40 MPH Check Engine Light

[xbornkilla2k4]

Hey Guys got a question. anyone now what would cause the "service engine soon" light to come on at 45 mph. its a automatic tranny. when i shut the engine off n restart it the light does not come on. i can drive all day, and as long as i keep it under 45 its stays off. it shifts perfectly no problems it will shift through all gears no problem. one thing is that it still has the stock transmission fluid in it since 94 with 78,000 on it do u think this would cause it?

[GMTuners.net]

first thing you should always do with problems of this nature is go to a pepboys or autozone and have them scan to see if you are throwing any codes. i believe a scan is free, and that would atleast help narrow down any and all problems.

[achievasc92]

Autozone probably won't be able to scan it since it's a 94, and uses a funky hybrid OBD system. I would guess it's either the EGR valve stuck closed or O2 sensor gone bad.

[Matt95GT]

You should have it scanned. Call your local AutoZone(s) and ask if they have a scanner that supports GM OBD 1.5.

My guess... TCC lockup solenoid/switch. IIRC TCC lockup is supposed to happen near 45 MPH.

[Danmuzicman]

i agree potentionaly the TCC solenoid an egr would do it at idle