Clogged fuel injector

Looking for suggestions to resolve a dirty fuel injector;
I did a search on this and have learned that a cleaning is in order, but wondering on fuel treatment or individual injector cleaning.
My 2000 gtp , w/139,000 miles has been experiencing increasingly poor idle and acceleration problems yielding a flashing "service engine" light. I took it to Auto Zone for a scan that revealed the following codes; p0203,p0303,p0420,p0446. The Auto Zone guy recommended Lucas injector cleaner for the #3 injector as indicated by p0203. I used the cleaner, and the car ran good for a couple days, but now has gotten worse than before.
So, should I try a different cleaner, like the chevron fuel additive, or clean the #3 injector.
On cleaning the injector, can I remove the fuel rail and clean just #3, or do I have to remove all of the injectors?
I read the link about the professional type cleaning method and am not sure I can put all the hardware together for that.
Looking for the easiest and least expensive solution here.
By the way,fuel filter, plugs, wires, have been replaced as part of normal maintenance. I know #3 plug is misfiring per p0303, but figured that would resolve with the injector problem. I'm assuming the other codes would fall in line with correcting the injector issue?
Thanks in advance for any help you guys may provide,
Dennis

p0203- injector circuit malfunction, cyl 3
p0303- misfire detected, cyl 3
p0420- catalyst efficiency below threshold
p0446- evap vent control circuit malfunction.

I don't believe any of these codes are being caused by a clogged injector. I would inspect the upper engine wiring harness. It sounds as though you have a corroded or broken wire/connection causing the p0203 and p0446, as these are both circuit malfunction codes. The p0303 is most likely caused by the injector not getting a proper signal and not firing correctly. The p0420 catalyst efficiency code could be a result of the messed up air/fuel ratios being created by the afformentioned problems.

Thanks for the suggestion Rich. I'll inspect the wiring, as you reccomend.

still having problems with this.
I checked the wiring & didn't see anything obvious. I also checked the voltage with the engine running at the #3 & #5 injectors. I picked #5 because it of it's clear accessability. Both measured about 8 volts. I've never had a code on the #5 injector so I'm guessing the signal received by #3 is adeqate, since it's the same. I picked up a bottle of the Chevron/techron cleaner with the hope that a clog can still be cleared, if that is indeed the issue. Any other ideas? If it doesn't clear up in the next few days, i guess i'll take it in. I drive about 40 miles/day on the freeway. How many miles should I give this stuff to work?
Thanks

I'd be looking at a possibility of a clogged CAT convertor instead of a clogged fuel injector. The P0420 DTC means your CAT convertor has less than desirable oyxgen storage capability and is failing efficiency. A clogged or damaged CAT may cause misfires DTCs to set.

What brand wires and plugs were installed? Also, any mods?

You can get the CAT convertor and exhaust system tested for backpressure and efficiency.

thanks for the reply.
no mods. Plugs & wires were a/c delco replacement@ 100k miles.
I hadn't considered the CAT convertor.
How or Where can I test the exhuast system checked?
Thanks

Plugs and wires should be OK. What AC Delco plugs did you use? Do you have the number off them?

The other thing I noticed is your mileage at 139K miles. Normally CATs are covered under GM's emissions warranty but you're way out of it. You can get the CAT convertor tested at any muffler shop and at the GM dealer.

Also, you can run a DIY test using an automotive vacuum gauge (0-30 hg). The test is run at idle and 2000 rpms. Depending on how bad the CAT is sometimes it will show up on this specific test.

i don't know the number for the plugs off-hand, i'd have to check. I know they were the iridium ones, though.
Would you know the acceptable hg reading @ idle & @ 2,000rpm, for the CAT test?
Thanks for your help

If AC Delco should be 41-101 which are made by NGK.

A typical SII 3800 SC or NA will show around 18 - 22 in hg on the gauge. Nothing ever less than 15. You run the test at idle to verify vacuum is in specs first plus to give you a reference point. Then you run it up to 2000 rpms. The gauge will be erratic but will stabilize as long as the rpms are steady at 2000. The reading should be around 18 - 22 in hg or nothing less than 15. If there is a problem with the CAT mainly a clog or high exhaust backpressure, the reading will start dropping towards 15 in hg or less. Any drop off is abnormal once engine vacuum is regained and steady.

On a backpressure test done by a shop, they hook up a fixture to the front 02 sensor bung and that reading should be 1.25 psi or less.

thank-you. if the problem is the CAT convertor, is there a high flow (low back pressure) convertor I should consider?

Some of the things you are experiencing, I did too on my 2000 GP 3.1. I did the complete tune-up and had the cat replaced and O2 before the cat replaced and still had the problem. I had the injectors cleaned professionally and the car ran fine for a short bit. I got the same missing on #1 injector. I finally broke down and bought 6 remaned injecotrs. The miss is gone and the car runs great, alot of pickup and easy idle. My GP has 99k and the injectors were just worn out. Just nickles worth of input here. Good Luck.

Thanks.
All input is appreciated

thank-you. if the problem is the CAT convertor, is there a high flow (low back pressure) convertor I should consider?

The Magnaflow brand hi-flows work well. $55.00 including shipping. Plus they have different inlet/outlet sizes if you have a custom exhaust or plan on going to one.

Only 8 volts to the fuel injectors? I thought it should be 12. I'll do a little research & post back. Is your meter showing 12 or more at the battery?

You could also check the resistance of the fuel injectors.

Should be 12 volts - how'd you check this?

Battery showed 11 volts.
On checking the injector; I removed the injector connector, started the engine, and touched a probe on each one of the two terminals within the connector cavity. Reading was 8 volts.
Where can I get the Magnaflow for $55.00?
Thanks

11 volts. Thats a little low. Typically you should get around 12 - 12.5. Is your multiimeter calibrated properly? Also, check the voltage at the battery or the alternator output while engine is running.

The 12 volts on one side of the fuel injecor coil (pink wire) is there when ignition is at run/start. Best to check it while engine is off not running.

just got the car back from a local shop. The #3 injector was replaced, and is running fine now. The old injector was showing 14.3 ohms resistance, according to the tech, excessive. Thanks for all your suggestions,
Dennis