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P0785 – Shift/timing solenoid -circuit malfunction
How to Clean the EGR Valve for GM Code P1406
P1870 (Transmission Component Slippage
Mine does that hard shift from 1-2 (a bit less hard also 2-3 and normal 3-4) and throws a P1870. It only does it after I drive on the highway for about 10 miles and shift from a stop. If I shut it off and turn it back on it goes away, unless I drive highway again, etc.
My understanding is that the line pressure stays high and doesn't come back down as it should to shift 1-2. You need to hookup a line pressure guage to verify and the problem is the valve body. Rockauto sells a valve body for around $200, and then you have to do the work. YT videos show this but seems a bit involved. I called a shop and they quoted me $400-$500 to swap valve body.
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On this video they show exactly the issue. I may be wrong on the problem, or the pressure is a result of the slip code? Don't know.
So I think you may have this problem, AND a shift solenoid issue. The shift solenoid is easy, just have to drop the pan and you can check continuity with a voltmeter.
But if it is slipping on all gears, probably needs a complete rebuild?