I have now inherited the job of taking care of my parents vehicles. Just ran the code reader on the 96 Explorer to find several codes. P0455 (Evaporative Emission System Leak Detected (Large Leak) , P0401 (Exhaust Gas Recirculation Flow Insufficient) , P0420 (Catalyst System Efficiency Below Threshold Bank 1) , P0171 (System too lean Bank 1) and P0174 (System too lean Bank 2) as well as Pending code of P0133 (O2 Sensor Circuit Slow Response Bank 1, Sensor 1). My gut is telling there are probably 3 things wrong. Likely a hole in the exhaust pipe, a clogged or disconnected Evap hose, and clogged EGR port(s). My question is ... What are the chances of there being only 1 or 2 issues that relate all of these codes? and if so what might they be?

 

I'd suggest starting by looking for a vacuum leak. Since you codes for both cylinder banks running lean, you're probably looking for something that would be common to both such as the intake air tubing or the connections at the MAF sensor and throttle body. Another clue might be the EGR flow code, which could be hinting at a leak with the EGR system. And a leak with the EGR system could also effect both cylinder banks. So now you should also carefully inspect the plumbing associated with the EGR system. Since you have a code for bank 1 catalyst efficiency, you might check to see if the exhaust inlet to the EGR valve is connected to bank 1. If so, and you happen to have a leak in the inlet tube to the EGR, that could wipe out 4 codes with just one fix.

As for the P0455, I would suggest you loosen, inspect, and re-seat the fuel cap. Make sure the seal is good and clean, and that you can still get a few clicks when tightening it. If so, that might fix the P0455 code. Ask your parents if they are the type that leave their car running when filling it with fuel. If so, tell them to stop doing that. It's unsafe and will trigger diagnostic codes.

-Rod

 

I'm with shorod and would look for a vacuum leak. A vacuum leak could occur through the evap system, but it's not the only source. Also, EGR valves typically use vacuum to actuate the valve, but the tube/hose to the EGR is fairly small so I wouldn't expect that to be a large vacuum leak.

I believe one measure of catalyst efficiency is exhaust gas O2 content. A properly working modern engine usually operates at next to stoichiometric air/fuel ratio, which means there should be almost no excess O2 in the exhaust gas. A vacuum leak could contribute to excess O2, which could set the catalyst efficiency.

If you've had a backfire, which can occur when your engine runs lean, vacuum lines or the EGR hoses could have come off.

 

My father is a mechanic... he taught me to fix cars, lol, he just physically can't move well enough to do it anymore. He would not leave the car running while pumping gas. He always tightens the gas cap till it clicks. He would have told me if it were backfiring. I already found 1 vacuum leak which was causing air flow control issues, so it would not surprise me to have another one. However, I am suspicious of an exhaust leak. I am afraid to look at the condition of the exhaust system. The poor car has some rust. My father seems to think it is a possibility that the Evap system may have rusted out near the fill tube. Can you check the Evap system with a vacuum tester? I can continue checking the vacuum system too if you think an exhaust leak may not cause the lean codes.

 

I don't think you can use a vacuum tester to test the EVAP system. I believe that system is designed to be checked using a smoke machine.

-Rod