Those misfires, and overall 181k mileage on the car ate the upstream cat, hence its replacement. Downstream cat works fine. Temperature delta across cat is fine, and downstream O2 sensor is responsive and 'flat' as expected. Both O2 sensors are fine and like I said, when the car idles/runs fine, all is ok.
A Mazda forum I found also tossed the brake switch under the bus because if the sensor tosses a bad position to the computers, the smart brake system goes nuts. Old switch replaced with new - problems still happen.
All 3 local Mazda dealers have no clue, or say they do. The last dealer on their report said "found no codes. Test drove and noted hesitation when accelerating". Me trying to explain to the service writer there's no codes because I deleted them else the car would not work to get there (I printed out everything I found with freeze frame data), and said their tech noted the hesitation upon acceleration (which is the precursor to sudden failure, for which they ignored me), is why I'm here. That did not stop that dealer from telling me I NEEDED ~$4k worth of their services, from fluid changes to tires. No thanks.
So what is the code combination of U0401-68, U0316-09, U0415-68 pointing to? I'm not ready to burn the car down as I believe with this taken care of, it could easily get another ~75-100k, but I am on the edge of dumping it for something else.
Thanks in advance
Wednesday, July 30th, 2025 AT 8:14 PM
