Chevrolet Malibu Forums banner

1 - 3 of 3 Posts

5 Posts
Discussion Starter #1
Threw a P0420 just 7 months after install by local ASE shop (long established). Local Chevy Dealer read the code, and performed all the data gathering required by HottExhaust. Relayed info to them - solid crickets for 6 weeks and counting...

Just had AAA run a battery check and it's good, as it had run down due to WFH for the last 2 weeks. Engine Light code cleared, but when I took it for CA Smog Check, O2 and "Heater" codes were "X". So I drove it around for another 20 minutes. Still "X" codes. Air filter is K&N, if that explains what look to be very good mass-air readings per liter.

First Q: Anyone else had issues with Magnaflow catalyzers going "bad" so soon?
Second Q: O2 sensor hasn't been replaced since at least 2015 - most likely some time in 2013/14 era. Would it be expedient to replace it (and is it an easy job!)?

Any help is appreciated - it's a $250+ job to put the old, braised, manifold and cat back on - and it needs another hairline crack braising, so add another $100 probably... I'm already "out" $550 for the manifold+cat, $400 for fitting - they sheared a manifold bolt - and another $350 brings it in spitting distance of Selman fitting a GM part!

So, thanks in advance, with hopes...

Criteria direct from HottExhaust's email, and Readings transcribed from Selman Chevy's tech, in case they're meaningful to a cognizant:
  • Be certain the engine is at operating temperature and in closed loop.
  • Use ONLY OBD II scan tools in Global or Generic mode – no VIN entry.
Raw, uninterpreted data is required here.

Readings taken at Idle Short Term Fuel Trim

2%__ Bank #1 NA__ Bank #2
Readings Taken at 2500 RPM Short Term Fuel Trim
4%__ Bank #1 NA__ Bank #2

Long Term Fuel Trim-IDLE

BANK 1_____2%__ BANK 2_____NA__________
Long Term Fuel Trim-2500 RPMS
BANK 1_____3%__ BANK 2______NA_________

Bank #1 Bank #2

MAF reading (if equipped) in grams per second at dead idle (no A/C or accessories on)
3-4_____ g/s Idle RPM___700___________
*If long term trim values don’t change between Idle and 2500 RPMs, take the vehicle on a road test with scanner attached to see if the values Change. More data may be necessary to locate the cause of the trouble

When you suspect a failed catalytic converter for ANY reason, follow these steps precisely before replacing the converter:
1) Hook up your OBD II scan tool in Global mode. Do not use vehicle specific (vin entry) mode or OEM scan tools. You want the rawest possible data, not data that has been interpreted by the vehicle’s PCM, and you must retrieve the data as prescribed below.
  1. 2) If any other codes are present address them first before continuing to diagnose for converter failure.
  2. 3) If the vehicle is equipped with a Mass Air Sensor, do a quick check of the values at idle.
(Very Important: the vehicle must be at operating temperature at dead idle. NO ACCESSORIES ON). Proper reading at idle will be very close to 1 gram of air per second (g/s) per liter of engine displacement.
  1. 4) Next, navigate to Long Term and Short Term fuel trim readings. A value of zero is perfect, any deviation from zero indicates the vehicle’s computer is adjusting the injector pulse width to maintain proper air/fuel ratio. Some adjustment is normal, but too much can either quickly cause damage to a catalytic converter, or possibly cause a false catalyst code to be set when in fact the converter may be in perfect working condition. A properly running vehicle should exhibit fuel trims within 5% of zero at most times.
Now, with the vehicle in closed loop and at idle and the transmission in park or neutral, record the Long Term and Short Term fuel trims for each bank (write them down!).
Next, bring the engine RPM to 2,500 and hold it there for about 15 seconds. While maintaining 2,500 RPMs, record the Long Term and Short Term fuel trims.
5) Now, analyze the data. If either the Long Term or Short Term trim is near or above 10% or near or below negative 10%, or if the combined trims on any bank are near these numbers, there is a problem that needs to be corrected before replacing any catalytic converter.
Additionally, if the “spread” between Long Term and Short Term trims on any one bank is near or above 10%, there is a problem. For instance, LTFT B2 = positive 7% and STFT B2 = negative 8%, overall trim would be -1% which is very close to zero, however the issue is LTFT is adding fuel while STFT is taking it away at a large rate. The spread between the two is 15%. This is a problem that needs to be corrected.
If fuel trims are high at idle but get closer to zero at 2,500 RPMs, it is likely there is unmetered air entering the engine (possible vacuum leak).
If fuel trims are close to zero at idle but rise at 2,500 RPMs, it is likely there is a fuel delivery problem (clogged filter, low fuel pressure, restricted injectors, etc...).
Positive fuel trims near or above 10% will likely cause “false” catalyst codes. Repairing the issue will often cure the converter code.

MY Conclusions:
  1. Regarding the percent values:
    1. All are well within the ranges described in the instructions;
    2. There is no significant "spread" as was also called-out - the spread is 0% and 1%, idle vs. 2500 rpm.
  2. Regarding the MAF readings:
    1. The reading of 3-4 is consistent with 2.4 liters of displacement, the range being 1.24 to 1.66, these are close to the 1 g/s/liter mentioned, indicating no issues.

Premium Member
1,663 Posts
I'd put a fresh upstream O2 sensor in first. In my '11 Tahoe I changed front sensors and picked up a average 1.9 mpg fuel mileage and the sensors where reporting no issues or codes. I only changed them as they had 100k miles on them and I'm glad I did. Now they are 80k replacement items on every vehicle in my fleet. Leaner exhaust will make my cats last longer also.
I was loosing 1.9 mpg fuel mileage so those O2 where definitely running the engine richer and working cats harder.
KN will put oil into the MAF no matter what they claim or how much you clean it. If O2 doesn't help I would then change the MAF next.
AC parts only no other brands with any sensor.
Aftermarket cat will not be as robust as OEM as factory has to last at least 80k miles due to Federal warranty on it plus ''high flow'' cat will not have as much catalyst in it as factory. Also CA has tougher standards than Fed. so when cat shopping most Co. only spec. meeting Fed and not CA .
That's how I would proceed if it was my car before putting stock exhaust back in. Sensors and MAF are not that expensive, Rock Auto was my source for parts.
  • Like
Reactions: SAinCA

5 Posts
Discussion Starter #3
Thanks, @repairman54 - that confirms what I subsequently read elsewhere - compounded by low voltage from a battery that just expired. Downstream is original - 170K. Upstream is at least 75K. Both being replaced today, and new battery. Cheers!
I would add that the exhaust helps acceleration and pulling up the 241 Toll Road hills here in the O.C. Pleasantly surprised by how willing it is now. Only replaced the front half, not the cat-back - didn't see the point as it is so improved already - and I don't need the extra noise...
1 - 3 of 3 Posts