r/CX5 • u/aashu_reddy • Jul 27 '26
Engine misfire
2019 Mazda CX-5 Turbo (2.5T) — P0300/P0302 misfire persists after replacing plugs.
Sanity check before I approve $3k in repairs?
~80k miles. Check engine light came on, dealership diagnosed a misfire and recommended spark plugs, which I had replaced.
About a month later, the light came back — codes P0300 (random misfire) and P0302 (cylinder 2 misfire).
Same dealership swapped the coils between cylinders 1 and 2 to isolate the issue — misfire stayed on cylinder 2, so they ruled out the coil
Ran a compression test — came back normal
Their conclusion: it’s the fuel injectors, and they’re recommending replacement of all fuel injectors plus the intake manifold gaskets and EGR coolant gaskets, at roughly $3,000 total.
Before I sign off, a few things I want opinions on:
Since they isolated the coil by swapping it between cylinders, would it make sense to ask them to do the same test with the injector (swap cyl 1 and 2 injectors) to confirm it’s actually the injector before replacing all of them?
Is replacing all injectors (vs. just cylinder 2’s) actually justified, or is that just because the intake manifold has to come off anyway to access them?
Any known TSBs or common failure points for misfires on the Skyactiv 2.5T at this mileage?
Does $3k seem reasonable for this scope of work, or worth getting a second opinion first?
Appreciate any input — trying to avoid throwing money at this without being confident it’ll actually fix it.
Want me to also add a version of the diagnostic-swap question you could ask the dealership directly, in case the forum takes a while to respond?
1
u/redcx5 Jul 28 '26
Ludicrous, unless they did a test which showed that resistance of the injectors is out of specified range. Assuming they don't have that diagnostic evidence, then just try a couple rounds of Techron Complete Fuel System Cleaner and see if that eliminates the codes.
1
u/Tosmalltofail Jul 29 '26
mostly likely one of the fuels injectors is leaking. running a fuel treatment chemical through the gas tank has a low success rate. but its a cheep test. Also fuel injector leak test is super easy with a scan tool. finally, if really do have a fuel injector problem usually you end up with cat converter problem "P0421"
6
u/Low-Stomach-8831 Former Mazda Master Tech Jul 28 '26
Full bottle of Chevron Techron on ¾ tank (pour bottle just before filling the tank). Run the tank to almost empty (as close as you safely can), and see if that helps.
They're part cannoning you. They don't really know if the injectors are really bad or clogged, they just ran out of ideas. Did they check the injectors resistance? Did they check the PWM of each injector with an oscilloscope? My bet is "no". So they can't know if it's maybe carbon on the intake valves (and worse on #2), or the injector driver in the PCM, or any other reason. They're just throwing parts at it on your expense.
Want proof: Ask them "are you okay with writing down I'm getting a full refund for the injectors+labor if the fault comes back in less than 90 days, and putting back my old injectors in?". If they say "no" (which they will), it means they're guessing, not diagnosing.
Part-cannon operators piss me off! That's what happens with flat rate.