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Dan, Technician
Category: Car
Satisfied Customers: 23939
Experience:  Independent Shop Experience, 2 Year Automotive Diploma, & Access to Information Database
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2001 Isuzu Trooper LS: Trac..egr Valve..4 o2 sensors..started running

Resolved Question:

I have a 2001 Isuzu Trooper LS Trac on Demand 3.5. Last month I replaced the EGR Valve, the 4 O2 sensors on the exhaust system. It started running better afterwards. Now it is running rough again and I'm getting codes P0420, and P0300 codes.
Submitted: 6 years ago.
Category: Car
Expert:  Dan replied 6 years ago.

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P0420 usually means the catalytic converter has failed and if its clogged it can cause vehicle to run rough, P0300 code, oxygen sensoor codes, and egr codes...You can remove oxygen sensor before converter to see if it runs better or install pressure guage, pressure shouldnt go over a few psi whiel reving engine.

Customer: replied 6 years ago.
It dose'nt run rough until I have driven at least 15 to 20 miles. On short trips less than 15 miles it runs smooth with no problems.
Expert:  Dan replied 6 years ago.
P0420 is caused by a converter 95% of the time...otherwise it can be caused by oxygen sensors which you already replaced, exhauast leak, or intake leak...So you can start by looking for leaks and also make sure fuel pressure is good...If fuel pressure drops replace filter if still low replace pump.
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Customer: replied 6 years ago.

could it be that the O2 sensors are not working properly. Or that I'm using gas that has ethonal in it. I put gas in it the past few days and i'm not having the problem right now.

I think it could just be a O2 sensor that I bought that is not working. I could not find any leaks in the manifold or exhaust system. I replaced the fuel filter last year.

Expert:  Dan replied 6 years ago.
Its not a E85 vehicle so I'd assume the gas is the problem.....If issue is gone I wouldnt worry about it for now. Let me know if you run into questions, thanks.