Trane XV90 shutting down after igniting

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Old 10-25-11, 05:56 PM
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Trane XV90 shutting down after igniting

I have a two year old Trane XV90 furnace that cycles properly right through to burner ignition, but then shuts down 5-10 seconds later. The red error code for past 4 faults is just two flashes (retries exceeded), so no help there. I suspected flame sensor and took it out and cleaned both sensor tip and contact wire. While testing results with chamber access cover off, furnace worked perfectly. I put chamber access cover on and cycle, it's back to shutting down. It's repeatable, chamber access cover off works perfectly, with cover on it shuts down 5-10 seconds after ignition. Vent pipes look clean, but they've got three 90 degree turns so I can't check end to end. I'm stumped, I'm not sure how open access cover would impact anything but venting. Would combustion venting issue cause shut down without a 3 flash venting code? Or does venting somehow impact performance of flame sensor switch?
 
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Old 10-25-11, 06:13 PM
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Have you gone outside and feel the intake to see if you have air flow?

the intake pipe on top of the furnace should lift up out of the hole, and I'd take a blower end of the shop vac and blow into the pipe and see if anything blows out outside.

How long of a run do you have on the PVC, and what size is it? 3 or 2" and what the furnace BTU rating?
 
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Old 10-26-11, 10:27 AM
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Unfortunately the intake pipe doesn't just "come out", it looks like it was attached with a plastic collar and the collar is screwed into top of furnace - with the screw heads below the sheet metal skin top of furnace. The intake pipe is 3.5" external diameter and roughly 10' long in total - with a total of four 90 degree turns and four 45 degree turns as it winds it's way from external access to furnance top entry. There is also a plastic "grid" that is part of the intake pipe collar in furnace that's obviously designed to stop critters.

I'd probably have to cut the intake pipe to be able to have access to hook up a vacuum cleaner blower.

When furnace is running it appears to pull in external air, but it's a "gentle" airflow.
 
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Old 10-26-11, 04:28 PM
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Was afraid that was glued in.. (doesn't need to be). So yeah just cut it and add a coupling on to it.
 
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Old 10-27-11, 06:12 PM
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I rigged up a fitting so that heavy duty shop vac was a tight fit with exterior vent pipes. When I connect vac to each pipe the suction pulls air rushing in the other pipe, right through furnace and into vacuum. So the vent pipes are completely clean/unobstructed.

If it weren't for the fact the furnace works with burner access panel off, common sense would say it's the flame sensor. Fire up cycle looks good right through burner ignition, but gas shut down occurs 5-10 seconds after jets light. And based on troubleshooting guide most common reason for two flash error code is flame sensor or flame sensor wiring.

At this stage do I just ignore the fact furnace works with burner access panel removed and replace flame sensor? Or are there any other suggestions to try first?
 
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Old 10-27-11, 08:06 PM
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I find it odd that it shuts down w/ the cover on but not with it off.. So that rules out the sensor to me.
 
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Old 10-27-11, 08:17 PM
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You are basically guessing and supposing with the methods you are using.

More precise diagnostic methods are needed.

Such as ---- measuring and correctly interpreting the pressure being measured by the pressure switch.

Measuring and correctly interpreting the DC microamps created by flame rectification and flowing along the wire from the flame sensor.

The likelihood is that there is a defect in the venting system or heat exchanger that is causing the pressure switch to drop out. But without being able to measure and observe the pressure, you are just guessing, which is not an adequate diagnostic method in this case.
 
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Old 11-03-11, 04:44 PM
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One to consider

SeatttlePioneer is correct on assessing pressure switches properly. If after this there is still an issue consider that the flame sensor is not seeing the flame in the required time. I have seen this in the past. The cause was that the gas pressure was slightly too low to propogate the flame accross the burner quickly enough for the flame sensor to detect the flame in the required time. The gas pressure required a slight increase on the regulator. This should not be addressed by an unqualified individual. Have a experienced furnace tech address.
 
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