Q. My 2016 Honda Accord has the four-cylinder engine and 112,000 miles on it. The car is throwing a P0420 code, catalyst efficiency below threshold. That is the only code, and the car runs great. What would you suggest checking before I have to replace the catalytic converter?

A. The first place to start is making sure there are no exhaust leaks. Even a small leak can affect oxygen sensor readings and trigger a P0420 code. Next, look at the data from the upstream and downstream oxygen sensors. If the sensors are not reading correctly, or if there is a wiring issue, the code can appear even when the catalytic converter is still functioning properly. If the exhaust system is leak-free and the oxygen sensor readings look normal, then the catalytic converter is the most likely cause. At 112,000 miles, catalytic converter failure is not unusual, but it is worth checking for exhaust leaks and reviewing oxygen sensor data before condemning the converter. If replacement is needed, I would strongly recommend using a Honda original-equipment converter, as aftermarket converters do not always provide the same long-term performance and may allow the code to return.

Q. I watch a lot of YouTube and notice that some mechanics use very inexpensive voltmeters for testing. In fact, on one channel, the meter they were using was a giveaway at the local tool store. Do these inexpensive meters do the job?

A. For most automotive testing, yes. Even an inexpensive digital voltmeter can accurately measure voltage, resistance, and continuity. When I was a working technician, I purchased a $400 digital volt-ohm meter, and it was great until it stopped working years ago. I replaced that meter with a........

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