OBD1 codes 25 and 26 show up causing the check engine light to turn on?

TIM HODGES

MEMBER
1993 TOYOTA PREVIA
2.4L
4 CYL
2WD
AUTOMATIC
140,000 MILES

Van runs fine, starts right up. It does have an exhaust leak maybe at the manifold. Can hear it but it is not sounding like a hole in the exhaust system. OBD 1 Codes 25 and 26 say too rich mixture and too lean mixture. Seems to me it should be one or the other. Will take it to a smog shop but if you have a recommendation of what I can do before hand I might be able to save on mechanics repair time. I don't smell unburned gas smell when it starts and runs. Might be not getting good gas milage. I guess the manifold leak is causing both codes due to the leak the sensor is feeding more fuel and the o2 sensor is sensing too rich mixture and the cat will be damaged if the exhaust manifold leak is not fixed.

3 Replies

JACOBANDNICKOLAS

MECHANIC
108,355 POSTS

Hi,

You are likely correct with your thoughts. If there is an exhaust leak pre-catalytic converter, it can draw air into the exhaust and cause the oxygen sensor to indicate a lean mixture. As a result, the PCM will add fuel.

I attached the diagnostics for both codes below. However, I feel if you would repair the exhaust leak, it will likely resolve the issue. Also, an engine vacuum leak can be the cause as well or the sub-oxygen sensor located right after the catalytic converter. See pic 4.

Let me know.

Joe

See pics below.

Images (Click to make bigger)