Here at All Tune and Lube Total Car Care Tempe customers commonly come in with a Check Engine Light (CEL or MIL Malfunction Indicator Light) and ask us what it means. Very often it’s nothing much, a bad or loose gas cap, maybe a thermostat but from time to time we’ll see the DREADED P0420 CATALYST EFFICIENCY BELOW THRESHOLD code. Oh No! OK, it can get pricey but it’s not necessarily your cat. Here’s the run-down.
A P0420 code is a trouble code on OBD-II equipped vehicles. If your car has multiple trouble codes (DTC’s), a mechanic is going to want to fix them in the order they come up on the code reader. For example, if you have these codes – P0300 (random misfire), P0420 (cat efficiency), P0171 (system lean), then we’ll want to tackle the P0300 Random Misfire code first, and then tackle the P0420 code and so on. The reason for that is that the first code could be causing further DTCs to be set.
So what’s a P0420 all about anyway? This code refers to a problem with the bank 1 cat and that’s all you’ll have if you have an inline 4, 5 or 6 cylinder engine. If you have a V6 or V8 or a Subaru you’ll also have a bank 2 cat but for the sake of simplicity we’ll just assume an inline engine. The catalyst system being referred to is your three-way catalytic converter; a device that looks a lot like a muffler in your exhaust system but performs an entirely different function. The cats’ function is to reduce pollutants coming from your engine. So the catalytic converter is not working properly, it’s not efficient.
A failing cat isn’t going to change the way your car feels, it’ll seem to run just fine. A P0420 code will cause no symptoms aside from the MIL. The reality is there’s still something wrong and your car needs attention in order to be in excellent running condition. A poorly maintained car will run less efficiently, burn more gas, and cost you more money in the long run. Plus, if your CEL is on for a P0420 code and you decide not to fix it, another more serious code could be triggered and you’d never know. The MIL lights up whether you have one code or ten!
There are a number of things that could be causing this P0420 code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other items could include exhaust leaks, damaged exhaust pipes or damaged O2 sensor wiring.
So our customer decides to have us diagnose the fault, what do we check? The first thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. Then we’ll check for what should be a substantial increase in temperature from the front to the rear of the cat. If any of those symptoms are there, the converter likely needs replacement. If all that checks out, we’ll check the operation of the O2 sensor. To do that we plug in to your cars’ computer with a scan tool and check the wave form on the two oxygen sensors.
OK, the oxygen sensors are performing as they should and there are no exhaust leaks. Sorry, you’ll need a cat. Now, emissions related equipment on most vehicles has a longer warranty than the rest of the car so we’ll check with the dealer to see if the cat’s covered. If it’s not then the customer has a couple options: OEM direct replacement is the best, highest quality part made to fit your car but it’s expensive. Next is a high quality direct replacement aftermarket part. Those will save you some money and usually have a 5-year warranty. The last one we’ll suggest is a high quality weld-in cat. With those we have to cut your old cat out and weld the new unit in its’ place. These cats also typically come with a 5-year warranty. There are cheaper parts but we won’t install them.
Hope this helps!
From your local mechanic, All Tune and Lube Total Car Care Tempe. Complete auto repair and maintenance.