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I have a 1998 Mazda Protege (1.5L) and the check engine light is on with two codes. Code P0421 (Warm Up Catalyst Efficiency Below Threshold - Bank 1) and P0170 (Fuel Trim Malfunction - Bank 1). I was told by two mechanics the problem was with the upstream O2 sensor which I had replaced by the Mazda dealer. They told me this would likely resolve the issue, but without them performing a their diagnostics, they would not guarantee the fix. A week after replacing the O2 sensor the check engine light came back on with the same two codes. I cleared the codes once, but the check engine light came back on almost immediately. Please advise. Thanks, Tony
Greetings Tony from Mikey at Just Answer!
Resolving all malfunctions on a car with multiple malfunctions is always a cat-and-mouse game. No one will ever guarantee that one or two or three or more repairs will be needed to rectify all problems. To make matters worse some malfunctions will only reveal themselves after other malfunctions are eliminated. You could fix one problem only to reveal another problem you didn't have before. The reason for this is the way the car runs its self-tests. Test are run in order under certain driving conditions. When a specific malfunction is detected, some future scheduled tests are suspended. So you could have additional undetected malfunctions.
Okay first of all we need to address the fuel system malfunction (P0170) and place the catalytic converter malfunction on the back burner for now.
You must rectify the P0170 and make sure it does not come back before any diagnosis or replacement of the front catalytic converter.
These cars have a well-known malfunction of the fuel system involving the big rubber boot that runs from the air cleaner to the throttle body. Even the slightest leak or crack in this boot will cause a false reading of the mass air flow sensor resulting in a faulty fuel delivery calculation by your the car's engine computer.
My suggestion is to order the air intake boot from the dealer and get that replaced. Check all hose connections to insure there are no vacuum leaks. Clear the computer and then drive cycle the car again.
We are looking for P0170 not to come back. You must pursue only this goal until that code is gone before addressing the catalytic converter.
If the catalyst code comes back by itself the front catalytic converter will have to be burned-off by getting the vehicle engine hot and creating an engine misfire on purpose to overheat the catalyst which will burn off any carbon or oil trapped in the converter.
Once the burn-off procedure is done then the computer will have to be reset again and the drive cycle completed.
If the catalyst code comes back again, the front converter will have to be replaced.
ASE Certified Master Technician
25yrs experience auto and truck emissions and repair.
Thanks Mikey... A follow-up question. You state "If the catalyst code comes back by itself the front catalytic converter will have to be burned-off by getting the vehicle engine hot and creating an engine misfire on purpose to overheat the catalyst which will burn off any carbon or oil trapped in the converter." Can you tell me how to create an engine misfire to overheate the catalyst?Thanks,Tony
Warm engine, then shut off. Remove one spark plug wire and place it on a bolt head or some other metal so you don't get stray sparks. Start the engine and let it idle 5 minutes with engine skipping. Stop engine and put that wire back on. Take off a different wire and then drive car for 5 minutes. We switch wires because we don't want to foul out a spark plug from inducing a misfire. Stop engine, put wire back on and clear the computer.
The unburnt fuel entering the exhaust from the misfire you create will overheat the catalyst burning off any unwanted residue and improving the converter's performance.
If its good enough to pass the vehicle's self-test is unknown but its the best and only chance you have of bringing the converter back to life.
Good luck and let me know how you make out.
Mikey,One more follow-up question... I was informed removing a plug wire would affect the ECM and thereore, should not be done. Is this true?Thanks,Tony
Hi again Tony. Yes, its true. The ecm will detect the engine misfire, the computer will need to be cleared after this part of the procedure. No harm will come from the procedure as we do this on a daily basis.