More codes.

[Blackstar]

Hey Guys. Having a problem here this evening. 2002 Envoy SLT with about 140,000 m. My other vehicle (Wife's 2012 Acadia SLT) is broke (power steering pump leak) so I was heading out to pick her up from work. Cruising on the highway at about 65-70 mph when suddenly I heard 4 chimes and got a CEL and REP. Shut the Envoy down and restarted about 2 minutes later and the CEL was still on but REP was not lit and truck ran good, for about 2 minutes then same REP.

Limped home and checked codes. P1221, P1271 and P1635. Can these three codes be related? This truck has been running fine for the last few months after I changed out 2 coil packs and a new set of plugs. Any easy ideas I can check?

P1221- fuel pump secondary circuit low
P1271- Accelerator pedal position (APP) sensor 1-2 correlation
P1635- 5 volt reference circuit

Thank you.

Glenn

 

[Mooseman]

Try disconnecting the cooling fan. It has been known to short out the 5v reference.

 

[Blackstar]

While trying to sleep last night I had an idea. When Mooseman mentioned the cooling fan I remembered a while back I had run low of coolant and had some different codes. Checked this morning and it was down almost a gallon. So I have a slow coolant leak (maybe 3-4 months it lost enough to set codes.) But at least right now She's running again. No codes. Thanks MM for the help.

 

[Mooseman]

You're welcome I guess? None of the codes are related to anything cooling except the fan possibly shorting the 5v reference, which is really independent of the coolant itself. You should find that leak or where it's going because if it overheats, it could cause engine damage.

 

[Blackstar]

I have checked for leaks in the past and can't find one. Possibly could be a bad rad cap realising too much pressure. But when I tried to clear the codes yesterday they came right back. After adding coolant I've had it around the block and warmed up twice and still running well and no codes.
I'm hoping head gasket isn't leaking.

 

[mrrsm]

As per @Mooseman ... Here is a Comprehensive Diagnosis and a Good Diagnostic Flow Chart to follow for eliminating or confirming the cause and Origin:

Don't mistake the Purple Plug and Harness that the VOP (Video Original Poster) is plugging in and out of that Connector at the front of the Radiator as being the source of the 5 Volt Reference Short Circuit in the Wiring.... THIS is actually where the problem can arise.

The problem develops after the operational motion from the Engine and Body Motions gyrate around enough to cause the Internal Fan Control Wires to rub repeatedly upon the inside of the protective PVC Tube at its bottom end. It is by this Friction that the Insulation on each Wire becomes worn enough to expose the Copper Wires inside and invite sketchy electrical contact(s) in between them that can interfere with the 5 Volt Reference Signal on the Class 2 Network: