PO304--- Possible Bad Ignition Coil on cylinder #4
PO136-- O2 Sensor Circuit Malfunction (Bank 1 Sensor 2)
Could be sensor or wiring or misfire or other items
You should have changed the front or upstream O2 sensors
on all your vehicles at 100,000 miles
The coil code may or may not be correct
You have to diagnose cylinder misfires
I would suggest a repair shop & they have to use a scan tool
in Mode #06 on a road trip of at least 20 miles, to check for
cylinder misfire counts or use an oscilloscope on a road test
If replacing the coil on #4 resolves the issue, it was just luck,
& luck only goes so far
I had a coil in June that no professional test equip found
I had to walk a new one one cyl at a time
SOURCE: Check engine light won't go away
i read it could be the mass air flow sensor and or faulty spark plugs. also be sure to check all the little vaccum hoses for cracks.
SOURCE: #5 cylinder misfire
I have a corolla and had #4 cylinder misfire also diagnosed at autozone. After changing the spark plugs, problem persisted, along with the same error code, so they recommended changing the spark plug wires, and that is what fixed it. I would have changed the rotor cap too, but was low on dough. Glad that was not the problem.
SOURCE: idles rough after engine is warm
check ALL vacuum lines for cracks/holes and replace as nesissary
SOURCE: 1999 mazda 626 4cyl runs rough at idle , engine
You may have to dig deeper for a vacuum leak. It's possibly the intake manifold gasket. Try spraying some starting fluid around the intake manifold. Also try disconnecting the brake booster and plugging that hose, the diaphram in the booster could be ruptured, causing a vacuum leak. Check the PCV valve and hose too.
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