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Q: I have a 2010 Honda CR-V. Last month its check engine light came on. I'm a retired mechanic and my scanner found code P0420 — catalytic converter efficiency. I tried a catalytic converter cleaner. It worked for about two weeks and the light came back on. I replaced the O2 sensor but the same thing happened again. I did find out if I drive for about 60 to 100 miles the code would not be there but then it's back again a few days later. Do you have any other remedy for this problem besides replacing the converter?

A: No. The P0420 DTC indicates a specific problem with the converter itself — it's effectively not fast enough during its test cycle while driving. If the system does not see this loss of efficiency for two more driving cycles, the light goes off but the code is stored. This is the cycle you are experiencing.

So, the catalytic converter needs replacement. The federal emissions warranty expressly requires carmakers to cover the performance of the converter for eight years or 80,000 miles, whichever comes first. If your Honda has fewer than 80,000 miles on its odometer, this should be covered under the emissions warranty.

Q: I have a 2003 Saab 9-3 convertible. On occasion my car will not turn over and will not start. It happens when I go short distances to the local grocery store, etc. In and out of the store, 15 minutes tops, and it will not start. I wait 15 to 20 minutes and in most cases it will start. I had to have it towed to the shop once, they said it was the starter, replaced it and two weeks later the car would not start again.

The shop has given up. When the car does not start, everything else in the car works — radio, up and down with the top, no problem. I would certainly appreciate any insight that you may have.

A: It seems the shop focused only on the starter motor. However, battery voltage must reach the starter motor with little or no resistance in order for the starter to function properly. My ALLDATA automotive database pulled up Saab service bulletin # 331-2628 from October 2006 that identifies a possible cause — corrosion on the battery cable connection to the starter motor. Surprisingly, the repair is rather involved and includes cutting and rewiring connections at the starter motor, alternator and fuse block. Apparently, moisture can enter and corrode some of these connections, disrupting battery voltage to the starter motor.

Q: My 1997 GMC Sierra 2500 with the 5.7-liter V-8 is giving me fits. Sometimes it starts and runs great all day long. The next day it will start but only run fine for 30 seconds until it dies. It restarts but does the same thing over and over. I had to have it towed twice. My code reader came up empty but when it was running it read EGR Decel Fail. Everything I have heard about EGR problems say the vehicle will miss and run rough. Mine runs terrific when it runs. Any ideas?

A: It's hard not to focus on the EGR (exhaust gas recirculation) system but don't overlook other possible causes — sticking sequential fuel injectors, loss of fuel pressure, fuel contamination, etc.

If the EGR valve sticks in the fully open position, it will introduce too much exhaust gas into the incoming air/fuel mixture, which can cause surging, poor idle and engine stalls. The EGR valve itself incorporates a control module that provides feedback, somewhat like the function of an oxygen sensor, to accurately regulate exhaust recirculation. Since a 12-volt signal from the powertrain control module is required to open the EGR valve, it may be mechanically binding or sticking. During one of these episodes of stalling, try lightly and carefully tapping on the EGR valve, located on the top front of the engine.

Paul Brand is the author of "How to Repair Your Car" and "How to Repair Your Truck and SUV," published by Motorbooks.