A check engine light can store more than one trouble code at the same time.

That does not mean every code should immediately lead to a parts replacement.

This 2013 Acura TSX came into Chuck’s Auto Repair – Shoreline running very rough.

The scan showed two important pieces of information:

A misfire on cylinder 4.

And a P0420 code.

The next step was figuring out which problem needed to be addressed first.

The Engine Was Running Rough

The rough running was not subtle.

The engine had an active misfire, and the scan pointed to cylinder 4.

That gave us a place to start.

But a trouble code alone still does not identify the failed part.

A cylinder misfire can involve ignition, fuel, mechanical condition or other causes.

So the technician needed to test the ignition side of the system instead of assuming the coil was bad simply because cylinder 4 was misfiring.

The Coil Swap Gave Us the Answer

The technician moved the ignition coil from cylinder 4 to cylinder 1.

Then the system was checked again.

The misfire moved with the coil.

That is a very useful test result.

The problem did not stay with cylinder 4.

It followed the ignition coil to cylinder 1.

That gave us direct evidence that the coil was involved in the rough running problem.

The repair plan then included replacement of the ignition coils and spark plugs.

What About the P0420 Code?

P0420 was stored at the same time.

That code often gets attention because it relates to catalyst system efficiency.

But this Acura was also actively misfiring.

The technician’s notes specifically called for repairing the misfire first and then determining whether P0420 still needed additional diagnosis.

That order matters.

An engine that is not running correctly can affect what the vehicle’s computer sees elsewhere.

So the presence of P0420 did not automatically mean the catalytic converter should be replaced.

The immediate problem we could prove was the misfire.

That came first.

Why We Did Not Start With the Catalytic Converter

It would have been easy to look at P0420 and jump straight to a converter conclusion.

But that would skip an important part of the diagnostic process.

The engine was running rough right now.

The misfire was active.

And the coil swap gave us a clear result.

When one problem is confirmed and another code may be affected by that problem, it makes sense to repair the confirmed issue first.

Then the system can be checked again.

That is much more useful than replacing parts based only on a code description.

The Repair Addressed the Confirmed Ignition Problem

The completed repair included new spark plugs and ignition coils.

The vehicle also received an induction service.

The important part of the story is how the ignition problem was identified.

The shop did not simply see “cylinder 4 misfire” and install a coil.

The coil was moved.

The misfire moved with it.

That test changed the situation from suspicion to evidence.

Why Spark Plugs and Coils Were Addressed Together

At approximately 140,000 miles, the ignition system had age and mileage on it.

The repair recommendation included spark plugs along with the coils.

The inspection also documented that the vehicle needed a complete tune up based on the ignition findings.

That does not mean every Acura TSX with a misfire needs every coil and every plug.

It means this repair decision was based on this vehicle’s condition, age, mileage and test results.

One Code Can Change How You Interpret Another

This is the part many drivers never get to see.

A scan tool can show several codes at once.

That does not mean every code represents a separate failed part.

Some faults may be related.

Some may be secondary.

Some may remain after the first repair and need more testing.

That is why the order matters.

On this Acura, the active misfire was something we could prove.

So that was the problem we addressed first.

If P0420 remained after the engine was running properly again, then it could be evaluated on its own.

A Trouble Code Is a Starting Point

A code gives a technician useful information.

It identifies a system or condition the computer has noticed.

But it still takes testing to figure out what caused it.

On this TSX, the useful test was simple and effective.

Move the coil.

See whether the misfire moves.

It did.

That gave the technician a clear direction.

Why This Matters for Check Engine Light Diagnosis

Modern vehicles can store several faults at the same time.

The goal is not to replace a part for every code on the scan report.

The goal is to understand which problem is active, which one can be confirmed, and what should be repaired first.

Then the vehicle can be tested again.

That approach can prevent unnecessary parts replacement and gives the next round of testing better information.

What Should You Do if Your Acura Is Running Rough?

If the engine is shaking, stumbling, losing power or running noticeably rough with the check engine light on, have it diagnosed.

A misfire can have several possible causes.

The scan provides clues.

Testing provides the answer.

On this Acura TSX, moving the ignition coil showed that the misfire followed the coil.

That gave us a clear reason to repair the ignition system first.

The P0420 code could then be reevaluated after the engine was running properly again.

That is the value of diagnosis.

Not just reading the code.

Understanding what it means on the vehicle in front of you.

ACURA TSX MISFIRE FAQ

What does P0420 mean on an Acura TSX?

P0420 relates to catalyst system efficiency. It does not automatically prove that the catalytic converter needs to be replaced.

Can a misfire affect other trouble codes?

Yes. An active misfire can affect engine operation and may influence other readings or stored faults. That is why it can make sense to correct the confirmed misfire before evaluating another code further.

How did the technician know the ignition coil was involved?

The coil from cylinder 4 was moved to cylinder 1. The misfire moved with the coil, which provided direct evidence that the coil was part of the problem.

Did this Acura need a catalytic converter?

The source documents do not establish that. The notes called for repairing the misfire first and then performing additional diagnosis if P0420 remained.

Why were the spark plugs replaced too?

The vehicle had about 140,000 miles, and the repair plan included spark plugs with the ignition coils based on the vehicle’s age, mileage and ignition findings.

ACURA REPAIR AND CHECK ENGINE DIAGNOSIS IN SHORELINE

If your Acura is running rough, misfiring or showing a check engine light, we can test the system and explain what we find.

Chuck’s Auto Repair – Shoreline
14501 15th Ave NE
Shoreline, WA 98155

206-368-0980

Schedule through the current Book Online / Schedule Appointment