Code 172 just won't go away...arrrggghh


Aug 23, 2005
I put in new Hpipe (removed cats) and exhaust and got a CEL. Ran codes and among other codes was 172 - too lean bank 1.

CEL is intermittent. Comes on here and there for about 10 seconds, then goes away.

Figured time for a tuneup anyway so I replaced: Plugs, cap, rotor, coil, ECT, MAF, and both O2 sensors. Did not replace wires yet, going to order online.

I've cleared the codes each time I replace something. Remove neg battery cable for 20 mins or so.

All the codes went away except 172. I also smell gas at the right tailpipe. Only thing left is maybe an exhaust leak at the manifold or the wires? Also read about fuel injectors.

What about MIL eliminators?

Any thoughts?
  • Sponsors(?)


SN Certified Technician
Jul 19, 2007
Vernon BC
MILs aren't required as the O2 sensors are before the cats (or no cats in your case). When the CEL comes on and then off that means whatever triggered the CEL (only emission related) has corrected itself and has fallen back into spec. I would look really hard to find an exhaust leak on bank 1.Is the H-Pipe a O/R pipe without cats? When you say you removed the cats it sounds like they were there and you physically removed them.


Aug 23, 2005
I installed a BBK offroad Hpipe after removing the factory cat mid pipe. I will go back under there today loosen and reset the manifold to H pipe flange and see if that helps. Thanks.


Founding Member
Jul 27, 2002
it definitely could be the wires since you have not replaced them yet. I know its frustrating to keep throwing money at it. Maybe swap the all the wires from bank 2 to bank 1 and see if you there is any difference?? Hopefully you'll get a code for the other bank and then you'll know if its the wires.


I have face herpes.
15 Year Member
Jun 14, 2004
Acworth, GA
That's usually an exhaust leak right by that O2 sensor. It will suck in air, and give it an incorrect reading. Does the CEL come on as soon as you start the car? You might want to take the H-pipe back off and clean the ball and socket joint to ensure a good seal. Can you hear an exhaust leak?



Aug 23, 2005
I don't hear an exhaust leak as best I can tell. And I don't see any black soot of any kind at the manifold flange....although I certainly have black on the inside of my tailpipes and they are only 30 days old.

The CEL does not come on for a while. I may drive 20 mins and after the car has heated up, it will come on for maybe 10-20 secs and then go off. I have a 40 min commute to work. It will go off and reappear every 10 mins or so for another 10-20secs and go away. The only code I get is 172

My car seems to run it best in that first 20 mins, then when the CEL light begins it's intermittent flash, my car seems to run not as good.

I have replaced everything except the wires. I read somewhere that a guy finally beat code 172 with new wires, which I have not ordered yet.

Also, I can remove and reset the hpipe to verify the seal, it is just a PITA to have to do all over again..:mad:

I also think I may have an air leak in the MAF housing. A while ago I dropped the housing on the ground and it cracked. I sealed it with duct tape and it has been fine for years. I recently removed the tape and resealed it. Could this be my too much air problem?

I skipped putting on new headers at this time, which may help the seal. I am worried about snapping off bolts in the heads. Is it common to snap the bolts and if I do snap one off, wtf do I do then?


SN Certified Technician
Jul 19, 2007
Vernon BC
Any air drawn in after the MAF is a problem as the computer is relying on the MAF to determine the fuel load requirements. That's why vacuum leaks cause problems. An exhaust leak at the flange will often draw air in and cause the O2's to read a lean condition. This in turn prompts the PCM to enrich the fuel load. Bad spark plug wires are more apt to give a rich condition if there is a poor spark and combustion.


