Help with CEL please.

[Terry]

I have a 2005 Chevy Cobalt LS 2.2L. I’ve been getting a P0420 code causing my CEL to come on. I’ve been reading about catalytic converters and O2 sensors. If I let my car idle in closed loop, the upstream O2 sensor will fluctuate rapidly between 0.1V and 0.9V, and from what I’ve been reading, this is normal. On the other hand, the downstream O2 sensor pretty much hangs around 0.75V with an occasional drop to 0.1V.

Some additional information which may or may not be relevant:

Every once in a while, my car will also throw these 2 codes:
P0106: Manifold Absolute Pressure (MAP) Sensor Performance – PCM
P01681: Control Module Throttle Position (TP) System Performance – PCM (causes limp mode)

So I can’t tell if this is for sure a genuine P0420 code. I have noticed that the MPG of my car has gone down since I’ve started to get this code. I believe it’s running rich.

Some freeze frame data:

[table]

Read Codes P0420 $07E8 Generic Catalyst System Efficiency Below Threshold Bank 1 View Freeze Frame DTCFRZF() P0420 FUELSYS1() OL FUELSYS2() — LOAD_PCT(%) 14.9 ECT(¡£C) 92 SHRTFT1(%) 0.0 LONGFT1(%) 8.6 MAP(kPa) 37.0 RPM(rpm) 787 VSS(km/h) 0 SPARKADV(¡£) -1.0 IAT(¡£C) 11 MAF(g/s) 3.77 TP(%) 24.7 RUNTM(sec) 1331 EVAP_PCT(%) 0.0 FLI(%) 56.9 BARO(kPa) 103 VPWR(V) 13.025 LOAD_ABS(%) 22.0 EQ_RAT () 0.916 TP_R(%) 11.8 AAT(¡£C) 4 TP_B(%) 24.3 APP_D(%) 17.3 APP_E(%) 9.0 TAC_PCT(%) 15.7 Modules Present ID: Protocol $07EA: CAN STD FAST $07E8: CAN STD FASTS

Read Codes P0420 $07E8 Generic Catalyst System Efficiency Below Threshold Bank 1 View Freeze Frame DTCFRZF() P0420 FUELSYS1() OL FUELSYS2() — LOAD_PCT(%) 14.9 ECT(¡£C) 91 SHRTFT1(%) 0.0 LONGFT1(%) 7.8 MAP(kPa) 40.0 RPM(rpm) 703 VSS(km/h) 0 SPARKADV(¡£) -4.0 IAT(¡£C) 0 MAF(g/s) 3.70 TP(%) 24.7 RUNTM(sec) 1393 EVAP_PCT(%) 0.0 FLI(%) 52.9 BARO(kPa) 101 VPWR(V) 14.599 LOAD_ABS(%) 23.5 EQ_RAT () 0.916 TP_R(%) 11.4 AAT(¡£C) -1 TP_B(%) 23.9 APP_D(%) 17.3 APP_E(%) 9.0 TAC_PCT(%) 15.3 Modules Present ID: Protocol $07EA: CAN STD FAST $07E8: CAN STD FAST

Read Codes P0420 $07E8 Generic Catalyst System Efficiency Below Threshold Bank 1 View Freeze Frame DTCFRZF() P0420 FUELSYS1() OL FUELSYS2() — LOAD_PCT(%) 13.3 ECT(¡£C) 90 SHRTFT1(%) 0.0 LONGFT1(%) 7.0 MAP(kPa) 37.0 RPM(rpm) 700 VSS(km/h) 0 SPARKADV(¡£) -3.5 IAT(¡£C) 4 MAF(g/s) 3.29 TP(%) 23.5 RUNTM(sec) 986 EVAP_PCT(%) 0.0 FLI(%) 71.4 BARO(kPa) 99 VPWR(V) 15.183 LOAD_ABS(%) 21.2 EQ_RAT () 0.916 TP_R(%) 10.6 AAT(¡£C) -3 TP_B(%) 22.7 APP_D(%) 17.3 APP_E(%) 9.4 TAC_PCT(%) 13.7 Modules Present ID: Protocol $07EA: CAN STD FAST $07E8: CAN STD FAST

Topic

[valde]

That P0420 wouldn’t be my main concern since all codes are from cold engine at idle. Maybe you should check throttle position sensor since it (normally) can be checked with multi meter, it has caused limp home mode, and it may cause MAP-code.

[skasten]

Hey there,

Your p0420 code could very well be the converter. These new gm’s seem pretty good when it comes to setting a trouble code fault for the o2 sensors, but with that being said it still is possible for being a downstream o2 fault. Good idea would be to get the vehicle nice and hot and if possible graph the upstream and downstream sensors and see how the down stream “stabilizes.”
As with your p1681 Throttle position and your p0106 MAP code there is a good chance that the throttle body needs replacement. These units are “fly by wire” and have known issues with them. Good place to start with this it to take off the intake tube and visually inspect the throttle plate for build up. I always try to clean these units before replacement because they are kind of spendy. Also there were a few T.S.B. and recall issues for the PCM’s so defiantly research that as well.

Good luck!

[Terry]

I am fairly certain something is wrong with my throttle body. Here are 3 pictures looking at live data:

TP and TP_B are the 2 sensors inside the throttle body(TP_R is relative). Notice how in image 2, TP and TP_B are in bigger disagreement than in the 1st and 3rd images. Also, notice how TP_B drops going from image 2 to image 3, even though more throttle has been applied. Perhaps this is close to the “bad spot” for the potentiometers. Although, I’m not quite sure how potentiometers work, so I could be wrong lol. The APP sensors are also known for going bad in these cars, but I’m leaning more towards the throttle body considering I have also gotten the MAP range/performance code. Does this sound like I’m on the right track?

[Terry]

Well I’m back with an update. Previously I did not have my own scan tool when the car would go into limp mode. 2 days ago, I was able to trigger limp mode after 2 months. There is in fact 2 codes occurring when the car goes into limp mode:

P1681 – Control Module Throttle Position (TP) System Performance
P2138 – Acceleration Pedal Position (APP) Sensor 1-2 Correlation

So basically when the car goes into limp mode, one code is for the TPS, and the other is for the APP. I have no idea if one is causing the other, and I don’t know which one would be the best to start with. Here is some information I dug up on both of these codes:

Sensed Parameter	Fault Code	Monitor Strategy Description	Malfunction Criteria and Threshold Values	Secondary Parameters and Enable Conditions	Time Length and Frequency	MIL Illum. Type
Control Module Throttle Position (TP) System Performance	P1681	1. Verify the PCM´s ability to detect a short between the TPS 1 & 2 circuits 2. Verify that the throttle control system position sensor short diagnostic is functioning.	1. TPS #2 Signal voltage > 2.05 V 2. No detection of the sensor short diagnostic active state	Ignition voltage > 5.23 V No PCM processor DTC. Ignition in unlock/accessory or run, not during TPS minimum learn active during intrusive portion of diagnostic execution	1. 2 cts, 156.25 ms w/immediate retest on an error, performed in the main μP 2. No sensor short diagnostic activity for 498 ms. Detected by the MCP	DTC Type A
.
Acceleration Pedal Position (APP) Sensor 1-2 Correlation	P2138	1. Detect an invalid minimum mechanical position correlation between APP sensor #1 and #2 2. Detect a short between APP sensors #1 and #2 circuits.	1. Difference between (5Vraw learned min. APPS#2 voltage)*2 and (raw learned min. APPS#1 voltage) > 0.25 V at min throttle position to 0.5 V at max throttle position 2. Difference between APP#1 and APP#2 < 1 V	1. Ignition in unlock/accessory, run or crank. Ignition voltage > 5.23 V No PCM processor DTCs No APP Sensor Circuit DTCs No 5VR DTCs 2. Ignition in unlock/ accessory, run or crank. Ignition voltage >5.23 V No PCM processor DTCs.	1. 15/35 Cts or 12 Cnts Continuous, 12.5 ms/Ct in the Main μP 92/217 Cts or 80 Cnts Continuous, 2 ms/Ct in the MCP 2. 2 Cts 156.25 ms w/ immediate test on an error, performed in the main μP.	DTC Type A

Here is a freeze frame:

DTCFRZF() P1681
FUELSYS1() CL
FUELSYS2() —
LOAD_PCT(%) 45.1
ECT(¡£C) 87
SHRTFT1(%) 3.1
LONGFT1(%) 9.4
MAP(kPa) 72.0
RPM(rpm) 2337
VSS(km/h) 109
SPARKADV(¡£) 23.5
IAT(¡£C) 19
MAF(g/s) 21.10
TP(%) 31.8
TP_B(%) 31.4
RUNTM(sec) 596
EVAP_PCT(%) 100.0
FLI(%) 57.3
BARO(kPa) 98
VPWR(V) 13.000
LOAD_ABS(%) 41.2
EQ_RAT () 0.999
TP_R(%) 18.8
AAT(¡£C) 19
APP_D(%) 42.4
APP_E(%) 19.6
TAC_PCT(%) 23.9

If any of you guys know where the best place to start would be, I’d appreciate your input very much.

