I recently purchased a used Jeep. I know the cat needs replacing. It was running fine but it just started to lose power and have trouble shifting to next gear. The check engine light also started to flash. Could this be a result of the cat or is it a separate more serious problem?

It could be a multitude of things... it could be the result of the clogged cat.

There's a way to pull the codes... run a search. I think you turn the ignition on, off, on, off, on...and leave it on and it'll start giving you codes (or something like that).

Or many auto parts places will scan pull the codes for free if it's drivable, though I'd try to avoid driving it as much as possible until you have the problems resolved.

I know I went to the Jeep dealership the other day and a guy had his Liberty in there and according to the dealer, you're not supposed to drive your Jeep at all if the check engine light is flashing. But I agree, try to find what codes you get and go from there.

A flashing MIL -- means there is a 'class A' misfire is present. It's a severe misfire, dumping raw gas into the converter.

It WILL ruin the catalytic converter(s), if the problem isn't fixed in a timely manner.

You can drive it, but only for a short distance.

If the check engine light is flashing, the ECM is telling you the fault will damage the cat. You need the code(s) pulled and the problems repaired. You might have a misfire. Your cat is probably bad but just replacing the cat will not correct the problem.

So while on the topic...does a flashing check engine light mean only that the cat will be damaged? Or is this based off the assumption that the cat is the cause (and if the cat is the cause, why not just get a dog? har har)?

Or does it basically mean that whatever is wrong could damage the engine? I'd never heard of it flashing before so that's good info to know.

according to the manual if the check engine light is flashing it means that serious damage could occur to the engine, trans, or cat, etc...

I had this happen to me last week. 2001 TJ. The check engine light stays on. (code 455) I have a gas cap ordered to start trying to cure the issue. Anyway, last week I tried to start the jeep and it didn't fire right off. I finally got it going and it was running rough. Dumb me tried to takeoff in 2nd and immediately the light started flashing. It stopped for a minute and started flashing again. After a mile of cutting out I pulled over and turned the ignition off and let it sit for a few minutes. It cranked right up like normal and ran fine after that...hmmmmm

My light flashes sometimes if I step on the gas hard and try to accelerate up a hill. It will flash for 10 seconds and then go away. What am I damaging?

Is your CEL staying on and only flashing for a few seconds or is your CEL off and then just flashes for a few seconds and then goes out agian?

It's off and comes on flashes then goes out after a short time, doesn't stay on.

The flashing is to get your attention. Check the codes and you'll see it has a bad misfire.
A bad misfire puts unburned gas in the cat. If the cat gets too much of it it plugs the tiny holes in it, rendering it useless. It can even cause a fire if there's enough unburned gas in it. It will run hot!

If the cat's just mildly plugged from it, sometimes driving it after the misfire is cured it can clear itself out, but it will get super hot when it's clearing. Try it!

I suppose it could affect an auto trans from all the heat.

The cat is not causing the problem, the misfire is. Plug, cap, wires, vacuum leak, injector, coil rail, etc.

If you try to fix it by replacing the cat without curing the misfire, your new cat will plug and burn too. $$$$

I'm surprised he's complaining of a flashing light and NOT a misfire. It makes me wonder if it's a later model 6 cyl TJ with the coil rail. A coil that's failed in the rail can kill 2 complimentary cylinders. If both are misfiring it won't feel like a misfire - no running rough, it just has low power. Being new to him he might not realize it's low on power. Wonder if that's why the previous owner sold it?

To check to see if a coil has gone south: Use a laser thermometer, go underneath right after it's been running a few minutes. It doesn't have to still be running, just very warm.

Use the thermometer to measure the exhaust manifold as close to the exhaust ports as possible. Write them down as you go.
If a coil has failed, you'll find one cylinder is much colder than the others, and another is either cold or very hot. One will be in the front 3, the other in the rear 3 cyls. Those 2 are the compliments of each other.

Typical temps of a 6 cyl running right are about 300 degrees at each port. But they are all the same.
One with a failed coil will have one a bit lower maybe 200-250, and it's compliment around 700 degrees! That's hot!

Autozone has the coil rails - they are Chinese junk!
CarQust and NAPA carry the same rail the dealer does made by Standard Ignition. Theirs run about $150. Dealer's is close to $250.

BUT CHECK IT FIRST!

Let us know.