Q: I have a 2009 Pontiac G3 with a 1.6-liter engine. The “check engine” light came on about six months ago showing a P0420 catalytic converter efficiency code. Since then I have replaced both oxygen sensors, the cat and battery. I have also cleaned all electrical connectors and checked the grounding points under the hood. The mass airflow sensor has also been cleaned. Any further guidance would be greatly appreciated. The car has 125,000 miles on it.

A: This can be a tricky code to resolve. Were there any other diagnostic trouble codes present with the P0420? If so, it’s really important they be addressed first. Catalyst efficiency is inferred by the powertrain control module by comparing the pre- and post-cat oxygen sensor waveforms. Was the replacement catalytic converter an original equipment manufacturer or after-market part? GM’s on-board catalytic converter testing specs are quite strict and some after-market cats simply won’t pass the on-board test. After-market cats, while less costly, typically don’t deliver the same level of exhaust treatment or last as long as the pricey original manufacturer parts.

Other possible causes of a P0420 code include oxygen sensor problems, exhaust leaks or an exhaust restriction, excessive oil burning, coolant passing through the combustion process (faulty head gasket, other), engine misfire, and incorrect air/fuel mixture.

I think the car needs to go to a savvy, emissions-certified technician who can sort out the possibilities in the preceding paragraph. The oxygen sensors and cat you replaced were logical repair attempts and the work performed under the hood does help narrow the field of possibilities.

Q: I’ve noticed a loud roar at times from the engine area of my Silverado when I am almost to the top of a long grade while pulling my travel trailer. I thought it might be the transmission slipping, but the tachometer stays pretty much the same before, during and after the noise comes and goes.

A: Finally, an easy one! The noise you’re hearing is likely the cooling system fan clutch engaging. Most longitudinal engines that run a mechanical fan employ a thermal fan clutch. This clever device allows the radiator cooling fan to loaf along most of the time at about 30 percent of water pump shaft speed, reducing noise and improving fuel economy.

When the air passing though the radiator reaches about 170 degrees (coolant temp is about 30 degrees above this) and hits the fan clutch’s bi-metal spring, silicon fluid is directed within the clutch to bring fan rotational speed up to about 80-90 percent of shaft speed. A firmly engaged fan roars like a jet taking off and can require over 20 horsepower! You may also notice the fan clutch briefly engages at engine startup due to the fluid entering the working area of the clutch during engine shutoff.

It’s amazing how quickly engine temperature falls when the fan clutch engages. Hearing it go quiet again is sweet music, indicating the cooling system has beat the hill!

— Bergholdt teaches automotive technology. Email questions to under-the-hood@earthlink.net.