[John Hugon]

The way I always checked the Acceleration Pedal Position (APP) sensor/ Throttle body-Throttle Position (TP) sensor circuit was with the scan tool connected; engine off, key on, open the APP sensor slowly and look for any drop outs with the TP sensor. If so with the codes you have, after checking the (wiring) circuit I would condemn the TP sensor. But looking at your freeze frame readings, (open and closed loop) you have two TP sensors in the APP sensor that don’t agree with each other, along with that, one APP sensor that agrees with engine load %…so after you check the (wiring) circuit from the APP sensor to the ECM and if ok I would condemn the APP sensor.

Hope this helps…..Great site, good Technicians on here!

[Terry]

Thanks JTF. The 2 TP sensors (TP and TP_B) are showing agreement, whereas the 2 APP sensors are showing inconsistency, so I’m inclined to believe the APP sensor is the problem.

Another reason I’m inclined to believe the APP sensor is at fault is because the P1681 is a performance related code, whereas the P2138 is an actual voltage correlation code. I think I’ll just say to hell with it and order an APP sensor.

[John Hugon]

After being a GM tech for longer than I want to think of, their terminal connections sometimes have issues….If it was me, I would verify your circuit wiring and connections and check the APP for resistance of OHMS before ordering parts. Also I did ECM/PCM reflashes for that issue. I wish I could help more, but I’m retired now with no access to the service manual and TSB’s.

If you can let me know the fix.

[Terry]

[quote=”JTF” post=93410]After being a GM tech for longer than I want to think of, their terminal connections sometimes have issues….If it was me, I would verify your circuit wiring and connections and check the APP for resistance of OHMS before ordering parts. Also I did ECM/PCM reflashes for that issue. I wish I could help more, but I’m retired now with no access to the service manual and TSB’s.

If you can let me know the fix.[/quote]

Well, I went with my gut and installed a new APP.

I posted 3 pictures earlier in this thread showing the various readings of my TP and APP sensors.
Here’s a before chart of my APP #1 and APP #2(as shown in those pictures):

APP #1	APP #2
17.3 (undepressed)	9.4 (undepressed)
28.6	13.7
32.9	15.3

After installing the new APP, I wanted to make some comparisons. Since APP sensor #2 is the lower of the 2, I decided to take my hand and push down trying to match the APP #2 on the new APP to the APP #2 on the old APP. Here is the new chart:

APP #1	APP #2
20.4 (undepressed)	9.4 (undepressed)
28.2	13.7
31.4	15.3

As you can see, even without me pressing on the pedal, there is a difference. Note: With regard to the APP sensors, I wasn’t too concerned with the percentage values being the same because APP #2 is in fact a backup sensor. On the other hand, the 2 TP sensors should read roughly the same with regard to percentage.

Now lets consider that for a moment: If you look at the second picture, look at how huge of a difference there is in TPS #1 and TPS #2. TPS #1 is reading 31.0% and TPS #2 is reading 39.8%. So I decided to compare TPS values just as I did with the APP sensors. I took my hand and pressed down on the pedal until TPS #1 was at exactly 31.0%. TPS #2 is now reading roughly 31%.

Now, I’m not really a mechanic, but I got to searching the internet trying to figure out why this occurred. I’m pretty sure, at least in the case of my car, TPS #1 and APP #1 share the same 5V reference circuit, and TPS #2 and APP #2 share the same 5V reference circuit.

This is why I went with the APP first. I’m 99% sure there was a short between the 2 APP sensors inside the APP. But like I said, I’m not very good with working on engines. Who knows, maybe this will get rid of my P0420 code, lol. I’ll post back with an update.

[John Hugon]

ChevyTerry,

Sorry I didn’t see the reply until now.

You’re getting above this old man’s head a little… I looked at a training manual…each sensor has separate signal, ground, and 5.0 volt reference circuits.APP Sensor 1’s signal increases as the accelerator pedal is depressed, from below 1.1 volt at 0% pedal travel (pedal off floor pan) to above 2.1 volts at 100% pedal travel (pedal fully depressed).APP Sensor 2’s signal decreases from above 3.9 volts at 0% pedal travel to below 2.9 volts at 100% pedal travel. APP Sensor’s 3 signal decreases from above 3.2 volts at 0% pedal travel to below 3.1 volts at 100% pedal travel.

To try to stay ahead of you…which I think is going to be hard…I’ll try to find some more of my training books to understand more about the electronic function of the system…or the internet.

The main thing I looked at after being a flat rate Tech for many long years for fast diagnosis was drop outs with the TP sensor while pressing the APP, if that passed if one APP sensor that agreed with engine load and one that did not I condemned it. I know one sensor is a back up for safety and I believe for government rules…so if one is out of specs…SES is on.

Did you do a resistance check between the two APP sensors?… Thanks for the update and going through the data…very…very interesting.

Oh and I don’t think the P0420 is going away… Please post more updates..

[Author]

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