There is a Technical bulletin on this problem, read below, you probably will not fix it by replacing parts, have a look:
CHECK ENGINE LIGHT ON WITH NO DRIVEABILITY CONCERNS, DTC P0120,
P0220, P1516, P1518 (CLEAR CODE)
TECHNICAL SERVICE BULLETIN
Reference Number(s): 03-06-04-032, Date of Issue: June 16, 2003
2003 Escalade, Escalade EXT, Escalade ESV with 4.8L, 5.3L or 6.0L Engine
(VINs T, N, U - RPOs LM7, LQ9, LQ4)
2003 Avalanche, Silverado, Suburban, Tahoe with 4.8L, 5.3L or 6.0L Engine
(VINs V, P, T, Z, N, U - RPOs LR4, LM4, LM7, L59, LQ9, LQ4)
2003 Sierra, Yukon, Yukon XL with 4.8L, 5.3L or 6.0L Engine (VINs V, P, T, Z,
N, U - RPOs LR4, LM4, LM7, L59, LQ9, LQ4)
SECTION: 06 - Engine/Propulsion System
Related Ref Number(s): 03-06-04-032
Some owners may comment that the Check Engine Light is illuminated and there is no driveability
concern. Upon investigation, the technician may find codes P0120, P0220, P1516 or P1518. Also,
technicians may note that low voltage codes P0562, P0563, P1637 or P1638 have been set.
The Tach module may set the code in error if it operates at a battery voltage of 4.5 to 5.0 volts.
If the vehicle has stored one of the low voltage codes, P0562, P0563, P1637 or P1638, follow the
appropriate diagnostics for the low voltage code. In most cases, this will lead to correcting the
concern without replacing any Electronic Throttle Control (ETC) components.
If codes P0120, P0220, P1516 or P1518 are present without any of the low voltage codes, the
technician should observe the Engine RPM in the fault record.
If the Engine RPM in the fault record is zero and the customer indicates that the vehicles has
experienced a low voltage condition, the code should be cleared and the low voltage should
be diagnosed and corrected.
If the Engine RPM is zero and the customer indicates that there have been no low voltage
conditions, then the P1516 diagnostic aids for intermittent concerns should be used to
diagnose and correct the vehicle.
If the Engine RPM is greater than zero, then check for an obvious loose ground at G103
located next to the right cylinder head just below the head and mounted into the block. Also,
check G104 located behind the left cylinder head. Then verify all electrical connections at
the TAC module.
If the concern is still not resolved, contact Technical Assistance.
CHECK ENGINE LIGHT ON WITH NO DRIVEABILITY CONCERNS, DTC P0120, P. Page 1 of 2
For vehicles repaired under warranty, use the applicable labor operation for the repair performed.
CHECK ENGINE LIGHT ON WITH NO DRIVEABILITY CONCERNS, DTC P0120, P.
Sunday, November 14th, 2010 AT 5:06 PM