I felt sorry for girls because
Founding Member
Jan 15, 2002
Dallas, GA
Visit site
The problem started as soon as you put on the O/R H pipe so it's probably related. I'm betting on that it isn't sealing correctly.
Thread starter Similar threads Forum Replies Date
rio95 Need some help - codes 121 and 172 1994 - 1995 Specific Tech 6
9 Help with code 172 1994 - 1995 Specific Tech 8
9 Lean Code -172 Fox 5.0 Mustang Tech 2
9 Lean Code -172 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 0
D Help With Engine Codes Car Code 172 HEGO (HO2S) sensor fault/lean 1994 - 1995 Specific Tech 17
94' Five.0 Code 172 and 173?? 1994 - 1995 Specific Tech 3
U codes 172 and 176. help? 1994 - 1995 Specific Tech 3
R Getting code 172 1994 - 1995 Specific Tech 12
9 code 172 1994 - 1995 Specific Tech 5
H Codes 113 157 172 and 173 on 94 GT 1994 - 1995 Specific Tech 0
M codes 172 and 176 1994 - 1995 Specific Tech 3
E code p0234 "A" overboost condition 2015+ Specific Tech 2
M P1233 CODE SN95 4.6L Mustang Tech 2
D On Board Computer several Codes 2005 - 2009 Specific Tech 1
B Engine Codes 81, 82, 85 and 41 - Where to start. 5.0HO/AOD transplanted into '67 Fox 5.0 Mustang Tech 14
B Code 95 in Memory - 5.OHO transplant into '67 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 2
DaveRog Black Leather Interior Trim Code 1994 - 1995 Specific Tech 0
C Electrical Codes with and without chip Fox 5.0 Mustang Tech 0
T Fuel pump code issues 1994 - 1995 Specific Tech 5
7 Need 1988-90 lx body parts passenger door and maybe passenger fender ? My paint code is 7n thanks The Welcome Wagon 12
79pace 2000 GT Strong exhaust smell possibly really rich but no codes 1996 - 2004 SN95 Mustang -General/Talk- 4
J Fuel Code 452. Fuel pump not working. Help 1994 - 1995 Specific Tech 5
J p0453 code 1996 - 2004 SN95 Mustang -General/Talk- 1
splinterddt Multiple Misfire Codes 2005 - 2014 S-197 Mustang -General/Talk- 56
C Does a 2015 Mustang have a radio code? 2015+ Specific Tech 0
C Electrical 90 LX 5.0 threw these codes... Any ideas? Fox 5.0 Mustang Tech 3
B Engine code p0118 SN95 4.6L Mustang Tech 4
M 1967 s-code fan problem 1965 - 1973 Classic Mustangs -General/Talk- 2
F new member 1965 Mustang Fastback GT A code 4 spd The Welcome Wagon 4
M codes on 01 mustange 1996 - 2004 SN95 Mustang -General/Talk- 1
S Engine Code 63 and more shuddering Fox 5.0 Mustang Tech 10
S Engine Bad Shudder and Bad Codes Fox 5.0 Mustang Tech 2
S Engine Need help with fox body code results. Fox 5.0 Mustang Tech 0
T 64.5 D code auto to 4 spd manual trans swap advice/info needed Classic Mustang Specific Tech 4
1 Engine Check engine light with no stored codes Fox 5.0 Mustang Tech 5
R 06 mustang reading p0353 code 2005 - 2014 S-197 Mustang -General/Talk- 3
K Engine Tps code 63 troubles Fox 5.0 Mustang Tech 10
titanium1990 YJ paint code please help 1979 - 1995 (Fox, SN95.0, & 2.3L) -General/Talk- 34
P Fuel code P0460 won’t pass smog SN95 4.6L Mustang Tech 11
M Please help with p1450 code 2015+ Specific Tech 0
4 Digital Tuning Backup Tune for 2012 Mustang GT, Computer Code ZFK0 2010 - 2014 Specific Tech 0
Eilermoon P0403 Error Code 1996 - 2004 SN95 Mustang -General/Talk- 2
Xenogenesis Engine Idles rough, misfires at idle, smells rich, but codes say it's lean on both banks SN95 V6 Mustang Tech 9
B 4.6L2V random misfire with no codes SN95 4.6L Mustang Tech 6
P Help new codes 4.6 swap SN95 4.6L Mustang Tech 4
P Help!! Open Injector Circuit Codes SN95 4.6L Mustang Tech 21
k19driver Engine ACT Code 54 "Sensor circuit open" Fox 5.0 Mustang Tech 7
F Drivetrain AOD to T5: random codes/issues Fox 5.0 Mustang Tech 12
C Airbag code 63 will not clear need help 1994 - 1995 Specific Tech 2
D Engine Codes P0155,P0135 SN95 4.6L Mustang Tech 5
Similar threads