What could be preventing overdrive from engaging? Overdrive works most of the time, but sometimes it just won't engage. I just replaced the tranny, so that is not the cause. The car is a 1995 Bonneville, tranny model 4t60e. I have another issue that may be involved. When I don't give it gas for more than a minute, the check engine light comes on. It could be at a stoplight or even going down a mountain. When it comes on at a stop the car'* idle drops and the whole car bucks forwards and backwards. I also noticed that whan I apply the brake at a stop, the rpms drop. This causes the check engine light to turn on that much quicker. It seems like a vacuum leak. I also replaced the manifold gaskets...upper and lower. Went to autozone to have the codes checked but the reader wouldn't work...error code!! Friend had a reader...error code...wouldn't work either. What is going on!!!!!!! I've been trying to fix this car for over a year now. Any suggestions?

You need to have a shop pull those codes. AutoZone won't be able to because you have an OBD I/II hybrid. It'* an OBD II connector, with the OBD I protocals. 94 and 95 model year Bonnevilles were that way.

The codes will tell us a lot.

Did you have the tranmission rebuilt, rebuild it yourself, or swap it in from another car?

Some Advance Auto'* can pull the 1.5 OBD codes. They use a version of the Actron 9150

Went to the shop that installed the tranny...mechanic pulled these codes
131...O2 sensor circuit, low voltage(bank1, sensor1)
171...system too lean/ fuel trim lean-low (bank 1, sensor 1)
300...random/multiple cylinder misfire detected
740...torque converter clutch, CIRCUIT MALFUNCTION

I had replaced the o2 sensor around the engine not too long ago...i understand there'* another in the exhaust system. Wonder how big a job that is...how much it would cost at a shop? How does that second o2 sensor affect the cars performance if its bad? I also installed a new fuel filter this weekend...one of the few things I can actually do. I blew through the old one ...clogged pretty bad. The car actually died on me this weekend when my gas tank was low. It had to sit for a few minutes before I could travel a mile...died again...repeated this till I got to a gas station. Was ok afterwards, but I realized my mechanic never replaced that filter...EVEN AFTER HAVING TO INSTALL A NEW FUEL PUMP 18 MONTHS AGO. I wonder if that was an HONEST mistake? I'm glad I was able to realize this issue before all that pressure blew out the fuel pump a second time.

Forget the rear O2. It is for emissions, and will have no performance effect.

What brand of O2? What brand of fuel pump? How old is your fuel filter? What'* your fuel pressure?

Don't know what brand my ex-mechanic used. He bought from NAPA, that'* all I know. The fuel filter I replaced myself this weekend had to have at least 40k on it, at least!! I had an agreement with the ex-mechanic to do all the routine maintenence because i'm so mechanically uninclined and he freaking fails to do something so basic and obvious for sooo long! I'm now looking for a new mechanic in Richmond, VA.

Would a scanner tell me the fuel pressure?
I was going to buy one. ACTRON OR AUTO-XRAY?

jr's3800...I hope the old one was bad. I blew a lot of money and there'* no going back now. FYI, the old tranny had issues...wouldn't engage the next gear till a high rpm, 3rd wouldn't engage at all sometimes, overdrive woudn't engage unless I dropped it into gear from 3rd with my foot off the gas at a high rpm. I had the fluid/filter replaced before I decided to replace it. The fluid was nasty. Only a month later, the fluid was turning brown again. Sound like a bad tranny. I at least hope so...what do you think?

What should I do about that last code...740..tcc circuit malfunction...? What does this mean to my wallet?

A scanner will not tell you the fuel pressure.

You need a mechanical fuel pressure gauge to attach to the schrader fitting on your fuel rail. It must work with GM fuel injected vehicles. Actron has one. Most Autozones or NAPA should have one for 20-30 something.

well, I took it to a new shop...they found that the metal tube? going to the EGR had a crack in it and was causing a vacuum issue. I wonder if the 740 code...the whole problem with overdrive could be caused by this crack. It was a dealer part. What do you guys think?

No. A vacuum leak won't cause an electrical circuit malfunction. It'* been there for two transmissions now, so I'd bet you have a wiring problem between the transmission and PCM.