I recently bought a 95 Riviera SC. When I purchased it, it had a rough idle and could tell the transmission was starting out of 3rd from a dead stop. At the time, no SES light was on. I got it home and started on some routine maintenance. New; coil packs, plugs, wires, coolant temp sensor, replaced damaged TPS plug and fuel filter. Also bought a salvage yard ECU for $20 swapped the PROM and still no change. About two days ago, the SES light and trac light came on. It idles really rough and is definitely down on power. Now here is the part I've never seen...the PRDN321 indicator flashes 5 times at startup and after it goes solid it indicates under every gear. Also, coolant temp sweeps and sits at 100 no matter what and fuel always reads full. Speedo and tach are functioning properly. I do not own an Actron or TechII so I have not had the codes read yet, hopefully this Wednesday morning I will be able to have them read by a local shop. I can't find a whole lot on what the gear indicator blinking could mean? This car had a motor replacement in 2020 and in the records the ICM was replaced. I've read that there should have been thermal grease applied on the underside of the module and if that wasn't done it could cause bad misfire conditions as well? I have not pulled the ICM yet to check this, but is there any truth to that? Thanks for any and all help!

You need to get the codes. Your tranny is throwing it into limp mode.

What carfixer007 said ^^^^^^^

. . . and check fuses and grounds.

Welcome aboard!

Got my service manuals ordered. The shop was too slammed this morning to run my codes. I’m going to go over any grounds I can find tomorrow. All fuses are good and I cleaned all of them and terminals with contact cleaner. Thanks!

It'* probably worth it for you to purchase the cheapest code reader from Walmart and just keep it in the glove box for occasions such as this.

Unfortunately, there is no cheap reader for OBD 1.5!

Oh yeah. 1995. I remember thinking 1995=pre-OBD2 when I replied the first time, then forgot before the second time.

If you're going to keep the car a while, it would probably be a good idea to get a meter that will read yours. They are a little pricey, but not really when compared to shop bills.