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Help Interpreting OBD1 Codes


New Member
May 13, 2007
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Year, Model & Trim Level
1994 4 door
I thought I would run this by you guys before I started buying parts. I am having a problem with my ’94 Explorer. The check engine light comes on and engine bogs down intermittently after driving several hundred miles. This occurred again recently when I drove it from NC to MI. I was able to pull the OBD 1 codes and this is what I got:

Key on Engine Off (KOEO) test:
Current codes – 335 EGR sensor circuit voltage higher or lower than expected.

Stored Codes (cont. memory) – 172 Heated oxygen sensor fault / lean bank 1 / right side
173 Heated oxygen sensor fault / rich bank 1 / right side

Key on Engine Running (KOER) test:
336 EGR circuit higher than expected / exhaust pressure high

All other test normal

I am guessing that the exhaust pressure high (and maybe the other codes) are due to a plugged catalytic converter. What do you think? Should I change or punch out the cat? Could the O2 sensors be related to the back pressure? If you get a chance let me know. If the O2 sensors are telling the EEC that I am rich or lean that would explain the bog down. Thanks for any recommendations.

John D. Thompson


I Drive By Brail
Elite Explorer
Moderator Emeritus
February 18, 1999
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City, State
Montrose, CO
Year, Model & Trim Level
'95 Bronco '80 Toy
Your EGR valve is most likely sticking open. This will cause a vacuum leak (make the engine bog, create lean O2 codes, etc...)


Explorer Addict
December 27, 2001
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City, State
Spanish Fork, UT
Year, Model & Trim Level
92 XLT and '87 Bronco II
Except that KOEO codes almost always have to be electrical rather than mechanical in nature. When the engine isn't running, there's no engine vacuum to operate the valve, and no exhaust backpressure to create flow through the valve. A stuck open valve wouldn't show up in the KOEO test.

I would first go through the PFE/DPFE sensor circuit before I looked at the EGR valve itself.